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MASSACHUSETTS WATER RESOURCES AUTHORITY PROGRAM MANAGEMENT DiViSldK V
BOSTON HARBOR PROJECT - DEER ISLAND RELATED FACILITIES LEAD DESIGN ENGINEER
TUNNELS; SHAFTS
VOLUME I
JUNE 16,1989
Submitted by IVLV. Metcalf & Eddy Metcalf & Eddy
Boston Harbor 4784
Geotechnical Interpretive Report Tunnels Shafts and Diffuser (v. 1)
DIG12RptV.1 c.1of1
MASS. WATER RESOURCE AUTHORITY I LIBRARY i
100 FIRST AVE. BOSTON, MA 02129
M:€ Metcalf & Eddy
I I B 8 I
The Schrafft Center 529 Main Street Charlestown, MA 02129 Tel: (617) 241-8850 Fax: (617) 241-9378
TASK 2 .4J3
Mr. Daniel P. O'Connor, Jr. Deputy Director, Engineering Services Program Management Division Massachusetts Water Resources Authority Charlestown Navy Yard 100 First Avenue Boston, MA 02129
SUBJECT: TASK 2.433 - GEOTECHNICAL INTERPRETIVE REPORT - TUNNELS, SHAFTS AND DIFFUSER
Dear Mr. O'Connor:
Transmitted herewith are ten copies of the final Geotechnical Interpretive Report -Tunnels, Shafts, and Diffuser, The report presents the preliminary recommendations for the design of the tunnels, shafts, and diffusers associated with the Deer Island Secondary Treatment Facility.
Please contact us if you have any questions or comments on the material presented.
Very truly yours,
Kenneth E. Mclntyre Project Director
cc: K. Willis (w/5 copies)
Attachment
TABLES OF CONTENTS
Page
SECTION 1 - EXECUTIVE SUMMARY 1-1
SECTION 2 - INTRODUCTION 2-1
2.1 Background 2-1
2.2 Previous Studies 2-1
2.3 Current Study 2-2
2.4 Related Documents 2-3
SECTION 3 - THE PROJECT 3-1
SECTION n - EXISTING INFORMATION 4-1
4.1 Literature Search 4-1
4.2 Geologic Setting 4-2
4.3 Bedrock Geology 4-7
4.4 Surficial Geology 4-28
4.5 Existing Tunnels 4-34
SECTION 5 - INTERPRETATION OF THE GEOLOGY 5-1
5.1 Outfall Tunnel 5-1
5.2 Inter-Island Tunnel 5-23
5.3 Conveyance Tunnel Shafts 5-32
SECTION 6 - GEOTECHNICAL PROPERTIES 6-1
6.1 Material Properties 6-1
6.2 Discontinuities 6-20
6.3 Permeability 6-25
6.4 In situ stress 6-29
6.5 Rock Mass Characterization 6-30
TABLE OF CONTENTS (Continued)
Page
SECTION 7 - SEISMICITY 7-1
7.1 Probabalistic Seismic Hazard Methodology 7-2
7.2 Geology 7-3
7.3 Stress Regieme 7-12
7.4 Seismic Activity 7-13
7.5 Seismic Zonation 7-17
7.6 Earthquake Recurrence Frequency 7-20
7.7 Regional Ground Motion Attenuation 7-21
7.8 Probabalistic Seismic Hazard 7-22
7.9 Seismic Design Recommendations 7-23
7.10 Conclusions
SECTION 8 - DISCUSSION OF ENGINEERING RECOMMENDATIONS 8-1
8.1 Tunnels 8-1
8.2 Shafts 8-15
8.3 Diffusers 8-17
8.4 Instrumentation 8-20
APPENDICES
Appendix A - References Cited
Appendix B - Borehole Summary Engineers Logs
1988 Marine Borings
• 1989 Shaft Borings
11
TABLE OF CONTENTS (Continued)
Appendix C - Laboratory Testing Results - Rock
• Robbins Report
• 1988 Campaign
• 1989 Shaft Borings
Appendix D - Geophysics of Boston Harbor
• Seafloor Contour Map - Outfall Area
• Top oF Bedrock Contour Map - Outfall Area
• Low Velocity Zone Contour Map - Outfall Area
• Top of Bedrock Contour Map - Inter-Island Area
Appendix E - On Island Geophysics
Appendix F - Permeability Data
Appendix G - Petrographic Report, Hager Richter Geoscience, 1989
Appendix H - Oriented Core Data
• 1988 Marine Program
• 1989 Shaft Borings
Appendix I - Memos on Reconnaisance Mapping of the Harbor Islands
Appendix J - Lineament Study, Weston Geophysical, I988
Appendix K - Seismicity Report
Appendix L - Laboratory Testing Results - Soil
• 1988 Marine Borings
• 1989 Shaft Borings
Appendix M - Borehole Geophysics Report - Shaft Borings
Appendix W - Pressuremeter Test Results - Shaft Borings
Appendix 0 - Falling Head Test Results - Shaft Borings
Appendix P - 1989 Shaft Boring Logs
Appendix Q - Inter-Island Marine Geophysical Survey
111
1.0 EXECUTIVE SUMMARY
The following report presents preliminary engineering recommendations for the
design of the tunnels, shafts and diffusers associated with the Deer Island
Secondary Treatment Facilities. The report is based on an examination of
published data and the collection and interpretation of data generated during
1988 and the spring of 1989. Information includes a review of the regional
geology, site geology, regional seismicity, seismic response spectras, and the
geotechnical characteristics of the soils and rock units to be encountered
during the construction of the inter-island tunnel and shafts; and the outfall
tunnel, shaft and diffusers. Much of the data developed during this study is
included in the appendices to the report.
The geology of the Boston Basin is quite complex and has undergone a number of
deformational episodes. The geology has been studied in detail for many years
and many questions are still unanswered. The best information on the geology
has come from the tunnels previously driven through the rocks of the Boston
Basin. The historical information of the rock types to be encountered as well
as the expected geologic features (i.e. faults, folds) are described.
M.P. Billings and CA. Kaye published many papers on the geology of the basin
and the problems encountered during construction of structures in and through
the Cambridge Formation. Owing to the limited amount of outcrops near the
tunnel alignments and the spacing of the borings, the results from these
papers may give us the best indication of what will be encountered while
driving the tunnels and constructing the shafts.
The geologic information obtained from the 1988 Marine Boring Program and from
the shaft borings is the first detailed investigation to be carried out in the
Harbor. The interpretation of the data generated is difficult to tie into the
geology established on land. The nature of the argillite makes it impossible
to correlate bedding or other geologic features encountered in one boring to
the next one. However, the core recovered has allowed us to generally
identify the geologic conditions that are likely to be encountered while
advancing the tunnels, sinking the shafts and drilling the diffusers.
1-1
A variety of in-situ and laboratory testing was carried out to characterize
the soils and rocks that will be encountered during shaft sinking, tunnel
boring and riser drilling activities. The data produced confirms the
feasibility of constructing the facilities conceived during the development of
the Facilities Plan (CDM, 1988) and should be used as input to the further
development of the design both at conceptual and detailed levels.
In addition to the quantification of strength, modulus, density and
boreability parameters of samples of the soils and rocks, an attempt is made
to predict the in-situ characteristics of the rock and soil masses by
consideration of such parameters as rock discontinuities, permeability,
existing field stress, rock mass modulus, and rock failure mechanisms.
Based on an evaluation of the historical seismicity of the region, the soil
and rock units encountered, and evaluation of the ground motions from east
coast earthquakes, response spectras were generated for structures founded on
rock and on various soil profiles that are expected at Nut and Deer Islands.
Following an overview of the data produced from the initial 1988/89 site
investigation program and the geotechnical interpretation thereof,
recommendations are made on tunnel alignment (plan and profile) and ground
conditions to be expected along these alignments.
Additional
excavation
.y, preliminary recommendations are made on the preferred method of
of the tunnels and shafts, their initial and permanent support and
recommendations given on probing ahead, ground treatment and instrumentation.
Finally, a
considered'
preliminary discussion is presented on the geotechnical factors
important for the design of the d i f fu se r at conceptual and detailed
level, giving recommendations on further geotechnical work required when the
diffuser site is finally selected.
I I
I I I I I I I I
2.0 INTRODUCTION
2.1 Background
In September 1985 the Federal District Court, Massachusetts District, ruled
that the discharge of "primary" treated effluent into Boston Harbor was
unlawful and ordered the Massachusetts Water Resources Authority (MWRA) to
provide full "secondary" treatment of its wastewater discharges. The MWRA
proposed a schedule containing specific dates for completion of particular
design and construction elements of the project, and this became legally
binding in May 1986.
The preferred plan consists of a pump station at Nut Island linked by a tunnel
to a secondary treatment facility at Deer Island. This plan will also process
wastewater from other parts of the Boston area that currently is delivered
through the two existing tunnels, the Main Drainage and the Worth Metropolitan
Relief Tunnels. Treated effluent will then be discharged through a deep-rock
outfall tunnel extending approximately nine miles offshore to a diffuser area
in Massachusetts Bay.
2.2 Previous Studies
As the first phase of this project, MWRA let a contract with Camp Dresser and
McKee (CDM) for a Facilities Plan studying the feasibility of designing and
constructing such a project and for determining an acceptable location for the
diffusers. Part of this study was a preliminary site investigation consisting
of boreholes and a geophysical survey. Another was an environmental impact
report which identified an area within which the diffusers should be
located. An independent environmental impact statement, produced for the
Environmental Protection Agency, essentially confirmed this location.
Based on the CDM Facilities Plan study, a second site investigation was
initiated consisting of a series of boreholes along the preferred tunnel
alignment and within the diffuser area. Because it was necessary to begin
work immediately and because a contract already existed with CDM, the drilling
2-1
services were contracted by CDM. Stone & Webster (S&W) performed the
geotechnical program planning and inspection for CDM (Phase I). At the same
time, MWRA requested proposals for lead design engineers. This contract was
awarded to Metcalf & Eddy (M&E). Once the contract was in place, overall
responsibility for the supervision of the site investigation was transferred
to M&E (Phase II).
2.3 Current Study
The purpose of the current study was to collect and analyze geological and
geotechnical information for conceptual design of the tunnels, shafts and
diffuser. There were five basic sources of information:
Available Data: papers and maps on the area geology and existing tunnels published in government bulletins, professional journals, society guide books and university publications. This included unpublished data by the late Clifford A. Kaye located at the U.S. Geological Survey's warehouse in Herndon, Virginia.
Geophysical Surveys and Seismicity Study: marine geophysical surveys of the outfall tunnel and diffuser area used to prepare contour maps of the seafloor and bedrock surface, to conduct a regional fracture trace analysis. Research into historical earthquake records to establish seismic design criteria.
Field Mapping: observations of bedrock outcrops on the Harbor Islands to determine rock types and structural features.
1988 Boreholes: twenty-five (25) along the outfall tunnel and four (4) along the inter-island tunnel with samples in soil and continuous core in bedrock. Also included oriented core measurements and packer (permeability) tests.
1989 Shaft Boreholes: three (3) boreholes, one at each proposed shift location on Deer and Nut Islands. Also includes oriented core measurements, packer tests, falling head tests, and pressuremeter tests.
Laboratory Testing: analysis of core samples for mineralogical content (petrographic analyses), strength parameters, hardness and abrasion.
This information was tabulated and analyzed to determine geological and
geotechnical conditions expected at the tunnels, shafts and diffuser.
Recommendations were then developed for design and construction.
2-2
This report describes the area geology, previous tunnel construction, geologic
conditions expected at the proposed structures, seismic design criteria and
recommendations for construction. Appendices are attached that contain logs,
data sheets and special reports pertaining to this work.
2.4 Related Documents
The following voliimes contain information related to this report:
• Coarse-Grid Marine Geophysical Surveys, Deer Island Secondary Treatment Facility by Weston Geophysical, September, I988
1988 Marine Drilling Summary Report by Metcalf & Eddy, February 1, 1989
• Boston Harbor Geological and Geotechnical References, Volumes I and II compiled by Metcalf & Eddy, May 1989
• Conceptual Design, Design Package 5, Inter-Island Tunnel & Shafts by Metcalf & Eddy, May 31, 1989
Conceptual Design, Design Package 6, Effluent Outfall Tunnel and Diffusers by Metcalf & Eddy, May 31, 1989
Conceptual Design, Design Package 6A, Effluent Outfall Shaft by Metcalf & Eddy, May 31, 1989
These are available at the MWRA Library and at the P/CM project library (at
Kaiser Engineers), both in Charlestown, Massachusetts.
2-3
3.0 THE PROJECT
The Deer Island Wastewater Treatment Plant and Facilities project was
developed to improve the existing effluent treatment and disposal facilities
for the Boston Area. The project comprises a number of individual components:
the Nut Island facility, the inter-island conveyance system, the Deer Island
Secondary Treatment Facility and the effluent outfall system. This report
deals with the inter-island conveyance system (DP-5), the effluent outfall
system (DP-6), and the tunnel shafts. Further reports will be produced
providing details of the geotechnical conditions for other components.
The inter-island conveyance system consists Of shafts at Nut Island and Deer
Island, both approximately 15 feet in internal diameter, and an 11-foot
internal diameter tunnel. The proposed Nut Island and Deer Island shaft
locations are 804,055.706 ft. E, 2,927,417.569 ft. N and 803,055.254 ft. E,
2,952,335.598 ft. N respectively (Mass. Grid, 1983). The proposed tunnel
alignment, which directly links the two shafts, is oriented approximately
north-south and passes beneath Rainsford Island and Long Island
(Figure 3-1). The anticipated tunnel elevation is between -200 and -225 feet
MDC datum, such that the entire tunnel length will lie within rock.
The effluent outfall system comprises a 30 to 50 foot internal diameter shaft
at Deer Island; a 24 foot internal diameter main outfall tunnel driven in an
east-northeast direction; and a 6,600 foot long diffuser section of
progressively reducing tunnel diameter connected to seabed by between 50 and
80 risers approximately 3 feet in diameter. These dimensions, as those of the
inter-island conveyance system components above, are determined by both
constructional and hydraulic constraints. The co-ordinates of the Deer Island
outfall shaft are 802,750.636 ft. E, 2,954,817.013 ft. N. The location of the
diffusers has been restricted by the EPA Record of Decision to an area
approximately 3 miles in diameter centered 9 miles from Deer Island, as shown
on Figure 3-1. The preferred alignment passes a short distance to the north
of a group of islands including the Graves, the Brewster Islands and Calf
Island. The anticipated effluent outfall tunnel elevation is between -225 and
-275 feet MDC datum such that the entire tunnel length will lie within rock.
3-1
I
I
4.0 EXISTING INFORMATION
This section summarizes available geological and geotechnical information
related to design and construction of the Deer Island project. Regional
geology, existing tunnels and geotechnical data will be discussed. This
information will be used as a reference to compare with new subsurface data
and to assist in the interpretation of conditions between data points.
^.1 Literature Search
A literature search was conducted to collect existing information. Sources
included the U.S. Geological Survey, journals of the American Society of Civil
Engineers and the Association of Engineering Geologists, guidebooks by the New
England Intercollegiate Geological Conference and the Proceedings of the Rapid
Excavation and Tunneling Conference. Computerized literature searches were
conducted using GEOREF and COMPENDEX. A bibliography of maps, reports and
theses is compiled into a separate report titled "Boston Harbor Geological and
Geotechnical References".
Professors at universities in the Boston area were contacted to determine the
status of any current geologic studies. Recent research consists of a
compilation of papers on the Boston Basin by Anthony Socci at Ohio University
(expected to go to press in 1989) and four unpublished M.S. theses from the
Department of Geology and Geophysics at Boston College (Cardoza, 1987; Munn,
1987; Sheridan, 1988; and Smith, 1985).
Clifford Kaye published the most papers on the geology of Boston. He spent
the last 25 years of his life examining excavations, mapping outcrops, leading
field trips and writing papers. Not all of this information was published.
In an effort to collect and review this data, visits were made to Woods Hole,
MA and Reston, VA to read his notes and look at his maps. Where relevant,
this data is reflected in this report. Abstracts of some of this work are
included in the references volume. We acknowledge the assistance of Dennis
O'Leary and Byron Stone of the USGS in making the information available to us.
4.2 Geologic Setting
Boston is located in the New England Province of the Appalachian Highlands, an
area characterized by complexly folded and faulted bedrock that has been worn
down by surface erosion and glaciation. A map showing major geologic features
of southeastern New England is presented in Figure 4-1. The Nashoba Thrust
Belt is believed to represent a collision boundary between blocks of
Paleo-African and North American plates (Barosh, 1984). This zone passes west
and northwest of Boston. Southeast of the thrust belt is largely Precambrian
igneous and metamorphic rock containing three sedimentary basins. Farther to
the southeast are submerged Cretaceous and Tertiary deposits that represent
the northward extent of the Atlantic Coastal Plain.
Locally, Boston lies in the eastern portion of a topographic and structural
depression known as the Boston Basin. Recent geologic maps of this area are
shown in Figures 4-2 and 4-3. On land, the Basin is about 25 miles long from
east to west and 15 miles wide. It extends eastward underneath Massachusetts
Bay and is believed to become wider (Kaye, 1982). The boundaries are a series
of faults that form steep escarpments at some locations. Bedrock within the
Basin is primarily sedimentary, mainly argillite, sandstone, and
conglomerate. Structurally, the bedrock is folded and faulted longitudinally
in an east-west direction. Secondary faults are also present trending
approximately north-south.
During glacial times, much of the topography we see today was formed. Massive
blocks of ice scoured the bedrock surface, eroding away the softer sedimentary
rocks that underlie the Basin. More resistant igneous and metamorphic rock
formed highlands surrounding the area. Today, bedrock inside the Basin is
covered by glacially deposited soil up to 200 feet thick. Present-day
topographic features such as Beacon Hill, Bunker Hill and some of the Boston
Harbor Islands are comprised of glacial debris.
4-2
Figure 4.1 MAJOR TECTONIC PROVINCES ANO STRUCTURES OF SOUTHEASTERN NEW ENGLAND (Barosh, 1984)
4-3
4 2* IS '
0 1 2 3 4 l l l l t I 1 I I 1 1 0 1 2 3 4 S 8
/ % r s
/J)i;>-^\ >; /VT^\ N
WEUESLEYV.-VV.V:
CNEEDHAM ^ ^ - ^
DEDHAM
5 MILES 1 i i
7 a KILOMETERS
71«00-
V , ^ ^ LYNN
uA/aWen /?;Ver ^ ^ - ^ ' ^ ' ^ ^ X
. , . MEOFOHO_A_—r-
/wATERTOWN/
J\^^ "r^tl —1 ^ .^AMBRlOCr^H^
X _ ^ ^ - ^ /-BAY *^ —^—BROOKLINE ROXBURY
NEWTON J A M A I C A ^ P L A I N - " ^
V . ^ WEST A \BOX0URY
/ \ - * ' ^ "^ ' * * ' V
V J < - ^
/ DORCHESTER^
•"^/MATTAPAje
\ « _ ^ ' " T ^ B • w
j ^ / R e v e r e .
REVERE I *'''^':
^ r r ^ 7^
• _ ^ ^ . ^
?^VT—T—^
A ^ NAHANT,
^ " " " ^ Massachusetts
-^^l^.^H'-'^^NTHRpP
' -—;,*^Iiostort . , ^ < : : ^ Harbor ^ / ^
er^COx^v ( ) } K ^ JJ
W e y m o u t h ' ' ^ - ' " ' C - i Fore R i v e r ^ r : ^ ^ ~ \ ^
'. • " - ' ' r ^ C N ^ n y - '
' I°P"i"^^\\s~ • : BRAINTREE. X } r i c
/ /
: o
- ^ ? ; i ^
o
)UTHN^
Bay
, / / 0
/ 5
^ ^ \ C i ^ ^ X
.Si--A ~ HINGHAM
\
Note: Limits of Boston Basin shown by ticked line and major faults by heavy lines. Plutonic rocks shown by hatches (Dedham Granite), crosses (Quincy Granite) and triangles (Nahant Gabbro).
Figure 4.2 MAP OF BOSTON AREA (Kaye, 1979)
4-4
7I»15' 7I°00'
I U l
y * ^ Thrust foulf
^ Normal fault
(8) Tunnel shafts Tunnels
420 15'
0 1 2 3 4 I I 1 I I
^ ^ ^'^ ^ ^ ^ I > ^
7n5' 7I''07'30" TrOO' Geological Sketch Map of Boston Basin and Blue Hills. See Legend for explanation
of symbols for formations. In Fig. 1 Brookline and Dorchester Members are combined as Prbd. Tunnels labelled thus: mt, Maiden Tunnel; c_te. City Tunnel Extension; c_C, City Tunnel; nmt, North Metropolitan Relief Tunnel; dbt, Dorchester Bay Tunnel; wrt, West Roxbury Tunnel; d£, Dorchester Tunnel.
Figure 4.3 Geological Map of the Boston Basin (Billings, 1976)
LEGEND FOR FIGURE 4.3
OJ
E
5^
Womsutfo Formation
PondviUe Conglomerate
Cambridge Argillite
(with "Milton" quartzite),,
iii M
C--L3
Pw
Pp
Pc (Pcm)
Squontum Member
Dorchester Member (with melaphyre)
Brookline Member (with melophyre lentil)
Mottapan Volcanic Complex
[=^Z^
o o o_o p o o o
Prs
Prd Prdm
Prb Prbm
Mm
c o o Q) in C
o c O CL
5 -_>» a>
2 o Q . <o
CO
Quincy Granite
Blue Hills Granite Porphyry
Volcanic Complex
Nahant Gabbro
Braintree Argil l i te
Weymouth Formation
V V V V V
V V M
A A A A A
A A A
SN ll : > < .
bgp
vc
€ b
€w
Lynn Volcanic Complex V V \«
Ml Basement Complex p-e
4-6
I I I I I I I I
4.3 Bedrock
Bedrock in the Boston area has been described in papers dating back to I8l8.
Despite this history, interpretation of the bedrock geology is still being
revised. This is due to the complexities of the area and the lack of outcrops
at critical locations. Probably the single most important contribution to
information on the bedrock geology has been the construction of tunnels
underneath Boston.
4.3.1 Stratigraphy
Underlying the Boston Basin and some of the surrounding area are fine-grained
volcanic rocks belonging to the Lynn and Mattapan formations. These units are
similar lithologically, but are named for their location geographically (Lynn
is to the north, Mattapan to the south). In the Maiden Tunnel, the Lynn
Volcanics were primarily felsite (52^), felsic tuff-breccia (38^), and
porphyritic felsite (10^). In the Dorchester Tunnel, the Mattapan was
pyroclastic felsite with minor basalt at one location and extensive basalts,
some amygdaloidal at another. Kaye (1984) describes these units as a series
of eruptions containing felsic material and mafic material such as sodic
andesites, keratophres, and spilites. The eruptive units are deposited as
coarse breccias, welded tuffs, layered flows, flow breccias, and amygdaloidal
lavas (Kaye, 1984). Originally, the contact between these older basement
rocks and the Boston Basin formations was thought to be an unconformity. More
recently, however, observations in the Dorchester Tunnel (Richardson, 1977)
and in field mapping (Kaye, 1984) indicate the contact is conformable, with
volcanic and sedimentary rocks interbedded.
The Boston Basin contains a series of interlayered sedimentary rocks intruded
by igneous rocks, mainly diabase. The Boston Basin was originally thought to
be a layered sequence of basal conglomerate, slate and tillite (Roxbury
Formation) overlain by fine-grained shales and slates (Cambridge Formation).
Formational names have been given to these units in the past. However, recent
4-7
I I I I I I I
Nclh Booth
Cort>oi«*
500 Mel«rj
KXQ Ue l t r i
i«m«nl Ll'<4i e< Csnlrol
SCratiRraphy of the Bckslun basin. CapKal letters indicate nature of control. A. North part of North Metropolitan Relief Tunnel. B. South part of North Metropolitan Relief Tunnel. C. Main Drainage Tunnel. D. North part of City Tunnel Extension. E. Surface geology in Maiden. F. Maiden Tunnel. G. South part of City Tunnel Extension. H. Surface geology in Brookline. I . Surface geology from Brookline lo Dorchester. J . Surface geology, north l imb of Mattapan anticline. K. Surface geology, south l imb of Mattapan anticline in Hyde Park. L. Surface geology, Dorchester Lower Mil ls. M . Furnace Brook at Adams Street, Quincy.
Figure 4.4 STRATIGRAPHIC SECTION THROUGH THE BOSTON BASIN FROM NORTH TO SOUTH (Billings, 1976)
4-8
work (Kaye, 1984) has shown that the relationship between formations is even
more complicated than is shown in Figure 4-4: there is a greater variety of
rock types than previously thought, and units change composition laterally.
Therefore, stratigraphic relationships are unclear. For the purposes of this
report, the formations will be briefly described since the terminology relates
to many of the geologic maps, papers and tunnel reports. Descriptions o f r ock
types, which is the current approach to discussing the geology, will follow
this section.
A revised summary of the geology of the Boston Basin was prepared in 1976 by
Billings, largely as a result of his mapping in the bedrock tunnels. Tables
from this paper listing the thicknesses and compositions of the units and a
partial lithologic description are presented in Tables 4-1 and 4-2,
respectively.
The Roxbury Conglomerate (Billings, 1976) occurs in the southern half of the
Basin and is comprised of three members. The oldest is the Brookline Member,
which is primarily conglomerate {U0%-52%) with argillite {M%-hS%), sandstone
(117»-31^) and basalt. The conglomerate is a gray, white or maroon colored
feldspathic sandstone (arkose) containing well-rounded pebbles and cobbles
one-half to six inches in diameter. This is the "puddingstone" visible in
various road cuts and outcrops in Brookline, Newton and Needham. The pebbles
are chiefly quartzite, quartz monzonite, granite and felsite (Billings,
1976). Argillite associated with this member is laminated like the Cambridge
Formation but is generally maroon in color compared to gray. Basalt occurs as
intrusives and as flows. Billings (1976) estimated a thickness of about 4300
feet for this member.
The Dorchester Member is similar to the Brookline Member except that the
relative abundance of rock types change. This unit is predominately argillite
(43^-8370 with some sandstone {8%~nS%) and conglomerate (97«-l8r=). The
argillite still tends to be white, pink, red or purplish gray in color.
Thicknesses up to 1590 feet were measured in the bedrock tunnels (Billings,
1976).
4-9
TABLE 4-1. LITHOLOGY OF BOSTON BAY GROUP (Source: Billings, 1976)
Thickness Conglomerate Sandstone+Argillites Tillite
(m)* {%) {$) {%) {%)
Cambridge A r g i l l i t e
Roxbury Conglomera te Squantum Member
Dorchester Member
Brookline Member
15 14 13 12
11 10 9 8 7 6 5 4 3 2 1
369 90#
2,060 1,162
41 19 92
122 187 485 399 405 146 423** 288**
• « o •
e •
o o
WO 95 • • 20 11 16 9
18 52 40 49
100 100 97 98
96 80
46 26 8 22 31 11 27
43 58 83 60 17 49 24
* Values for thickness are given in meters to nearest unit for reasons explained in the text.
+ Includes argillaceous sandstone. § Includes some arenaceous argillite. # Only bottom of formation penetrated. ** Only the upper part of this member was penetrated by the tunnels. 1. City Tunnel Extension, stations 255 + 11 to 333 + 74. 2. City Tunnel, stations 146 + 20 to 287 + 13 (includes tunnel from shaft 7
to 7B). •3. Hingham, surface data. 4. City Tunnel Extension, stations 333 + 74 to 368 + 97. 5. Main Drainage Tunnel, stations 0 + 00 to 119 + 6. City Tunnel, stations 24 + 60 to 146 + 20. 7. Hingham, surface data. 8. Main Drainage Tunnel, stations 119 + 16 to 137 9. City Tunnel, stations 11 + 60 to 24 + 60. 10. City Tunnel Extension, stations 368 + 97 to 37' 11. Hingham, surface data. 12. Main Drainage Tunnel, stations 137 + 94 to 375 + 86. 13. City Tunnel Extension, stations 398 + 98 to 627 + 72, 14. City Tunnel, stations 0 + 00 to 11 +60. 15. Maiden Tunnel, stations 2 + 00 to 24 + 57.
16.6.
1- 94.
+ 08.
4-10
TABLE 4-2. SAMPLE OF LITHOLOGY OF PART OF BOSTON BAY GROUP* (Source: Billings, 1976)
Formation Description Thickness
(m)
Cambridge A r g i l l i t e Argillite; gray slabby. 18 Sandstone; red, fine grained. 38 Argillite and sandy argillite; 8 gray, buff, red, and purplish-red; a 1-m bed of gray grit at top.
Roxbury Conglomerate Squantum Member
Dorchester Member
Conglomerate. Pebbles well rounded 2 to 8 cm in diameter, some 15 to 20 cm. A few beds of quartzite and sandstone, each 5 to 8 cm thick. Argillite, some quartzite, and conglomerate. Argillite red, pink, gray, and greenish gray. One conglomerate bed 1.5 m thick. White shale with some buff and green quartzite beds 5 mm to 20 cm thick. Sandstone; pink, fine grained. Bed of quartzite 1 m thick at top. Argillite; gray and greenish gray. One bed of fine grained sandstone 1 m thick.
Argillite; pink and red. Conglomerate. Pebbles rounded, 2 to 5 cm in diameter, maximum 12 cm; mostly quartzite and granite. Pink, sandy argillite, red sandstone, and pink argillite.
16
12
20
39
4 68
50
* From City Tunnel Extension, stations 351 + 7 4 to 398 + 98 (stations in feet).
I I
4-11
The Squantum Member is probably the best known and most controversial rock
formation in the Boston area. It has traditionally been described as a
tillite, a lithified glacial till, based on the angular shape of some pebbles
and cobbles. Bailey (1976) presents a list of nine different stages in the
interpretation of the origin of this unit. Today, most geologists believe it
is a subaqueous slump or turbidity current deposit. Kaye (1984) suggested
this could have been caused by vibrations from volcanic eruptions. He has
reclassified the unit as diamictite, which is a poorly-sorted, non-calcareous
sedimentary rock with a wide variety of grain sizes. Papers describing the
composition, source and deposition of this member are still being published.
The rock is a dark gray, purple or greenish-gray sandstone or shale containing
subrounded to angular rock fragments 2 to 24 inches in diameter. A block of
argillite 20 feet long was described in this unit in the City Tunnel
(Billings, 1976). Bailey (1976) also mentions large deformed pieces of
penecontemporaneous mudstone and siltstone up to several inches in length near
the base of the Squantum. The maximum thickness of this unit is given by
Billings (1976) as 400 feet and by Bailey (1976) as 440 feet.
Cambridge Argillite occupies the northern half of the Boston Basin and
overlies the Roxbury Conglomerate in the southern half of the Basin. It is
characterized by laminated bedding with alternating layers of light gray,
sandy and dark gray, clayey argillite. Thicker beds up to about 3 feet also
occur, and the composition occasionally grades to sandstone. A maximum
thickness of 15,000 feet for the Cambridge Argillite is shown by Billings in
Figure 4-4.
Igneous intrusions, not mentioned as a formation, are also abundant in the
Boston Basin. They occur as sills, intruded along a weak bedding plane, as
dikes, intruded along a fracture plane, or as irregularly shaped bodies that
melt into place (Rahro, I962). The composition is mainly diabase - a dark
colored, fine grained igneous rock containing feldspar and magnesium rich
silicates. Also present are basalt and aplite. The basalt and diabase are
chemically similar but have different textures. The basalt usually has no
visable crystals. In the City Tunnel, basalt comprised about 2k% of the rock
excavated and diabase comprised about h% (Tierney et. al., 1968). Igneous
4-12
intrusions, mainly altered basalt, comprised about ]0% of the rock excavated
for the Porter Square Station in Cambridge (Dill, 1986).
4.3.2 Rock Types
The descriptions given in section 4.3.1 illustrate that certain rock types
occur in several formations of the Boston Basin. This section describes the
composition of these rock types.
4.3.2.1 Argillite
Argillite is perhaps the roost common rock type in the Boston Basin. It is
comprised of clay-size particles of quartz, feldspar, sericite, chlorite and
kaolinite. Mineralogical analyses of eight argillite samples from the Main
Drainage Tunnel are given in Table 4-3 (Rahm, 1962). Darker argillite
contains more sericite and chlorite while the lighter colored argillite
contains more kaolinite (Kaye, 1967). The argillite is typically gray, but
purple, purplish brown, tan and green colors also occur. Some mineralogical
variations in argillite have been described by Kaye (1984). These include
calcareous argillite interbedded with normal argillite (Boston, Boston Harbor,
Somerville, Dorchester and West Roxbury), sideritic argillite (Somerville and
till pebbles on Harbor Islands), gypsiferous and dolomitic argillite (Boston,
Cambridge and Inner Boston Harbor), red argillite (Lynn, Milton, Quincy-Houghs
Neck, and Weymouth) and black argillite (Somerville, Charles River Syncline,
Nut Island and Long Island).
The argillite is typically hard and well indurated, more consolidated than
shale but not fissile like slate. Fresh rock tends to break across bedding
planes according to Kaye (1979). When partings do occur along bedding, they
have smooth, planar surfaces (Rahm, 1962). Bedding is typically laminated,
consisting of alternating light and dark colored layers 0.1 to 0.2 inches
thick. Bedding up to 3 feet thick is also present. Grain size can vary
locally to sandy or silty. Sedimentary structures, particularly slump folds,
are common in this unit. Slump folds are formed at the time of deposition
when soft, fine grained sediments with high water content tend to shift and
4-13
TABLE 4-3. MINERALOGY OF SHALES AND ARGILLITES (Quantitative X-Ray Analyses, Source: Rahm, 1962)
Specimen Quartz Sericite Chlorite Kaolinite Albite Zoisite Zo-epidote
M-D 3 (Shale) M-D-R 118 (Argillite) BH-30A (Shale) M-D-R 115 (Shale) M-D-R 83 (Shale) M-D-R 44 (Shale) M-D-R 121 (Argillite) M-D-R 120 (Argillite) M-D-R 119 (Argillite) M-D-R 105 (Argillite) M-D-R 90 (Argillite) M-D-R 97 (Argillite) M-D-R 100 (Argillite)
42 62 46 31 45 52 40 47 50 62 37 48 55
30 14 32 39 23 18 25 25 25 18 20 27 22
— 5 — 14 11 16 17 13 6 8 37 14 11
28 19 17 16 11 9 8 8 8 4 — --—
— — 5 — 5 5 6 5 6 5 6 7 8
— ----__ ----4 2 — 3 --— —
Analysis by Fred Layman - February, 1959
I I
4-14
slide. A diagram of t h e s e f e a t u r e s is shown in Figure 4-5. Ripple marks,
cross beds and flow clasts were also described by Rahm (I962).
In some areas of the Boston Basin, argillite is altered to a soft, whitish
rock or even to clay. This is due to secondary kaolinite replacing almost all
the minerals present (Kaye, 1967). The alteration process is not clearly
understood, mainly because it is known to extend so deep (over 300 feet) below
the top of bedrock. It may be due to surface weathering of the previously
exposed bedrock or due to hydrothermal alteration (Kaye, 1967). A map of
areas in greater Boston where kaolinization is known to exist is shown in
Figure 4-6.
4.3.2.2 Sandstone
Sandstone has been described as lenticular beds in the argillite (Kaye, 1981)
and as units 0.9 to 4l feet thick associated with tillite, conglomerate and
argillite in the Squantum and Dorchester members of the Roxbury Conglomerate
(Rahm, 1962). Its composition is chiefly sand-size particles of quartz,
feldspar (up to 35/ sodic plagioclase) and rock fragments in a matrix of clay-
size sericite, kaolinite and chlorite (Rahm, I962). Sand fragments are mostly
subangular in shape and medium to coarse in size. The color is typically tan,
green, reddish or flesh and can be mottled. Bedding in the sandstone is
generally thick, although some units have very thin shale partings with
undulating surfaces.
4.3.2.3 Conglomerate
Conglomerate occurs mainly in the central and southern portions of the Boston
Basin. Three units of conglomerate 5 to 97 feet thick were mapped in the Main
Drainage Tunnel (Rahm, I962). Thirty-eight percent of the City Tunnel was
excavated in conglomerate (Tierney et. al., 1968), and about 65 percent of the
Dorchester Tunnel was in conglomerate (Richardson, 1977). It is typically
gray-green, tan, gray or purple and comprised of rounded to subrounded, pebble
to cobble size clasts of felsite, quartzite, granite and basalt in a sandstone
matrix. Clasts comprise 30 to 50 percent of the rock (Rahm, 1962). They are
4-15
SLUMP FCLOS IN
CAMBRlOCE FORMATION
• OSTOW MAIW 0 W * I M * 6 g TUNNgL
Figure 4 .5 SLUMP FOLDS IN THE CAMBRIDGE ARGILLITE (Source: Rahm, 1962)
4-16
EXPLANATION
ll LHJL
Ai-filUu and tAndAtOfM: minor coAylofn*rmU utd vekuuc rocks DMtmd timra »rr mnkt tr^tU* k^mt4
x a t s > o
Concto*n«rkt«. minor arffilUt*. and volcmnie rocks " J
Approximate foncset l>«ew««n dominsntty eonctomsrsue f^cks and fin«-irrain*d rocks
Sthk* and dlrtetion of dip of bods
[Cnowa kaolinixatioa
Known oBsltartd rock
Figure 4.6 AREAS OF KAOLINIZED BEDROCK IN GREATER BOSTON (Source: Kaye, I967)
4-17
I I I I I I I I I I I I I I I e I I i
1 to 3 inches in diameter, but locally reach 12 inches (Tierney, et. al.
1968). The matrix is feldspathic sandstone similar in composition to the unit
described above. Sometimes bedding is evident from clasts oriented with their
long axes parallel (Kaye, 198O). More often, however, the clasts are random
and the structure is massive (Tierney et. al., Rahm 1962, and Richardson
1977). Outcrops tend to be large, rounded and sparsely fractured.
4.3.2.4 Tillite or Diamictite
Tillite or diamictite is found in Quincy at Squantum Head, in areas of Roxbury
and Jamaica Plain (Arnold Arboretum), and along the Massachusetts Turnpike
Extension in Mewton. It was also found in the City Tunnel, Main Drainage
Tunnel and Dorchester Tunnel. It is similar to conglomerate in that it
contains clasts of granite, quartzite, felsite, flow-banded volcanics, basalt,
slate and siliceous argillite (Bailey, 1976). But the clasts are subrounded
to subangular in shape and the matrix is a mixture of sand, silt and clay
instead of just sand. Poor sorting is another distinctive feature of this
unit. It has been described as a heterogeneous mass of clasts of various
Uthologies with rounded boulders up to 50 centimeters in diameter
(Wolfe, 1976). This similarity of the unit to glacial till lead early
geologists to label the unit a tillite. Subsequent observations of graded
bedding, soft sediment deformational structures, a lack of dropstones and the
local origin for clasts has led more recent geologists to favor deposition by
a gravity flow or turbidity current deposition (Bailey, 1976). The term
"diamictite" has therefore been adopted for this unit in recent years.
Controversy also surrounds the stratigraphic occurrence of the unit. Some
geologists believe that there is only one layer of diamictite, while others
believe that there are several that interfinger and grade into the other rock
types.
4.3.2.5 Diabase
Diabase is the most common intrusive rock in the Boston area. It is medium to
dark gray or greenish gray in color, dense, and comprised of sodium-rich
feldspars and mafic silicates (labradorite to oligoclase, diopside, augite,
4-18
1 I I I I I
I I D n
g fl
A
and uralitic amphibole) (Kaye, unpublished). Its most common occurrence is in
dikes that cut across other bedrock units in the Boston area. In the bedrock
tunnels, these dikes trend approximately north-south, ranging from N45W to
N60E. Dips are 60 to 90 degrees. In many cases, the dikes have intruded
along existing faults. In tunnels, the average width of dikes in tunnels is 5
to 10 feet with a maximum of 134 feet in the Maiden Tunnel. In outcrops, the
widest dike is the Medford Diabase which is up to 500 feet wide (Billings,
1976). Average spacing of the dikes in tunnels ranges from 170 feet (City
Tunnel Extension) to 1250 feet (Main Drainage Tunnel). This spread may be due
to differences in the extent to which dikes have been mapped.
_ Diabase also occurs as sills. In the tunnels, the average sill is 5 feet
jp thick, with a maximum thickness of 74 feet. On the Harbor Islands, however,
layers of fine and coarse-grained diabase form two thick sills
II (Kaye, unpublished). The Great Sill is about 300 feet thick and is comprised
of The Graves, Calf, Little Calf, Middle Brewster and Outer Brewster
lj Islands. The Lesser Sill is about 180 feet thick and is comprised of Great
™ Brewster, Little Brewster and Shag Rocks. These are the thickest and longest
I diabase sills known in Boston. The two chains of islands form a U-shape,
conforming to the synclinal structure in the area. The presence of offsets
and shoals between islands suggests that faults underlie those areas (Kaye,
H unpublished).
4.3.2.6 Basalt
Another common intrusive in the Boston area is basalt. Basalt occurs as dikes
and sills on the south limb of the Charles River Syncline in the City Tunnel
Extension (Billings and Tierney, 1964) and as dikes in outcrops in Nahant
(Bailey, 1984), Medford, Newton, Quincy, Nantasket and many other areas.
Balsalt in the City Tunnel Extension is dark green to yellow green and fine
grained. In places it contains small (0.1 to 0.2 inch) vesicles filled with
ealcite, epidote and chlorite. Petrographic examination shows that the basalt
has been extensively altered to secondary minerals - albite, hornblende,
chlorite, epidote and ealcite. Bailey (1984) mentions that many of the basalt
dikes at Nahant have well-defined chilled margins and coarser grained
4-19
I I I I I 0 I I D I I I I I I I I I I
interiors. In the City Tunnel Extension, 29 basalt dikes and 3 basalt sills
were mapped. Dikes average 46 feet thick, with a maximum of 93 feet. Basalt
dikes strike about N50W at Nahant and N45W in the City Tunnel Extension.
4.3.2.7 Felsic Intrusive Rocks
Other rock types occuring as dikes have been mentioned in tunnel reports.
Aplite was identified in 3 dikes and 2 sills ranging from 2 to 15 feet thick
in the City Tunnel Extension (Billings and Tierney, 1964). It is described as
a pink, fine grained intrusion. In the MBTA Red Line Tunnel, intrusions were
described as mostly felsic but details were not given (Cullen, et. al.,
1982). Kaye (1979) described a medium to light gray, aphanitic trachyte as
forming sills in the argillite.
4.3.2.8 Tuff
Tuff, an extrusive volcanic rock, is also interbedded with the sedimentary
rocks of the Basin. Three tuff units are described in the City Tunnel
Extension (Billings and Tierney, 1964). A white tuff comprised of siderite,
quartz, albite and chlorite intersects the tunnel for a length of 158 feet.
It is fine grained (clay to silt size), with beds 0.05 to 0.50 inch thick. A
second tuff interbedded with argillite intersects the tunnel for a length of
315 feet. It was white and fine grained, speckled with light orange siderite
crystals. Petrographic analysis indicates that the tuff is composed of
quartz, siderite, chlorite and goethite. The third tuff is a light yellow,
fine grained rock speckled with black magnetite crystals. It was composed of
magnetite (157»), quartz (257o) and kaolinite (6070.
4.3.2.9 Other Volcanic Rocks
Kaye (I98O) describes a variety of other extrusive igneous rocks as
interbedded with the sedimentary rocks of the Boston Basin. These range from
felsic to mafic in composition. Rhyolites are described as porphyrofelsic and
welded ash flows, various tuffs, flow breccias, breccia pipes and extruded
domes. They are black, red, white, cream and gray in color. Keratophyres are
• 4-20
I I I I I I I I I I I I I i I I I I I
found as massive flows, breccias, pillow lavas and laminated tuffs. These are
dark gray, dark greenish gray or reddish gray. Spilites are described as
vesicular (amygdaloidal) flows, pillow lavas, feeder pipes and vents and
pyroclastics. They are greenish (chlorite) or reddish (hematite) in color.
4.3.3 Structural Geology
Structural features include bedding planes, folds, faults, shear zones and
joint sets. The dominant trend o f bedrock s t r u c t u r e in the Boston area in
nearly east-north east, ranging from N65E to N85E. Offshore, this trend is
believed to shift to W45E (Kaye, 1984). A series o f fo ld a x e s spaced 1 to
3 miles apart occurs from north to south across the Basin. Regional faults
with a similar strike have been identified in between the folds. The
resulting pattern is a series of elongated fault blocks trending nearly east-
west and containing folded or tilted bedrock. Additional faults oriented
nearly north-south complicate the picture. Two maps showing major structural
features in the Boston area are presented in Figures 4-7 and 4-8.
4.3.3.1 Bedding
The strike of bedding in the Boston area is typically east-west, but can be
oriented in virtually any direction due to local structural changes. In
bedrock tunnels where numerous measurements have been made, strikes range from
N65W to N90W and N6OE to N90E. Minor folding produces local strikes in a
northerly direction according to Rahm (1962) and some data on the Boston
bedrock map (Kaye, 198O). A sampling of data from surface outcrops includes
N45E at Nahant (Bailey, 1984), N8OE at Orient Heights in Revere (Kaye, 198O),
N45E and N8OW in Wellesley near Routes 9 and 128 (Kaye, 1980), and N45E to
N70E at Squaw Head in Quincy (Wolfe, 1976). Dips are generally moderate,
ranging from 25 to 55 degrees to the north or south.
4.3.3.2 Folds
Nine major folds trending N6OE to N84E are shown on the bedrock map by
Billings (1976). Kaye showed 5 folds on his map in I98O (unpublished) and
4-21
I
Figure 4.7 Tectonic Map of the Boston Basin (Kaye, 1984)
-P-I
LO
42* 15*
7 n 5 '
MALDEN
7I°(X3'
7I°15 7I''07'30" 71° 00'
0 1 2 3 4 1 • I I • I I
. Miles
J f
Figure 4.8 Tectonic Map of Boston Basin and Blue Hills (Billings, 1976)
I I I I I I I I I I I I I I I n fl
I fl
4 folds on his map in 1984. The locations are not the same due to differences
in geologic interpretations. A list of regional folds with a rough
correlation between authors is presented in Table 4-4.
Folding is less complicated in the northern half of the Basin, which is
dominated by the Charles River Syncline and the Central Anticline. To the
south, Billings shows four folds (Roslindale Syncline, Mattapan Anticline,
Hyde Park Syncline and Milton Anticline) converging at Squaw Head in Quincy
and not extending offshore. Most of the fold axes plunge east at 10 to
20 degrees. Numerous local folds are imposed on the larger scale regional
structures. These have been interpreted as drag folds and related warps or
buckles (Rahm, 1962).
Structural features that intersect the proposed tunnel alignments are listed
in Table 4-5. Fewer folds intersect the outfall tunnel because its alignment
nearly parallels the regional trend. According to Kaye (1984) two folds would
intersect the outfall tunnel and five would intersect the inter-island
tunnel. Billings' picture is simpler: one fold in the outfall tunnel and two
folds in the inter-island tunnel.
4.3.3.3 Faults
Both large and small scale faults have been mapped in the Boston area. Most
of the Basin's boundary is comprised of faults. The Northern Boundary Fault
was penetrated by the Maiden Tunnel and was found to be a moderate-angle
thrust with a strike of N80E and a dip of 55 degrees north. Instead of the
wide shear zone that was expected, the fault was "knife-sharp and tight" on
one wall of the tunnel and "an opening about one inch wide" on the other wall
(Billings and Rahm, 1966).
Within the Basin, a series of east-northeast trending regional faults divide
the bedrock into elongated slices. There is considerable disagreement on the
number and locations of these faults. Kaye shows 11 and Billings shows 5.
These are also listed and correlated in-Table 4-4. Kaye's 6 additional faults
include 1) two north of the Northern Boundary Fault in Wakefield (Walden Pond
4-24
fl
I I fl
i i
TABLE 4-4 COMPARISON OF STRUCTURAL FEATURES BETWEEN BILLINGS (1976) AND KAYE (1984)
REGIONAL FOLDS
Billings. 1976
1. Charles River Syncline 2. Central Anticline 3. Roslindale Syncline* 4. Mattapan Anticline 5. Hyde Park Syncline 6. Milton Anticline 7. Wollaston Syncline* 8. Houghs Neck Anticline* 9. Hingham Anticline
1, 2. 3. 4. 5. 6. 7. 8. 9.
Kaye. 1984
Charles River Sycline Central Anticline
Squantum Syncline
Brewster Syncline
*Author states evidence for this structure is questionable
REGIONAL FAULTS
fl
I i fl
1 fl
i
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Billings, 1976
Northern Boundary
Mt. Hope Neponset
Blue Hills Thrust Ponkapoag
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Kaye, 1984
Walden Pond (Lynn) Northern Boundary (Somerville) (Cambridge) Unnamed (Mt. Hope) (Long Island) (Peddocks Island) Unnamed (Cohasset)
( )Author used these names on unpublished map, about 1980
4-25
i R i i I i I i i
TABLE 4-5. STRUCTURES ALONG TUNNEL ALIGNMENTS
Outfall Tunnel:
Folds:
Regional Faults;
Billings, 1976
Central Anticline
Mt. Hope Fault Neponset Fault
Inter-Island Conveyance Tunnel:
Folds:
Faults:
Central Anticline Wollaston Syncline
Mt. Hope Fault Neponset Fault
I 1 i R
i R
I
H
4-26
Kaye, 1984
Brewster Syncline Central Anticline
Brewster Fault Unnamed Faults (Near Outer Harbor Island)
Hull Anticline Brewster Syncline Central Anticline
Squantum Fault Long Island Fault Peddocks Island Fault Unnamed Fault (Trends M-NW)
fl
R R fl
H U V R 0 fl
I H fl
R H fl
R fl
R
and Lynn), 2) one from Medford through Revere and converging with the Walden
Pond Fault in Lynn (Somerville), 3) one from Brighton through Cambridge to the
north end of Winthrop (Cambridge), 4) one from Newton through South Boston
under Castle Island (Unnamed) and 5) one from Quincy Bay off the'north end of
Hull. The other five faults are on both maps, in roughly similar locations.
Causes of these differences are the scarcity of outcrops and complexity of the
geology. Both authors spent considerable time mapping the area and were
familiar with the various tunnel projects. In addition, Kaye had seismic
reflection profiles from the Harbor to help extend his work offshore.
Most faults observed in the field and described in tunnels are thin, rehealed
and show minor displacements. In tunnels where faults were mapped, they were
spaced 370 feet apart, on the average (City Tunnel, City Tunnel Extension,
Dorchester Tunnel and Main Drainage Tunnel). Fault zones are typically only a
few inches wide and contain fragments of rock cemented together by subsequent
mineralization. The Mount Hope Fault intersected by the Dorchester Tunnel was
estimated to have a displacement of 10,000 feet and is only 2 to 3 inches
wide. A few wide faults have been described, such as a 20-foot wide unnamed
fault in the Dorchester Tunnel (Richardson, 1976) and a 170-foot wide regional
fault in the MBTA Red Line Tunnel (Kaye, unpublished).
Billings (1976) summarized data on faults from three of the bedrock tunnels.
A total of 318 minor faults were mapped, 186 normal, 51 reverse, 4l vertical
and 40 undetermined. The most frequent strikes are N20E, N10W, and N50W.
Dips are typically 80 t o 90 degrees, but are as low as 50 degrees.
4.3.3.4 Shear Zones
Shear zones consist of a series of nearly parallel fractures. They tend to be
wider than a fault zone, and the additional fracturing produces weathered
bedrock and groundwater inflows.
Shear zones have been described in several of the Boston area tunnels. In the
Maiden Tunnel, 40 shears in the Lynn Volcanics were described, striking
Northeast with a dip of 45 NW and N80W with a steep dip to the South. Shear
4-27
fl
R R fl
R fl
fl
I I H fl
H H fl
H fl
I fl
fl
zones were also found in the argillite of the MBTA Red Line Tunnel. They are
oriented east-northeast, parallel to the regional structural trend. A large
shear zone - 4700 feet wide - accompanied by altered bedrock, groundwater
inflows and diabase intrusions was described in the Dorchester Tunnel. This
structure was oriented nearly north-south as compared to the regional trend.
4.3.3.5 Joints
Joints are patterns of fractures that develop in bedrock in response to
regional stresses such as folding. As a result, their orientations will
change from place to place. Joints tend to occur in groups of at least three
sets which define block boundaries. If several episodes of stress are applied
from different directions, then several groups of joints will occur.
Joints were mapped in the bedrock tunnels. Their orientations are variable,
but the most prominent sets are approximately North-South with 80 to 90 degree
dips and approximately East-West with 80 to 90 degree dips and 20 to 45 degree
dips (bedding planes). A listing of the joint data is given in Table 4-6.
4.4 Surficial Geology
Bedrock outcrops are rare in Boston. Surficial deposits can be over 200 feet
thick. Most surficial material is Pleistocene in age, deposited during the
last glacial epoch (Wisconsin stage) that ended about 12,000 years ago. These
units are glacial till, marine clay and outwash. Since that time, more recent
processes have deposited organic silt/peat, artificial fill, alluvium, and
reworked sand and gravel. These units are Holocene (Recent) in age.
Glacial deposition produces discontinuous and nonhomogeneous units. For
instance, glacial till already an unstructured mixture of sand, silt, gravel
and clay, can also contain pockets, wedges or deltas of stratified sand.
Exploratory borings need to be site specific under these conditions.
Buildings in the Boston area have had different subsurface conditions in
different corners of the foundation.
4-28
fl
H fl
fl
fl
R D fl
fl
fl
H fl
fl
fl
I fl
H I H
TABLE 4-6. JOINT DIRECTIONS MEASURED IN BEDROCK TUNNELS
Tunnel Strike Dip
City
City Extension
Maiden
MBTA
Main Drainage
N 10 E
N 10 W - N
Diverse NNE-NNW
NNE E-W
N 10 E N 75 W N 45 £ N 90 E
30 E
steep
65-90
30-90 steep
90 20-45
Sow 90 80NW 803
4-29
fl
fl
R fl
fl
fl
fl
H H H fl
fl
R R R fl
H fl
R
4.4.1 Glacial Till
This unit is a variable mixture of sand, silt, clay, gravel, cobbles and
occasional boulders. Typically, it is very dense, unstratified, and contains
at least 15 percent silt and clay. Clasts in the till are angular to
subangular in shape because they were moved and deposited by glacial ice
rather than water. At Beacon Hill, the till was found to contain sheets of
older sedimentary deposits, structural deformation such as thrust faults and
folds, and local variations such as stratified sand, sand pockets and gravel
deltas (Kaye, 1976).
Glacial till ranging from 5 fco 25 feet thick, occurs as a discontinuous
blanket on top of bedrock. In places where the till is missing, glacial
outwash in the form of sand or sand and gravel may be present.
Till also forms elongated hills (drumlins) that form onshore landmarks and
offshore islands. Offshore seismic profiles have also revealed submerged
drumlins that have been eroded by wave action to form platforms and drumlins
buried below the seafloor by other glacial deposits (Kaye, 1976). In some
drumlins glacial till is interlayered with marine clay. A map of drumlins in
the Boston area is shown in Figure 4-9. The orientations of their long axes
has been used to determine the direction of glacial ice flow. In the Boston
area, the drumlin axes are oriented from southwest to east, a range of 135
degrees. Kaye (1976) has interpreted this to mean that during the last
glaciation the Boston area was located in an interlobate position where the
directions of ice currents were not strongly expressed.
4.4.2 Marine Clay
Among surficial units, the marine clay (Boston Blue Clay) is probably the best
known surficial unit because of its tendency to consolidate and cause building
settlements if the groundwater table is lowered.- It was deposited in low
areas between drumlins. This unit is glacial rock flour deposited in a quiet
marine environment, without the characteristic graded bedding and varves of a
lacustrine deposit (Kaye, 1976). It is comprised of clay-sized particles but
4-30
•p-
Part of Boston basin showing: 1) drumlins (black); 2) boundary fault on north side of basin (heavy broken line); 3) major outcrops of conglomerate and volcanics within basin (fine dashed line, ticks towards these rocks); and 4) major areas of made-land (stippled).
Figure 4.9 Map of Drumlins in the Boston Area (Kaye, 1976)
fl
I n n I fl
n fl
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becomes sandy or silty locally or is interbedded with thin fine sand layers.
The clay unit is composed primarily of illite with some chlorite and a little
mixed-layered smectite/illite (Kaye, 1979). It maybe over 200 feet thick.
Kaye (1979) has distinguished 3 units of marine clay in the Boston area. The
oldest is part of the pre-existing sedimentary deposits that were thrust over
glacial till in the Beacon Hill area. This clay contains numerous dropstones
and masses of till. His second unit is the typical soft marine clay, gray in
color and moderately plastic. The top is marked by a layer of sand, gravel
and/or till 2 to 8 feet thick. This represents the Beacon Hill readvance and
is found at depths of 60 to 100 feet. A third, younger clay is characterized
by an oxidation zone 6 to 10 feet thick at the top where the clay is stiff and
yellow-colored. This unit is overlain by as much as 15 feet of till, which
represents the Back Bay readvance.
4.4.3 Outwash
The outwash consists of stratified sand and sand and gravel deposited by
meltwater streams during the retreat of the ice front. It overlies the marine
clay in most areas of Cambridge and Back Bay and maybe up to 15 feet thick
(Kaye, 1979). It is typically medium dense.
4.4.4 Organic Deposits
As sea level fell in post-glacial time, estuaries and marshes formed along the
shoreline. These areas were much more extensive than is evident today because
all the rivers were tidal and extensive filling had not taken place. Dark
brown to black, fine to coarse grained sands are interbedded with organic
silt, shells and peat. This unit is up to 20 feet thick (Kaye, 1979) and is
soft to medium dense. It is often used as a marker horizon to indiate the
base of fill or top of natural ground.
4-32
H R fl
I R fl
H fl
fl
fl
fl
fl
fl
H fl
fl
fl
fl
fl
4.4.5 Alluvium
This unit includes sand, gravel, and silt deposited along stream beds. Runoff
from seasonal rainfall erodes and transports material to these streams. When
flooding occurs, the enlarged streams carry alluvium down gradient, eventually
to the sea. Since topography is relatively flat in the Boston Basin, alluvial
deposits are thin and narrow. They consist primarily of sand with varying
amounts of silt and/or gravel. In areas of poor drainage, swamps may develop
containing deposits of organic silt or peat in addition to fine grained
alluvium.
4.4.6 Reworked Sand and Gravel
In coastal areas, wave erosion and longshore currents are transporting and
redepositing surficial materials, mainly glacial till. These processes have
been described on the Harbor Islands (Kaye, 1967), Winthrop Beach
(FitzGerald, 1984) and Thompson's Island (Rosen, 1984). Exposed drumlin
cliffs are visible all along the Boston coast and offshore on the Harbor
Islands. Storm waves, rainfall and freeze/thaw cycles erode these steep and
often unvegetated slopes, causing gradual loss and an occasional landslide.
This material is then transported by longshore currents, creating new
landforms elsewhere.
Spits are long, narrow ridges of sand and gravel that extend out from the end
of a peninsula or island. Rosen (1984) described three spits on Thompson's
Island. Great Brewster Spit extends for a mile to the southwest off that
island. Great Fawn is a spit extending 0.7 miles east of Deer Island.
Tombolos are spits that connect one island to another or to the mainland.
Yirrell Beach, which connects Deer Island to Winthrop is a tombolo
(FitzGerald, 1984). Deer Island was an island until 1934 when the opening at
Shirley Gut closed. Actually, Winthrop is comprised of a series of drumlins
interconnected with reworked sand and gravel tombolos (see Figure 4-10). Nut
Island is also a leveled drumlin connected to Great Hill in Quincy, another
drumlin, by a tombolo. Several of the harbor islands such as Peddocks, Long,
Spectacle, Grape and Great Brewster are drumlins connected by tomobolos.
4-33
I I R R R R H fl
fl
R fl
^SEACHMONT
DRUMLtN LOCATIONS
N
t
Point, Shirlay
^ \ D E E f t «ISLAND
• !-...- r- wM V7«
Figure 4.10 Map of Drumlin Locations - Winthrop, MA (Fitzgerald, 1984)
4-34
fl
fl Deposits on the seafloor are also reworked sand and gravel. The upper 5 to 10
feet of these deposits shift due to seasonal variations and storm waves. They
are generally loose and difficult to recover with a split spoon sampler. In
still-water areas, they become silty and may contain organics.
Any construction in shoreline areas will have to be compatible with coastal
processes. Projections (breakwaters, groins, piers) or fill areas that
interrupt the longshore currents may produce erosion or deposition outside the
project boundaries. The coastline is a dynamic environment affected by
seasonal cycles, storm events and long-term processes.
4.4.7 Artificial Fill
Comparing modern maps with colonial maps of the original Boston Peninsula
shows the extent of artificial fill subsequently used to develop the area.
The entire waterfront, Back Bay and much of Charlestown and East Boston have
been created by filling (see stippled areas on Figure 4-9). The fill material
is primarily sand mixed with ash, brick, glass, wood or concrete and is highly
variable in density. The fill is an average of 15 feet thick (Kaye, 1979).
In it are embedded remnants of previous construction, such as wooden piles,
granite wharfs, and foundation walls. These can be obstructions to new
construction and cause costly delays if encountered unexpectedly.
Historical records of a project site should be reviewed to determine the
potential for buried remnants of previous construction. Maps of Fort Dawes,
earlier stages of the wastewater facilities, and old lighthouse or buoy
locations should be collected.
4.5 Existing Tunnels
Eight major bedrock tunnels have been constructed in the Boston area. The
locations are shown on the map in Figure 4-11. Papers describing the
projects, including the geologic conditions and construction methods, have
been published in professional journals. This information has been summarized
into three tables that contain general, geologic and construction data. Each
of these tables are discussed below.
4-35
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• » » » » »
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MALDEN TUNNEL
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' \ — J ^ ^ " ' 1 v ^ / •'•
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DORCHESTER BAY TUNNEL
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TU-NORTH METROPOLITAN 'm; : RELIEF TUNNEL
^ ^ ^ k i S h o f f f
•^ -^ t tShof t C
0«<r Itlan4
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TUNNCL » f • »
QUINCY
«'I3'
Figure 4-11
MAP OF BEDROCK TUNNELS IN THE BOSTON AREA
(Source: B i l l i n g s , Geology of the North Metropolitan Relief Tunnel)
4-36
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4.5.1 General Data
Basic information on the bedrock tunnels is given in Table 4-7. The tunnels
were constructed over a 100-year period from 1885 to 1983, but most were built
from 1946 to 1958. They were constructed mainly for water supply or sewage
conveyance, but also for flood control (Maiden) and for subway transportation
(MBTA Red Line Extension). Tunnel lengths range from 0.9 mile for the MBTA
Red Line Extension to 7.12 miles for the Main Drainage Tunnel. Finished
diameters are generally 10 to 12.5 feet. The largest is 19 feet for the MBTA
Red Line Extension. Because the Tunnels are located in various areas of
greater Boston, they provide a range of geologic conditions. Nationally
recognized contractors have been responsible for their construction.
4.5.2 Geologic Data
The bedrock tunnels intersect every geologic formation and most major
structures in the Boston Basin. In fact our geologic knowledge of the area
has been developed largely from this information. The tunnels provide a
continuous view of lithologic and structural features and contacts that
determine the stratigraphic relationships. A list of geologic information
recorded in each of the bedrock tunnels is given in Table 4-8.
Every geologic formation in the Boston Basin has been penetrated by a
tunnel. In addition, the Maiden Tunnel crosses the Northern Boundary Fault
and was partly excavated in the Lynn Volcanics. The Dorchester Tunnel
intersects the Mattapan Volcanics.
Regional folds and numerous smaller scale folds were measured and described.
Most tunnels intersects only one regional fold, but the Dorchester Tunnel
intersects six. Most folds trend east-northeast and plunge at 12 to
18 degrees. Superimposed on these folds are numerous minor folds, some
congruous with the regional structure (drags) and some in other directions
(buckles or warps).
4-37
BEDROCK TUNNELS - GENERAL DATA TABLE H - 1
Tunnel Purpose Yr. Built Length Diameter Location (Shaft) Contractor
4^ I
00
city Tunnel Water
City Tunnel Water Extension
Dorchester Water
Dorchester Sewage
Halden Flood Control
MBTA Red Subway Line Ext .
Ha In Sewage Drainage
North Het. Sewage Relief
19U6-1951
1951-1956
1970-1973
1885 est.
1957-1958
1979-1983
195U-1959
1952-1956
5.U3 ml (28,682 ft)
7.10 ml (37,511 ft)
6.33 mi (33,t37 ft)
1.15 mi
1.0 mi (5,266 ft)
0.9 mi» (4,810 ft)
7.12 ml (37.586 ft)
3.93 ml (20,772 ft)
12 FTID (5to7) 10 FTID
10 FTID 13.5 FTOD
10 FTID 12.1 FTOD
No Data
12.5 FTID
19 FTID 22 FTOD (2 Bores)
10 FTID (A to B) 11.5 FTID (B to C)
10 FTID (Tunnel) 11 FTID (Shafts)
Weston (5) to Newton (6) to Brookline (7)
Brookline (7) to Boston (8) to Somerville (9) to Halden (9A)
Brookline (7B) to Mattapan (70) to Dorchester (7D)
Columbia Point to Squantum
Maiden R (A) to N. Maiden (B)
Harvard Sq. to Porter Sq. to Davis Sq.
Brookline (A) to South Boston (B) to Deer Is. (C)
Chelsea (2) to Deer Is. (1)
Marlnuccl Bros. (Sha f t s 6 ,7) P e r l n i , Haney, Walsh & Hugo
(Tunnel & Shaft 7A)
Morrison, Knudson, Kewitt & Haney
Not Given
No Data
Coker Const ruct ion Co.
Morrlson-Knudsen, White, Hergentlne (Harvard t o P o r t e r )
P e r l n i Corp. ( P o r t e r t o Davis)
Morrison, Knudson, Kewitt & Haney
S.A. Healey
• = Length of tunne l s In bedrock.
BEDROCK TUNNELS - GEOLOGIC DATA TABLE t-a
4^ 1
Tunnel
City Tunnel
City Tunnel Extension
Formations
Cambridge Squantum Dorchester Brookline
Brookline Dorchester Squantum Cambridge
( 1») ( 5») (12» (19*)
(35»
(65»
Folds
N. Limb, Central Anticline
both limbs, Charles R. Syncline
28 minor folds on N. Limb, plunge 8°, strike N87E
20 half folds on S. Limb plunge 20° NW
Bedding
N65E-N80W Dip 22-38W
E-W, dip up to 15°
Locally overturned to vertical
Faults
71 Mapped: 18 normal 20 revers 6 vertical
N30E steeo duo 29 with gouge or breccia, ave. 3ft thick
22 with offset over 12 feet
106 mapped 70 normal 11 reverse 22 unclass
S. Limb: N15W, 60NE
N. Limb: NICE, 50NW-90
17 with offset over 12 feet
Igneous Intrusions
109 diabase dikes N30E-30W, 80-90 ave width 5ft max width 85ft
298 mapped: 221 dikes 58 sills 19 unclass
261 diabase, 32 basalt. 5 aplite
dike thickness diabase ave 10, max 91 basalt ave 16, max 110
sill thickness
Joints
251 mapped N10E, Steep dip
130 mapped N10W to N30E dip 65E/W to 90
Dorchester Bay
Maiden
Dorchester (29*) Squantum (16*) Cambridge (55*)
Lynn Volcanics Cambridge (37*) Diabase ( 8*)
S.Limb,Central Anticline
N.Llmb, Charles R. Syncline
about 50°SE
NE, 10-80SE
No Data
diabase ave 6.5, max 68 basalt ave 55, max 93
dikes, N. Limb: N15W, 60NE N-5, steep E/W
No Data
(shaft 2) NE, 85SE to 75NW (overturned near fault)
Northern Boundary 16 dikes & 7 sills mapped N80"W, 55N thrust 1" contact
10 shears In Lynn Vol NG, 15NW N80W, steep S
16 ealcite veins G-W, steep
gash veins N80W, 15S
all diabase 1.1ft ave, lift max
dike thickness 20 ft ave 131ft max
NE, dip steep considerable spread in data
No Data
Lynn Volcanics diverse strike dip 30-90
Cambridge strike NNE to NNW dip steep
BEDROCK TUNNELS - GEOLOGIC DATA (Continued) TABU 1-8 (Continued)
4> I 4> O
Tunnel
Dorchester
MBTA Red Line Ext.
North Het. Relief
Format ions
Brookline Dorchester Squantum Cambridge Hattapan
Cambridge
Cambridge
(71*) ( 5*) ( 2*) (11*) ( 8*)
(100*)
(100*)
Folds
Central Anticline plunge 12°E
Roslindale Syncline plunge 65°NE
Unnamed Syncline Mattapan Anticline plunge NE
Unnamed Syncline plunge 20°SW
Lwr. Fall.s Anticline plunge 18° at N67E
N.Llmb, Charles R. Syncline
Both limbs, Charles R. Syncline plunges 18° at. N81E
minor syncline 4
Bedding
N60E, 60S (S.Llmb.CA)
N60-90E,70-80SE
(N.Llmb,RS) N30W, 70NE
(RS.near fault) -, 60NW (SE limb, US)
-, 60S (N.Llmb, US)
-, 21-51NW (5Limb, US)
E-W,20-15S
N50E, lOSE (shaft 2)
N70E, 70NW (S.Llmb, CRS)
Faults
67 total 8 larger: N30E, 60NW
Mount Hope N52E, 85N 2"-3" wide offset, 10,000ft
Unnamed N75E, 55SE 20ft wide
Unnamed N60E, 60NW
Stony Brook N10E, 90 1700ft wide altered rock water Inflows diabase
Many faults and shears trend ENE some contain gouge that reduces stability
Not mapped
Igneous Intrusions
37 diabase dikes N10W, 90 (50*) N20E, 70S (35*) N90E, 65N (15*) ave width 1 ft
Most felsic, also mafic N15-15E, 60-90 some contacts sheared
Not mapped
Joints
Ho Data
Bedding E-W, 20-15 S/N 12"-20" spacing
major set strike NNE,90°
shear zones strike ENE
Not mapped
anticline on N.Llmb
BEDROCK TUNNELS - GEOLOGIC DATA (Continued) TABLE 1-6 (Continued)
Tunnel
Main Drainage
Formations
Dorchester Squantum Cambridge
(32*) ( 5*) (63*)
Folds
N.Llmb,Control Anticline
87 minor folds - moat plunge ENE at 10-30° (congrous drag)
- some plunge 10-50°NW or SE (later warps)
- slump folds during deposition
Bedding
N65W-70E, dip 20-15N
Faults
158 total 68 normal 17 reverse 27 vertical 16 unclass
roost strike N-S offset 1" to 28' mean 2.5'
Igneous Intrusions
Numerous dikes, sills & Irreg. bodies
all diabase dikes trending N-S generally fresh
dikes trending NW, irreg bodies & sills more alteration
dikes up to 26.5ft thick ave 2-10ft,
sills up to 71ft thick, ave l-8rt irreg bodies up tp 120ft thick
30 dikes mapped most strike N-S dip steep to 90°
Joints
638 mapped Dorchester N10E, 80W
Squantum N75W, 90
Cambridge N15E, 80NW N90E, SOS
I -1^-
I I I I I I I I I I I t I I I I I I I
Bedding orientations were also measured in the tunnels. Where possible, such
as in the City Tunnel, these were correlated with strikes and dips in surface
outcrops. Bedding strikes primarily east-west with moderate dips (20 to
40 degrees). Other orientations occur, especially due to folding. Beds can
be steepened or overturned near faults.
Numerous faults are recorded in every tunnel. In most cases, these are a few
inches or feet wide and are healed with breccia or mineralization. Shear
zones presented a greater construction problem since they consist of numerous
fractures, sometimes weakening the bedrock from alteration and producing water
inflows. Most faults trend northeast, have steep dips and small offsets of
10 feet or less. Some faults also trend east-west (Maiden Tunnel), northwest
(City Tunnel Extension) and north-south (Main Drainage Tunnel).
These are extensive igneous i n t r u s i o n s in the tunnels. Almost 300 were mapped
in the City Tunnel Extension. Most are diabase but some are basalt (City
Tunnel Extension), and some are felsite (City Tunnel Extension and MBTA Red
Line Extension). They occur mostly as dikes but sills and irregularly shaped
bodies are also present. The intrusions are typically 5 to 10 feet thick;
however, they reach maximum thicknesses from 26.5 feet (Main Drainage) to
410 feet (City Tunnel Extension) (Table 4-8). Dikes are oriented in all
directions, primarily north-south in the City Tunnel and Dorchester Tunnel,
and primarily northeast in the Maiden and MBTA tunnels.
Joint sets were measured in some tunnels. In the City Tunnel, where 251
joints were mapped, and in the MBTA tunnels the primary joint direction seems
to be east-northeast. In the Main Drainage tunnel, joint orientations change
from nearly north-south in the Dorchester Member of the Roxbury Conglomerate
to east-west in the Cambridge Formation. The change in joint orientation is
more likely due to changes in the position of the tunnel relative to the
Central Anticline structure. Joint spacing was only mentioned in one case,
the MBTA tunnels, where bedding plane joints are typically 12 to 20 inches
apart. A flaggy fracture pattern due to bedding plane joints in the Cambridge
Formation was also found in the Main Drainage Tunnel.
4-42
I I I I I I I I I I I I I 1 I I I I I
4.5.3 Construction Data
Information on construction of the bedrock tunnels is given in Table 4-9. All
excavation was done by conventional drill and blast except for a section of
the Dorchester Tunnel. A tunnel boring machine - the Alkirk Hard Rock
Tunneller, Model T-7, manufactured by the Lawrence Company - was used to
excavate 3777 feet. Progress was good in the Cambridge Argillite, exceeding
300 feet a week at 12-foot diameter. However, progress was only 135 feet a
week in the massive and hard Mattapan Volcanics. For this reason, the TBM was
removed and the tunnel was completed by conventional methods. Similar
problems would undoubtably have been encountered in large portions of the
Roxbury Conglomerate but it was never reached by the TBM. A raise drill
successfully excavated Shaft 7D in Dorchester Lower Mills to a diameter of 6
feet; an innovative technique at the time.
The percentage of tunnel length needing steel sets for support varied from
0.06^ (City Tunnel) to 52f» (Maiden Tunnel). A variety of reasons is given for
the use of steel support. In the Maiden Tunnel, it was due to a general
weakening of the rock near the Northern Boundary Fault, although the fault
itself is narrow and tight. The Stony Brook Fault was also cited as the main
reason for steel support in the Dorchester Tunnel. Except for these major
regional structures, fault zones generally did not require support. The use
of steel was due more to weaker rock composition (City Tunnel Extension and
Dorchester Member, Main Drainage Tunnel) and localized jointing related to
folding (Dorchester Tunnel). Shotcrete was used in the Dorchester Tunnel but
was discontinued due to poor bonding to the argillite. Roof bolts were used
successfully in the Main Drainage Tunnel near Shaft C to pin back flaggy
partings in the Cambridge Argillite.
All tunnel linings are cast-in-place concrete. The most controversial was the
Dorchester Tunnel, with 200 psi of outward pressure, and a 1-foot thick, and
unreinforced lining. After the tunnel was placed in operation, a few homes
with basements in the local bedrock began to have water inflows. The tunnel
was dewatered and examined. Longitudinal cracks had developed in the tunnel
for a length of 2500 feet along the springline and at construction joints in
4-43
BEDROCK TUNNELS - CONSTRUCTION DATA TABLE 1-9
Tunnel Excavation *Length SUPPORT USED Type Cause/Comments Lining Groundwater
City Tunnel Drill & Blast 0.06* Steel Sets
City Tunnel Drill & Blast 5.6* Steel Sets Extension
Dorchester 11*-TBM 9-9* Steel Sets, 89*-Drill & Blast 1ft centers 7D-Ralse Drill
300 ft Shotcrete
-(
Dorches te r Bay
Maiden
MBTA Red L ine Ext .
D r i l l & B la s t
D r i l l & Blas t
D r i l l i B l a s t
No Data
52*
37* 63*
No DaU
S t e e l S e t s
S t e e l S e t s
Not especially fractured or weathered
Weak shales (33*), fracturing in dikes (33*), shear zones (16*), fracturing/jointing in sedimentary rocks (18*)
Stony Brook Fault Zone (80*) Jointing related to folding (20*)
• Discontinued due to poor bond in argillite
No Data
Northern Boundary Fault
Depth of cover, rock quality interesting shafts
Cast In place concrete No data
Cast in place concrete No data
Unreinforced cast in place
No d a t a
Cast in p lace concre te No d a t a
Cast in p lace concre te Max pumpage: 1015 gpn 1,162,000gpd 1,162,0O0gpd/mi
tunnel
Reinforced c a s t in p lace concre te
I n f i l t r a t i o n from b e d r o c k / t i l l i n t e r f a c e v i a f r a c t u r e s in shear zones
Main Drainage
North Met. R e l i e f
D r i l l & Blas t
D r i l l & Blas t
35*
Near Shaft C
21*
S t e e l Se ta
Roof Bo l t s
Wood p o s t s & s t e e l r i b s
39 s e c t i o n s , 16-I78f t long
1ft centers
Dorchester (87*), Diabase (53*) Cambridge (11*), Squantum (9*)
Bedding plane separations in argillite
Not given
Cast In place concrete No data
Cast in place concrete No data
Cast In place concrete Max pumpage: 1900 gpm 2,763,000 gpd 691,000 gpd/mi
tunnel
I I the invert. The lining had apparently cracked due to outward pressure and
water had migrated through the fracture system of the surrounding Cambridge
Argillite to basements in contact with the bedrock (Ashenden, 1982). A
grouting program was undertaken to repair the problem (Dugan, 1982).
Groundwater data were recorded in only two cases. The maximum pumpage rates
ranged from 694,000 gallons per day per mile in the North Metropolitan Relief
Tunnel to 1,462,000 gallons per day per mile in the Maiden Tunnel. Conditions
were described as "unusually wet" in the Maiden Tunnel, which penetrated the
Northern Boundary Fault and 40 shears in the Lynn Volcanics ove r its length of
just under one mile.
4.5.4 Summary
Records from existing bedrock tunnels have revealed the complexity of geologic
conditions typical of the Boston area. A variety of rock types were
encountered, as well as numerous folds, faults and intrusions. This
information will be useful in anticipating conditions in the proposed Deer
Island tunnels. Records on construction are helpful but not as detailed and
reflect older tunneling methods. There is little experience with tunnel
boring machine excavation or supports other than steel sets. There are no
laboratory analyses of rock strength or abrasion characteristics. The reasons
for using steel sets were not recorded in detail and terms like "weak rock"
are unclear. Information on groundwater inflows such as the number of
locations, geologic cause and individual flow rates was not documented. Data
that are given are useful but limited.
4-45
I I I
5.0 INTERPRETATION OF THE GEOLOGY
During 1988, a coarse grid seismic reflection and refraction survey was run
within the proposed outfall and diffuser areas. In addition, 25 borings from
275 to 450 feet deep were drilled in the outfall diffuser areas. The emphasis
of the 1988 boring program was in the southern portion of the outfall study
area because of a "low velocity zone" identified by the geophysical survey in
the northern portion of the study area (Appendix D). A low velocity zone
could indicate the presence of weathered or highly fractured rock. The
borings in the northern portion of the study area indicate that the rock is
more highly weathered. For this and other reasons, the investigation of
diffuser location option 3 was discontinued and efforts were concentrated on
Optimizing locations in the southerly zones, options 1 and 2. The borings
were spaced so as to get information throughout the southern portion of the
area.
Four borings were added along the proposed inter-island conveyance tunnel.
There is little information about the quality of rock in this area and most of
the previous geophysical surveys were run either east and west of the proposed
alignment and or the south, near Nut Island. One of the four borings (88-28)
was placed in a suspected bedrock low and the remainder spaced evenly along
the tunnel route. An additional 11 miles of seismic reflection and refraction
surveys were run in February I989 between Nut and Deer Islands.
A discussion of the I988 drilling program is given in the I988 Marine Drilling
Summary Report (Feb. 1989, Volumes I and II).
Information on the geology of the three shafts was obtained from three
borings, one at each shaft (Appendix P), with seismic refraction surveys in
each shaft area (Appendix E).
In addition to the seismic survey and drilling program, additional information
on the bedrock of Boston Harbor and Broad Sound was obtained by reconnaissance
mapping of the Harbor Islands that had known bedrock outcrops (Appendix I).
5-1
5.1 Geology Along the Outfall Tunnel
5.1-1 Overburden Materials
Approximately 22 percent of the material sampled, drilled, or cored in the
outfall and diffuser areas was soils (Figure 5-1). The soils consisted mostly
of a gray clay/silty clay and a dense glacial till (Figure 5-1). As discussed
in Section 4, the Boston Basin area experienced glaciation during the
Pleistocene, and it was this final glaciation that deposited and shaped much
of the surficial material in the area. The sea floor contour map from the
coarse grid seismic work indicates that there are drumlins in the eastern
portion of the study area (Appendix D).
Generally, proceeding from the sea bed down to the top of rock, the soils
consist of recent sediments over dense marine clays, which are underlain by
glacial till.
A thin layer of recent sediments blankets most of the area. These sediments
are usually 2-15 feet thick and consists of an organic rich clay and fine
sand, sand, or sandy gravel. These recent sediments comprise of 6.5^ of the
overburden materials sampled.
The dense marine clays (clay and silty clay) varied in thickness, range in
thickness from 10 to 152 feet with an average thickness of 50.6 feet. In the
diffuser area (Borings 88-14, 88-3, 88-18, 88-15, 88-9, 88-10, 88-11, 88-12
and 88-4) the clay averages is 43.5 feet in thickness. The clay is usually
plastic to slightly plastic, and ranges from soft to stiff. It is probably
contemporaneous with the Boston Blue Clay but no upper dessicated layer was
apparent,
A layer of dense glacial till usually underlies the clay. The thickness and
nature of the till are variable (Appendix B). The till is 6 feet to 119 feet
thick with an average thickness of 34.3 feet for the whole area and 36.4 feet
in the diffuser area. The till usually consists of fine to coarse gravel
(mostly metamorphic Uthologies), coarse sand, some to little fine sand and
5-2
FIGURE 5-lA - MATERIALS RECOVERED OUTFALL AND DIFFUSER - 1988
TILL ( S . i * ) " ^ " ^ ^ i f ^ L — _ SANDY ARGILLITE (8.9*)
SAND (0.8K)
SILTY CLAY (6.6X)
DIABASE (6.0SK)
INTRUSIVES (0.7*)
ALT.ARGILLITE (4.3X)
ARGILLITE (55.9!«)
FIGURE 5-lB - ROCK TYPES RECOVERED
DIABASE (7.7*)
INTRUSIVES (0.9«)
ALT.ARGILLITE (5.5K)
OUTFALL AND DIFFUSER - 1988 TUFF (3.3*)
SANDY ARGILUTE (11.3*)
ARGILLITE (71.3*)
5-3
I stiff gray clay. It was difficult to sample the till, and recoveries were
generally poor.
5.1.2 Lithologles Encountered
5.1.2.1 Argillite
Approximately 71^ of the rock core recovered from the outfall borings was
argillite (Figure 5-1). The argillite tends to be gray to dark gray,
moderately hard and unweathered. Generally, bedding within the argillite is
prominent and less than 0.4 feet thick. In many instances the color of the
argillite within a bed varies giving it a varved appearance. The apparent dip
of the bedding ranges from near vertical to horizontal, but typically ranges
from 30 to 60 degrees. The bedding is sometimes contorted with deformation
ranging from small slump features to rip-up clasts to small-scale crenulation
folds. The plastic deformation exhibited by the bedding indicates that the
environment in which the sediments were deposited was sloped and may have been
tectonicly active. The variety of depositional environments discussed by Kaye
(1982) could explain the variations in the argillite encountered. The
argillite recovered from some of the borings (88-20 and 88-8) has a poorly
developed slatey cleavage. A slatey cleavage forms from low-grade
metamorphism due to heating or regional deformation. The borings that exhibit
the cleavage are near The Graves, which is part of a large diabase sill
(estimated thickness 300 feet: Kaye, unpublished) and is also near the axis
of the Brewster Syncline.
5,1.1.2 Sandy Argillite
Many of the borings contain a coarser grained argillite, often grading into a
fine-grained sandstone. Approximately 11^ of the core recovered was
classified as a sandy argillite. This sandy argillite is gray to dark gray,
with beds ranging in thickness from 0.5 to 2.0 feet. The bedding is typically
not as well-defined as it is in the typical argillite. Most of the sandy
argillite is found in the eastern portion of the outfall study area (east of
and including Boring 88-21).
5-4
The sandy argillite is often interbedded with and grades into the typical
argillite. The interlayered nature of the two rock types may reflect
deposition on a slope or as a part of channelized submarine fan.
5.1.1.3 Altered Argillite
Approximately five percent of the core recovered during the 1988 program was
described as altered argillite. The alteration varies in appearance, extent
and degree. A majority of the altered argillite occurs in two holes, 88-2 and
88-5, both in the low velocity zone identified by Weston Geophysical.
(Figure 5-2). The most complete alteration is in boring 88-2 where
kaolinization is complete and only relict bedding remains. A thin section
from this boring confirms the extensive alteration to clay and fine opaques.
Petrographic analysis shows that the material is an altered tuff rather than
an altered argillite (Appendix G) indicating that at least some of the altered
argillite in the section is a tuff instead. The altered material in Boring
88-5 is heavily jointed and underlies a 150-foot thick layer of fine grained
andesite and/or basalt.
Alteration occurs on a much smaller scale in the borings away from the low
velocity zone. Here the altered zones range from 0.5 to 22.2 ft., with an
average thickness of 10.2 feet. This alteration is reflected by a change in
the color of the core, usually due to higher chlorite content. Much of the
alteration is found near igneous intrusions, indicating that the cause may be
hydrothermal alteration along joints or zones of weakness (Kaye, 1982). Other
indications that the small scale alteration is due to hydrothermal activity is
the presence of pyrite on many Joint surfaces.
5.1.1.4 Diabase
Mafic igneous rocks were found within the study area. Approximately 7.5
percent of the core recovered was diabase (Figure 5.1). The diabase was
generally fine grained with some porphorytic zones. The diabase bodies
5-5
8
O
H
B
<0 ES6 ROCK
O fe-^'
8
>-.
\
MASSACHUSETTS
LEGEND
O 88-4 BORING Pth'-'-'JHt^LD DURING 1988 MARiNF ^nc-iN-s PROGRAM
TUNNELS
PROPOSED TUNNEL 'AND DFKUStR ALIGNMENT
LOU VELOCITY /ONE (LESS THAN UOOO FT./SEC)
FROM UESTON GfOPHYSlCAL CORPORATION/ tfC.^
FIGURE t6 tN "COARSE GRID MARINE GFOPHYStCAL SURVEYS", SEPTEMBER, t988; REVISED APRIL. 1989
FAULTS
FOLDS
TUFFACEOUS SEDtMtNi
CONGLOMERATE OR TILLITE
DIABASE U/ARGILlITt
ARGILLrTE U/SANOSTONf
THE BEDROCK IN THE OTHER AREAS IS INFERRED 10 BE ARGILLIIE
SOURCES: PUBLISHED AND UNPUBLISHED MAPS OF CA KAYE, USGS
BOSTON HARBOR PRO.IECTS BY METCALF & EDDY FOR MDC & MASSPORT->9G7 TO ' ' i AND '938 MARINE EXPLORATION
PROGRAM FOR MWRA.
2000 1000 6000 8000 10000 FT.
GRAPHC SCALE
GR€ IS MA'" ' "'-"t GP'L), 1983
H
2880000
!2970000
2360000
2950000
'29^0000
2930000
B
2920000
SCALE
CHWCEO BY nECnCDLCTOt
DRAWN BY
J. POTENZA
DEPT, CHECK
PP£)J. CHECK
METCALF k EDDY
REG. PfDF ENGH. 0*T6
REG. PRCF. ENGR. DATE
S / 8 / 8 9
i / 1 ? / B 9 SBV
;BV
NUMBER DATE MADE BY CF€CKED BY
CORRECTIONS
CHANnr LOW vELP'-TTY 70Nr
DESCRIPTION
REVISIONS
MASSACHUSETTS WATER RESOURCES AUTHORITY DEER ISLAND CONCEPTUAL DESIGN DP-&..6 <S< 6A
FIGURE 5-2 COMPILATION BEDROCK GEOLOGY MAP
BOSTON HARBOR
JOB NO.
'CAOD FILE
MWRA CONT.
MWRA ACC-
5HEET
001 004132-0003-002
GEOTECH_. SEISMIC
5534
10
Ktay h 'fd||!BS!»o^Ns4a:l'a^. Tlt ^ d|: ;fc jig, . h§^j bj^g-.. alfce JT eji -an# f| s ;feurre|l , wi%,
o a i ^ l ^ a>l iQ|if qfaarrbs; fiil'l..;jl|j^ \ ^ ^ ff^^biijies^. F| |:ipQgr;_^yi5iij| anp^f^fsi off? s^f^,
djl^l^;^^ ggfipi'S;^ lp.#ic?^^5; n^:e^.t^j aaT.|tsf^|^g|B tK^ qh|.|3|\i|^, spi>diatJ6f| and;, crl'a;| ,-
(A|jEps#|J? GI).., %RinS5 8 i | l | csii^t^l-j^^ ^ 1-B21^ sm^^B?!; (f16Q3, fie^-tp)i qft" fs in^^pain^
" T | ^ ^ %#?F-* ttt i%, if? e i t h - ^ ^ j ^ i l / a j ojy ba;: |5tr5,. %3vin^ ^ % ^ ^ ' i ^J^'9i Gmt?^m
R$fi!l3,ji ^1^5 ap# ^--"l 9| 3trm rr^nf TSte GhavsEB and:] ma '. t^f f^ntt o;^ cgrpj oi| tsllej uHL s
im tti«5 ama i.a H<?e ? eirfM t*i«5 df^t^f^ ITO fc^jpih^ ^f^ waB| ofJi^/ 13S2J f^fe thick wtMi
Q,vei?- Ippj- f^^te off si^^^lo-Pm QJii eii^en^ si^l>; npi*- ^ ' ^ 3P|§- fles^: s^g^ppe^i by Kayf
(•uaipteLii^^#.)..
THs».:d|,-fgE . ei^^ ijTi agv^giail ^ # Rffe#i^%?li sip|l|- t^li-if^l^^ ^B^ ^ ^©pauap: the- s i l l
thriJjfetie^S obwffB^ om GM-W r@;l ndi i -, a raaxijmro; sjLil tshickn^ss • GKay§;,;
u, jibJ4i.£ ; i>.; egy l::fes .sffe iite hms b^n) %pri^aedi bj v fciulfeing ;.. ijtite^rnatively tb^
' d i ^ ^ m fp^^M i|*3. bojaiji^ 8.81 .8 ma y/ tt^ panbr, oif t*te.' OMt ??? saiyi rjf •i'j!]?..?; * '- ^^
s^^f^smM^i^Y 1%) f^ffe tSti3p. i, • G ' ttiffi ^ilill ejroJjMi^jj-^ ifn tojrn-gj Q 'k^ may/ n^ .
.bet patt;? . o ^ e-3|5tje^ Q^' \$im t-wp? .^Sltfe di «S .)??s®di 1: / a e? (ijiixpiihliiished.').
Thej- G®RpjL s|#sj' tiKi43ijiye3| o Q,€r .tts^,dp--;^asB? itn i^-Rihg- 8B>-IW w :a-. tmfe: de tee^mned'.-
.Bps PjSef: q ;' t^:^ slje^nestj n tty jes q:if- thee tsa ig wl;! the? nesjiiUteiiag.: .^uxgmient probleps
-•ti|%; bjp)3?2i;j3'gf, wa-s? c^^pdj^^? aitg; 2iliSf fi&pt!-,^, &#• ijji diibtl' l'^^ fifeKtx wa^ da^'^'^^^ °^
•W<'S mwKW sfceaBejSs i|gp ?jjiis:?; r oefe (j pq^ riji r/ a^dBsaitfe? ppr da;abas,e);v Thiis diabase
ma " bss i^^^ o:^ t!tm (&,. ? . sjiiMl- ddi^^Uj^^^ by/ K§y,&. (iungt ^bljdshg ))..
• ^t^lPiB^ oS' t#.es Gja;t? i|j- B p i!Xi!»!- I&la^ss (PCJ^M;., -Ghe^^,, a-n±';©uten- inewat^);
im^js^'B^i t^%te sissnee c^f t ^? , ig|If !!J:i: a;G ii i•W•" n y" ^^i^^' Qi'9?'! RCs§d--- a? - "i ?" same t:|ine
. fi'd isi iUSli ipn <i#r 1?t:i8P' awipi'SiillLijg; ,. S -. (^tm tMandj ai^iiMfe'g^ siabs.-: are^ fbund-
B itisyX! t-ha-:: (Mj m.&m'- tiiMlvmil / IS-tiyia? iiff ^py/ MiteTi^lon^.. ^ •' y^ritlcaiL artgi-lillit^i
cltke ,,, 5> tfe! 3' S^l?: w :ele•; wliich-' ctj,t .fci|i:r!0,igfhi ^ha'. diabt§ef\. asgste liikely" formed
6ipil,95 tjcttfeBi p,ili]iijfc;03 asd:!-cSlat^sp; viiei ? rieia^o-ii^^y flUisil. Ihi efhST' sfreas. on
Caglf!;, a«5 Welti a m e®i.(|teen'; afi s Qijtefer- gfiewstfert- tegaiitt .§ndi dlaba^ge-- are;
iijta2r!la^,©mitl. ^jm'. oft" the;: i,i[vtes-pi;a^Kih|i maj?v fe dge} feB-' f xthemely. .thin fauit:
giajimsiMii^i t-M®;|iC!,ifegt@PtJ- QJ?oeiJ| in|i dutKlnp ^yrrclinail faii^ing- (Kaye^
E«iapa^:I"^ee||),. The? i^peous-- PQQ^P (?fr Th§s (Irtgve^i do:, no.fc. .ffchiblt: the - layei'ing.
fSuBSsti ami fe-' otKtfr islandi?:^.
5.1.1.5 Other IgQisous Lithologles
As expected froin previous work in the Bo'stbh ferbbr and within the Bbstbh
Basin (such as Kaye, 1984) other igneous materials were 'encountered. These
include altered ash and tuff •deposits aftd felsites. Most o-if these mat'erlais
are quite altered with high clay conteht and rahge frOra 1 inch tb 15 feet ih
thickness. Most of these deposits 'are parall'el to bedding, irtdicatihg either
ash deposition or intrusion along a weak bieddiftg plane.
Boring 88-1 contains a large affiount of altered tuffaceouS material.
Petrographic analysis of different sections ol" the core indicate extehsive
alteration to clay and chlorite (Appendix G) . Boring -88-6 also contains ah
extremely altered (chloritized) 4sh bed.
Most of the thin layers bf altered igneous rock are termed "felsite". their
extensive alteration makes it impossible to determine whether they are of
intrusive or extrusive origin. Much of the felsite is hot bedded arid light
colored. Eleven of the 25 borings from the 1988 pfograffl contain felsite
(Table 5-1). Petrographic analyses of the felsites indicate an extremely fine
grained quartz and feldspar matrix which is extensively altered to clays
(Appendix G).
5.1.3 Structufal Geology - Outfall funhfel
5.1.3.1 Regional Structures
As pointed out in Section 4 and elsewhere, the str^ucture of the basin is qtiite
complex. When the complexity is combined with the low numbed of outcrops near
the harbor, it is difficult to draw conclusions about the regional structures
from the information obtained during the 1988 drilling progrart; Oh a regional
scale, large-scale folds and faults tend to trend northeast tb east-west.
Secondary structures, specifically faults, generally trend north to
northwest. The compilation map (Figure 5^2) shows the regional geology and
structure in the Harbor area, the location of land faults extended into the
harbor may change after the 1989 geophysioal surveys;
5-8
BORING: OVERBURDEN:
t,78 ft.: 2 ft. SILT 133 ft. C U Y 18 ft. SAWOY CLAY 25 ft. T i m
277 ft.
BEDROCK
10.6 ft. SAPROLITE 8.4 ft. DIABASE 8.3' ft. ARGILLITE 15 ft. MO RECOVERY 93 ft. TUFF 10.7 ft. ALT. ARGILLITE 23 ft. DIABASE 104.4 ft. TUFF
ROD IN TUNMEL
(%)
69-100
APPARENT OIP
OF BEDS
50 - 60 slisnped
A'PPftRENt DIP .
OF JOINTS
20 - 40 70 - 85 30 - 50
88-2
88-3
88-4
I vo:
88-5
88-6
181. f*:: 153' ft. SiLTY CLAY 28 ft. TlUi
104 ft: 90 ft. SILTY CLAY
14 ft. nu.
55 ft. 10 ft. SAND 45 ft SILTY CLAY'
61 ft.: 5 ft. SAND 48 ft. SILTY CLAY 8 ft. COBBLES
11.2 ft: 11.2 ft. TILL
217 ft: 20 ft. SAPROLITE 197 ft. ALTERED. ARG.
197 ft: 9 ft. ARGILLITE 7'ft. FELSITE 181 ft. SANDY ARC.
22fcft:- 35 f h ARGILLIT6 185' ft'. AUG W/ SANDY ARG.
22-100
71-100
51-100
239 ft: 149 ft. BASALT/DIABASE 90 ft. ALTERED ARG.
409:8 ft:66.6 ft. ARGILLITE 5 ft. TUFF 51.7 ft'. ARGILLITE 6.4 ft. TUFF 28 ft. ARGILLITE 15' ft. TUFF 146'.7 ft. ARGILLITE 15.5' ft. DIABASE 36 ft. ARGILLITE 1.6 ft. FELSITE 37.3 ft. ARGILLITE
0-93
0-100
50 - 60 slumped
50 - 60
25 - 35
indistinct
50 - 60 45 - 60
bdg. pl, 70 - 80
20 - 30 bdg. pl. 70 - 80
25 - 45 60 - 70 bdg. pl. 80 - 90 10
45 - 60 70 0 - 20
bdg. pl
88-7 64 ft: 10 ft. SAND 34 ft. SILTY CLAY 20 ft; TILL
256 ft: 22 ft. ALTERED ARG. 234 ft. ARGILLITE
68-100 2 0 - 3 0 3 0 - 4 0
40 - 50 20 - 40
BORING
fi8-8
OVERBURDEN
TABLE 5-1 - SUMMARY Of DATA FROM 1988 BORINGS - QUTfALi A(iD PIF.FUSER AREA? " COMT.INUED.
BEDROCK
11 ft; 11 ft. TILL 374.5 ft:139.5 ft. ARGILLITE 145 ft. DIABASE 90 ft. ARGILLITE
RQD itt TUNNEL
(%>
50-100
APPARENT DIP
OF BEDS
30 - 45
APPAREiNT DIP
OF JOINTS
40 - 70 3b - 45
88-9A 120 ft^ to ft. GRAVEL/SAND 15.5 ft: 15.5 ft. ARGILLITE 20 ft.. SILTY CLAY HOLE ABANDONED 90 ft. TILL
30 60
30 - 60
I
88-9
88-10
ifta-'M
a8-'12
SEE BORING 88-9A 179 : f t : 179 f t . SANDY ARG.
56 f t . : -35 f t . SILTY CLAY 251.5 f t . : 251.5 f t . SANDY 20 f t . TILL ARGILLITE
•108 f t . : 9 f t . SANDY GRAVEL ,75 f t . SUTY^CLAY 24 f t . TILL
30.5 ft: 30.5 ft CLAY
192 ft.: .54 i:t. ARGILLITE 138 ft. SANDY ARGILLITE
270 ft.: 15 ft. SANDY ARG. .255 ft. ARGILLITE
•76-?) 00
'78-100
0-100
0-100
20 - .40 60
.60 - 70
0 - 10 ',10 - .20
slunped :io - 20 30
,60 - 80 bdg. p l , 25 - .40
50 - ,70 ,20 - 45 •0 - 10
,60 - 80 10 - 30 35 - 55
.60 - 80 , 2 0 - 4 5 5 - 15
88-13
;S8-13A
'88^14
:89 - f t . :
m f t . :
59 f t ; :
15 f t . SAND 74 f t . CLAY
11 ; f t . 'T ILL
5 f t . SAMD 10 f t . -CLAY '36 f t . SAND ;;8 f t . ' T ILL
'te-w '^64.5ft.: :64'.5!ft.!TlLL
rHOLE ABANDONED 8 89 ft. CONTINUED IN 88-13A
190 ft.: 72 ft. ARGILLITE 10 ft. SANDY ARG. 108 ft. ARGILLITE
313 ft.: 143 ft. ARGILLITE 80 ft.:SLATY ARG. 90 ft. ARGILLITE
284.5 ft:74 ft. ARGILLITE 2 ft. ASH FLOW 208.5-ft.-ARGILLITE
D-T0O
,100-100
15-100
30 - 60 0 - 30 40
30 -.45 50
•45 -'50 .slumped
45 -.70 •20 - 30
50 - 60 •bdg.,:pl •30 - 70 60 -;fiO
,bdg.,fl l 40 - 65
BORING
TABLE 5-1
OVERBURDEN
SUMMARY OF DATA FROM 1988 BORINGS - OUTFALL AND DIFFUSER AREAS - CONTINUED.
BEDROCK RQD
IN TUNNEL (%)
APPARENT DIP
OF BEDS
APPARENT DIP
OF JOINTS
83-16 98.3 ft: 12 ft. GRAVELLY CLAY 18 ft. SILTY CLAY 68.3 ft. TILL
246.2 ft:32 ft. ARGILLITE 5 ft. SANDSTONE 4 ft. ARGILLITE 46 ft. SANDY ARG. 13 ft. SANDSTONE 11 ft. ARGILLITE 0.5 ft. FELSITE 10 ft ARGILLITE 3 ft. FELSITE 8 ft. ARGILLITE 3.5 ft. FELSITE 110.2 ft. ARGILLITE
92-100 40 - 60 30 - 40
60 - 96 50 - 70
88-17 27 ft.: 27 ft. CLAY 318.5 ft:65 ft. SANDY ARG. 90 ft. ARGILLITE 50.5 ft. SANDY ARG. 113 ft. ARGILLITE
95-100 30 - 40 20 - 45 slunped 20 - 50
70 - 85
88-18 58.5 ft: 58.5 ft. TILL 321.5 ft:31.5 ft. ARGILLITE 10 ft. ALTERED ARG. 280 ft. ARGILLITE
98-100 30 - 50 45 - 55
70 - 90 bdg. pl. 60 - 80
88-19 19.5 ft: 19.5 ft. TILL
88-19A SEE BORING 88-19
226.2 ft:24 ft. ARGILLITE 10 ft. ALTERED ARG. 64.5 ft. DIABASE 11 ft. ALTERED ARG. 116.7 ft. DIABASE HOLE ABANDONED
146.2 ft:146.2 ft. ARGILLITE
30 - 50
92-100 30 - 60
70 - 80 30 - 40
30 - 40 70 ' 90 bdg. pi.
88-20 63 ft.! 2 ft. GRAVEL 53 ft. CLAY fi ft. TILL
333 ft.: 35.5 ft ARGILLITE 3 ft. FELSITE 21 ft. ARGILLITE 5 ft FELSITE 20.5 ft. ARGILLITE 2.5 ft. FELSITE 53 ft. ARGILLITE 3 ft. FELSITE 10 ft. ARGILLITE 15 ft FELSITE 24 ft. ARGILLITE 12.5 ft. FELSITE 128 ft. AJ?GILLITE
100-100 30 bdg. pl.
BORING
TABLE 5-1
OVERBURDEN BEDROCK
338.5 ft:171 ft. ARGILLITE 28 ft. SANDY ARG. 44.5 ft. ARGILLITE 4 ft. ALTERED ARG. 2 ft. ASH 40 ft. ARGILLITE 30 ft. SANDY ARG. 19 ft. ARGILLITE
RQD IN TUNNEL
a)
73-100
APPARENT DIP
OF BEDS
20 - 40 25 - 35
APPARENT DIP
OF JOINTS
20 - 30 60 - 75 bdg. pl.
88-21 25.5 ft: 14 ft. CLAY 11.5 ft. TILL
88-22 75.8 ft: 75.8 ft. ARGILLITE 296 ft: 10 ft. ARGILLITE 8 ft. FELSITE 83 ft. ARGILLITE 0.6 ft. FELSITE 93 ft ARGILLITE 1 ft. FELSITE 24 ft. ARGILLITE 0.6 ft. FELSITE 51 ft. ARGILLITE 10.5 ft. FELSITE 14.3 ft. ARGILLITE
36-100 40 45
60 bdg. pl. 60 - 80
1 88-23 119.3 ft:119.3 ft. TILL 330.7 ft:215.1 ft. ARGILLITE 6.5 ft. ALTERED ARG. 72 ft. ARGILLITE 2.5 ft. ALTERED ARGILLITE 34.6 ARGILLITE
58-100 65 - 85 35 - 45 sli^tped 40 - 60
10 - 30 60 - 80
88-24 174.5 ft:90.5 ft. SILTY CLAY 65 ft. CLAY 19 ft. SANDY CLAY
274.5 ft:228.5 ft. ARGILLITE 2 ft. TUFF 44 ft. ARGILLITE
86-100 50 - 70 60 - 80
10 - 30 70 - 80 0 - 20
88-25 45 ft. 45 ft. TILL 437.7 ft:14 ft. ALTERED ARG. 135 ft. ARGILLITE 17 ft. DIABASE 271.7 ft. ARGILLITE
90-100 indistinct 0 - 30 45 - 60 70 - 90
10 - 30
Tunnel Zone for RQD values at El -200 to -275 MDC Datum bdg. pl. = Bedding plane separations.
From this map it appears that the Brewster Fault will cross the preferred
tunnel alignment near Deer Island (Figure 5-2). Work by Billings (1976)
indicates that two faults will possibly intersect the tunnel (Figure 4-8). In
feoth cases the faults trend northeast, oblique to the proposed .'tunnel
alignment,.
Local faults will most likely cross the tunnel alignment. These faults will
probably treTid east-northeast., north-northwest, and east-west. The nuinber and
width of the faults is not known. The fault pattern shown around the outer
islands associated with the Brewster Syncline gives an indication of the
.possible fault density that may be encountered along the outfall tunnel
alignment (Figure 5-2). The faults in the outer island area were determined
by Kaye (unpublished). Kaye's work was based on to;pographic lineaments,
seismic reflection surveys, and island mapping.
Mapping of the outer harbor islands confirmed the presence of a synclinal
feature and also showed a fair amount of faulting in these areas, including
large shear zones (Appendix I). Large fault zones may also exist under linear
bedrock lows bordered by sheared rock (as on Calf Island). These faults trend
east and north-northwest. The extent of individual faults found in the outer
island area is unknown, as is whether they will intersect the proposed
tunnel.
There are no islands to the north of the tunnel alignment that could be used
to trace the structures through the alignment. However, 215 miles of seismie
reflection and 94 miles of seismic refraction were performed in the spring of
1988 (Weston Geophysical, 1988). This geophysical study produced a sea floor
contour map and a top-of-rock contour map (Appendix D).
The top-of-rock map (Appendix D) highlights many topographic lineaments
including linear bedrock lows and offset linear ridges that may indicate fault
or shear zones, changes in sedimentary lithology or the presence of igneous
bodies (dikes and sills). The majority of these lineaments strike north and
northeast, with a few prominent lineaments in the center of the area striking
to the northwest (Appendix J). Combining- the bedrock velocity data (from the
5-13
refraction surveys), with the lineament study shows, that many of the lineaments
are: associated, with, low velocity zones. The low velocity zone along- the
eastern, edge: of the bedrock depression may represent a large shear zone that
has been preferentially eroded..
Apparent dip angle, changes of more- than 20-degrees were observed in almost
every boring:. The change, in. apparent dip may not necessarily- be due. to
faulting or folding in the- vicrnity. It may be due to the depositional slope-
on slumping of soft sediments..
Oriented; core. was. sampled: for- approximately 30. feet in most borings-
(Appendix H) . The sampl-e intenval started 15 feet ab'ovff the proposed: tunne:!
crown.. The elevations over- which the- oriented core was sampled range from-
-214 and -34'7 feet in- the outfall study area with most, readings- occurring,
between -225' and. -275 feet. (Figure 5-3)- Strike and dip vary in. this zone;;
however the- strike" is.; typically to the east-northeast... Bedd:ing.: s.trikes' to the;
northwest were found in 5 borings^ (88-24, 88-1,. 88-23.,. 88-21',. and 88-:-25,)'..
Borings 88-24, 88-V, 88-23:-, and 88-25'are closest to Deer Island, (B.'orings: 88;-
1, 88-24, and 88-25 have their oriented core sampled lower than -3.00 feet
elev.. (MDC sewer datum)). The variation in bedding orientation within a-
single boring typically is not large. However, Borings 88-1,. 88--24, and"
88-21,. show two different strike directions... Strike of bedding may vary
depending; on the- depos-itional environment. For example, if the material; was.
deposited, within a large submarine fan complex,, bedding- strike- could:, vaery,
depending on its location and the ooncavity o f the fan..
5.1.3.2 - Fractures; and: Joints
Joints patterns along the proposed tunnel alignment are-, also; dl-ff iculfe to--
determine.. There.- were fewer oriented core readings on jo-ints than on the-
bedding' surfaces (Appendix H). Most readings were of healed- jb;ints:.. Useable:
information- from the. outcrops, in the Harbor' Islands-; have- been- d;iscu.s3;ed.
earlier-. The joint patterns found on, these- islands.-; may give- some.- rndicat.ion;.
of what will, be found., along the- tunnel; alignment:.. The bedrock; on'; the. Calf,.
Gre.en, and Outer Brewster islands is: characterized by a: blocky
5-14-
U l I
o>
C
0
3 C
Z
o
m a: »-
FIGURE 5-3 - JOIt T STRIKES OUTFALL ANO DIFFUSER AREA - 1988
I I I ' • ' ) " " " ) • • "I | -" • "1 ' • 1 ' ' "I ' • "I ' 'I T ' 1 • • -J• 'i "1 N N10EN20EN30EN40EN50EN60EN70EN80E W N80\WN7GWN60WN50\W40WN50NW20VVN10W
STRIKE DIRECTION
fracture pattern Consisting of 2 or 3 sets of high-angle joints and k
low-angle set, that parallels the bedding (Table 5-2). Average joint Spacing
varies from 0.5 to 5.0 feet and is typically 1 to 2 feet; surfae"es tehci to be
irregular. There are some similarities in joint sets from island to island^
mainly in the orientation Of the high-angle joints (Table 5-2). The Ibw-angld
Joints that appear related td bedding and layering vary from island to islandj
although those at Calf and Green islands are similar (Appendix I).
Numerous joints and fractures were found ih the 25 borings within the outfall
area. The number usually decreases with depth, as reflected by the higher
Rock Quality Designations (RQDs) (Appendix B and Figure 5-4 thru 5-6).
However, numerous healed fractures and brecciated zones were encountered even
where there were no open jdints. The average widths of these zones rahge from
0.1 to 3.9 feet (Table 5^3) arid are typically 1.2 feet. Ih many borings the
healed fractures are filled with ealcite, a hard calcareous Clay, or quairtz
with secondary chlorite or epidote. The quartz is usually found in areas near
igneous bodies. Numerous c'Slcite veins also cut across the cores. S6me of
the veins are quite thin (hair width) while others are wider (typically 0:1 to
0.5 inches). Most of the wider ealcite veins are interpreted to be healed
fractures.
The pattern of healed fractures resembles that found on the harbor' island's'.
There are typically two high^angle conjugate joints with an appareht- dip' of 66'
to 90 degrees, and a third fracture oriented nearly' parallel to the
predominant bedding angle. Often bedding planes^ are offset fciy the he'aied-
fractures;. The offsets are usually small (less than- 1- i-rich)-. in a few
instances the healed- fracture's which offset the bedding are fehem'sel-ve's offset'.-
Joint spacing is usually variable both within a boring: and between b'bririgs
(Appendix B). Figures 5-3 to 5-5 graphically displaiy the RQD values aiohg- the
borings and Table 5-4 gives'- the-- average RQD arid- recovery (REC) for each-
boring. Typica;lly, j.oint spacing is-moderately close' to-close- C3 feet td'
2' inches).. The rock' ih boring 88- -19' which has a- relativeiy loW' avfe'ra'g.'e' RQD'
(44 %), is- extremell-y sheared' a:rid slightly altered i dica-ting- that- bh¥ boring'
may have- passed, through a- fa'Ult. The- sheared- nature' df the- bedrock-" cause'd' thfe'-
5H&
TABLE 5-2 i GiSMP&RATIVE SUMMARY OF BEDROCK JOINTS SETS (M THE MftRMJR ISLANDS
JOIMT SET
Cleavage;
High Angle;
Low Ahgie:
RAIHSFORD
N65-86E,
N25^35W, N 0-lOE, N50-65Wj
Mg6-40Ei
7G-85fJW
75N£-75SW 96^80NW
50" 80SW
CALF
Mone
!ii55-70Wj
t5-3i5SEj35NW N20W^25E, N70-80W,
90-70^5W 60SW-50NE;
GREEN
None
N05~30W 60-85SW N10-40E, 9CJ-75NW N70W< 9£J
10-^30NE,20^3CiSE N20W-20E, lONE-lOSE 20-30NE Si70-80W,
OUTER
None
1^G(5W-40E, N6b-80W,
W20-60E,. 25SW
BREWSTER _
70SW-70NW 65-75SW
15-25NW'
Ul'
I
10 8
H
B
ELEVATION
(FT MDC DATUM)
0-t-OO
KEY
• RECENl SEDIMENTS
BOSTON BLUE CLAY
SAND AND CLAY
«-} TILL
ARGILLITE
5 j ALTERED ^ ARGILLITE
DIABASE
'. V " ^ SEDIMENTS
ROCK
0 TOO
RQD %
H
0
BOREHOLE 8 8 - 2 5 OFFSET 710FT S
BOREHOLE 88-2^ OFFSET 350FT S
BOREHOLE 88-1 OFFSET 550FT N
- BOREHOLE 88-23 OFFSET 91 OFT S
EOREHQuE.. 88-22 OFFSET 590FT N
NOTES: 1. SEA BED AND TOP OF BEDROCK PROFILES DERIVED FROM PRELIMINARY GEOPHYSICAL INFOMATION
B 0 soo 1000 :5oo 2000
0 >0
HORIZONTAL SCALE ( IN f^EET)
100
VERTICAL SCALE ( IN FEED
ISO 200
J
10
Mott Hay Inc SCALE
IN.ES5 OTVtnvrSE WTED CP OWCED BV fCPfUaCTOt
DRAWN BY
DEPT. CHECK
PROJ. CHECK
8
MS METCALF k EDDY
HEG. PHOF. ENGR. DATE
f€G PROF ENGR, DATE
NUMBER DATE MADE BY CHECKED BY DESCRIPTION
REVISIONS
MASSACHUSETTS WATER RESOURCES AUTHORITY
GEOTECHNICAL REPORT TUNNELS, SHAFTS AND DIFFUSER
FIGURE No. 5-4
T
JOB NO.
CAOD FILE
MWRA CONT.
MWRA ACC.„
SHEET
10 8
H KEY
M I RECENT SEDIMENTS j
BOSTON BLUE CLAY
ALTERLU
ARGILLITE
• 1**1 • DIAOASE
^'ftl t SEDIMENTS
H
I
I
D
ELEVATION (FT MDC DATUM)
150-
12b
200+00 250-^00 300+00
00
75—1
5C
25-^
0 --4—-
-25 -H I i
-50 -i i t
-75-j
- 1 00 —
- 1 25 -
-1 50 --J
-1 75 --] I (
-200-
-225-
-275 -
-300 —
350F00
BOREHOLE
88-6 OFFStT 1020FT N
Z__ BOREHOLE 88-7 OFFSET 1270F
L
N
BOREHOLE
88-20 OFFSET 1370Fr S
BOREHOLE
88-16 OFFSET 1780FT N
BOREHOLE
88-8 prrcrT ^QOFT S
BOREHOLE
88-19
OFFSET 1550FT S
BOREHOLE
88-18 OFFSET 20^0FT N
B O R E H O L L
88-21 OFFSET 530FT N
88-17 OFFSET 1G30F!
BUKEHULE
8 8 - U OFFSET /.20FT S
D
B 0 500
_J„J,
1000 SCO
HORIZONTAL SCALE (IN FEET)
?000
I . . J
B
10
Mott Hay Inc SCALE
ItLESS OTtCnwS tOTEQ OR CHW4E0 BT REPnooucrnN
DRAWN BY
N.ANSLEY DEPT. CHECK
PROJ. CHECK
8
0 :.0 00 150
VERTICAL SCALE ( IN FEET)
200
MS METCALF k EDDY
REG. PfOF. ENGR. DATE
HEG, PROF. ENGR. DATE
NUMBER DATE MADE BY CHECKED BY DESCRIPTION
REVISIONS
MASSACHUSETTS WATER RESOURCES AUTHORITY
GEOTECHNICAL REPORTS TUNNELS, SHAFTS AND DIFFUSERS
FIGURE No. 5-5 T
JOB NO.
CADD FILE
MWRA CONT.
MWRA ACC
SHEET 6-C-7
4'-ff;iJ:>*-Kl^
8
KEY
RECENT SEDIMENTS
BOSTON BLUE CLAY
SAND AND CLAY
TILL
R I ARGILLITE
ALTERED ARGILLITE
DIABASE
0 100
SEDIMENTq
ftUL-K
ROD %
H
D
B
ELEVATION (FT MDC DATUM)
150
125
100—1
75
50
25—1
0
-25
-50-1
-75
-100
-125
-150-
-175-
-200-
-225-
-250 H
-275-
-300-
350+00 ^00+00 lSO^-00
MEAN SEA LEVEL 105.65'
NOTES: \
SEA bLA BEL AND. BEDROCK PROFIL..:..
FROM PRELIMINARY GEOPHYSICAL '.^A •- i V 1 V :D
BOREHOLE 8 8 - 9 OFFSET 2020FT
V._
o
BOREHOLE 88-13A OFFSET IG-dOFT N
BOREHOLE 88-15 OFFSET 2170FT
V
S
- BOREHOLE 8 8 - 1 ' OFFS[_: i jJOFT N
.BOREHOLE 8 8 - 1 0 OFFSET 1710 S
-BOREHOLE 8 8 - 1 2 OFFSET 300FT N
BOREHOLt ST'- /.
500+00
OFFSET ^OOF r T <-
INFOMATION
0 SCO 1000 1500 2000
0 50
HORIZONTAL SCALE:
( IN FEET)
100 150 200
J J VERTICAL SCALE
( IN FEET)
p 150
- 125
100
- 7 5
- 5 0
25
0
- 2 5
- 5 0
L- _75
| - - 1 00
L 25
h - 1 50
- -1 75
J--200
- - 2 2 5 I
L- - 2 5 0 I
| - - 2 7 5
I l - - 3 0 0
D
B
10
Mott Hay Inc SCALE
IM.E95 (mCFWS MOTCD GR CHWCEO BY REfflaajCTiON
DRAWN BY
N.ANSLEY DEPT. CHECK
PROJ. CHECK
8
MS METCALF k EDDY
REG. PROF. ENGR. DATE
REG, PROF ENGR. DATE
NUMBER DATE MADE BY CHECKED BY DESCRIPTION
T REVISIONS
1 4
MASSACHUSETTS WATER RESOURCES AUTHORITY
GEOTECHNICAL REPORT TUNNELS, SHAFTS AND DIFFUSERS
FIGURE No. 5-G
1 ;
JOB NO.
CADD FILE ,
MWRA CDNT.
MWRA ACC._
SHEET. _
U-&.
'si'M' msfLum im: ni' u^nQes-j m
I
TABLE 5-4 AVERAGE ROCK RECOVERY AND RQD FOR EACH BORING
BORING FOOTAGE CORED
(ft)
AVERAGE RECOVERY
%
OUTFALL TUNNEL AND DIFFUSERS: 88-1 88-2 88-3 88-4 88-5 88-6 88-7 88-8 88-9 88-9A 88-10 88-11 88-12 88-13 88-13A 88-14 88-15 88-16 88-17 88-18 88-19 88-19A 88-20 88-21 88-22 88-23 88-24 88-25
INTER ISLAND 88-26 88-27 88-28 88-29
256.4 197.5 196.0 213.0 239.0 402.5 256.0 371.5 179.0 11.0
281.0 193.0 270.0
0.0 187.0 272.0 281.0 246.0 313.5 319.0 219.0 146.5 330.5 330.5 296.5 325.7 256.0 463.0
CONVEYANCE 298.0 343.0 202.9 270.7
93 94 97 97 90 99 96 99 92 93 88 96 85 ND 94 99 99 97 99 99 93 98 96
100 86 89 95 92
TUNNEL: 100 96 93 95
AVERAGE RQD
%
83 79 91 77 60 89 83 93 88 85 81 68 52 ND 69 96 89 92 95 89 44 94 92 98 65 81 85 89
98 89 87 91
ND - No data
5-22
boring to be abandoned at a depth of 24? ft. and redrilled approximately 50
feet to the east-northeast. The rock recovered in boring 8S-19A is a good
quality argillite (average RQD 94 % ) .
The joint surfaces encountered within the borings are usually coated with a
thin layer of gray clay. In many instances pyrite was found along the joints,
as well as along surfaces that are weak and separated during the core
logging. Calcite and quartz were also found on joint surfaces, generally with
a brown (Fe02) ^^^i"- Slickensides were found on some joint surfaces,
commonly on those surfaces coated with quartz or calcite indicating faults.
Slickensides were observed on a number of surfaces encountered on the Harbor
Islands (Appendix I).
Bedding plane separations and weak bedding planes are also important in
determining shape and type of blocks that may be encountered while driving the
tunnels and opening the shafts. Bedding plane separations are quite common
and in some cases very closely spaced (less than 2 inches). The separations
are very similar to Joints, with slight clay coatings and some staining and
mineralization. Many borings, such as 88-18, have weak bedding planes with
calcareous clay or kaolinite between the beds. Although these areas have high
RQDs the core would break easily, typically along bedding. Stress relief
openings were also common along bedding planes, occurring as the core was
being drilled or as it was being removed from the core barrel.
5.2 Inter-Island Tvmnel Alignment
This route is about 25,000 feet long and trends nearly north-south between Mut
Island and Deer Island. Rainsford Islands, composed partly of bedrock, lies
about midway along the alignment. An island composed of two drumlins, Long
Island, is located between Rainsford and Deer Island. The proposed elevation
for this tunnel is about -200 MDC Datum.
Four borings were drilled for this tunnel in 1988. These are designated 88-26
through 29 (see logs in Appendix B and profiles in Figure 5-7). A summary of
data from these boreholes is presented in Table 5-5.
5-23
TABLE 5-5
ajMMARY QF DATA FflOM 1988 BORINGS INTEM-ISLAND TIMIffiL
Boring Number
88-26
88-27
88-28
Boring Location
Pres iden ts Roads
Long I s . to Rainsford I s .
South of Rainsford I s .
Overburden
134 f t t 59 ' 55 ' 20'
45 f t : 38' 7'
95 f t : 50 ' 45 '
T i l l Clay T i l l
Clay T i l l
Clay T i l l
Bedrock
308 f t :
350 f t :
212 f t :
308'
297' 45 '
8 '
212'
A r g i l l i t e
A r g i l l i t e Diabase* A r g i l l i t e
A r g i l l i t e with Sandy A r g i l l i t e * *
RQD in Tunnel
Dip of Beds
Dip J o i n t s
N> •P-
88-29 Morth of Nut I s .
66 f t ; 19' Clay 47 ' T i l l
8535-100?
54f.-100?
22^-100^
281 f t : 281 ' A r g i l l i t e * * * 82^-100.3^
5-20 20-50 60-90
0-20 20-30 60-75
45
5-25 10-20 10-45 30-60
50-70 60-90
30 bedd ing
30-50 beddin,g
60-70 bedding
10-20 60-70 bedding
0-20 65-90
Motes: * una l t e red , with l aye r s a r g i l l i t e one igneous in t rus ion 0 . 5 ' t h i ck four igneous i n t r u s ions , 0 . 4 - 1 . 3 ' th ick two ash l aye r s 4.6^ and 0 . 3 ' th ick two f e l s i t e l aye r s 0 ,4 ' th lek
Tunnel zone for fiQD values a t El -175 to -255 MDC Datum
.Vfe- !.\ 1fck-i1* '.-fJ.>H,t'ftg.qjl '!t«yi^iMi:l iiUt.4k'tf>.!IJ8&Wfei*!JLMa^ ^iJgiJWlfleg
r H
•
B
KEY
UJ
RECENT SEDIMENTS
BOSTON B L U E
CLAY
SAND AND CL AY
TILL
ARGILLITE
! ALTERED ARGILLITE
ROD %
ELEVATION (FT MDC DATUM)
150 - ]
1 2 5 -
100
?5
50
25
0 --
- 2 5 -
- 5 0 -
- 7 S -
" 1 0 0 - |
- 1 2 5 -
- 1 5 0 -
-1 75 -
-200 -
-225 "
-250
-275
- 3 0 0 -
-325 -
-350-- f
DEER ISLAND 0-*00
LONG ISLAND 50+00
RAINSFORD ISLAND
150+00
<^<#^^K
200+00
NUT ISLAND
250+00
MEAN SEA LEVEL 105.65 '
n
BOREHOLE 88-26 OFFSET 6 4 0 n E
BOREHOLE 88-2? OFFSET 320F? E
BOREHOLE 88-28 OFFSET 120FT U
1 - 150
- 125
I - 100
•?5
- 5 0
- 2 5
-50
I - ••75
' MDO
?5
•- : ^ 5
- "200
- -225 I
j~ -250
l - -2?5
I" -300
k- 325
L- -350
bUKtHULL 38-29 OFFSET 120FT U
NOTES:
1 . SEA BED PROFILES FROM PRELIMINARY GEOPMYSICA INFOMATION
2. TOP OF BEDROCK KROM INTER ISLAND BEDROCK CONTOUR MAP UESTON GEOPHYSICAL. MAY 1989
Mott Hay Inc SCALE
UMJSs ancnwtx WTED GR CHMEO 9r ret-pooxitu
DRAWN BY
DEPT. CHECK
PROJ, CHECK
MS METCALF k EDDY
REG. PnOF. ENGR. DATE
HEG. PRCF ENGR. DATE
S / 9 / 8 ' J GAP
NUMBER DATE MADE BY CHECKED BY DESCRIPTION REVtSlDNS
"1
MASSACHUSETTS WATER RESOURCES AUTHORITY
GETECHNICAL REPORT TUNNELS SHAFTS & DIFFUSER
FIGURE 5-7 T
JOB NO
CAOD FILE.
MWRA CONT.
MWRA ACC.
SHEET.
10 8
I A few'borings into bedrock had previously been d r i l l e d i n t h i s area.. In 1982,
.one Jsoriing iWas dr i i lec l .-.at -the inortiheast end of 'Long. .Island (M&E, 1981) and
.several ^ftf.fsho.re-..from-Nut a.sland-K^Stone ••&'Webster Ehg. -Corp. ;, 1980).. In 198-1:,
!th'ree :borings were d r i l l e d negr cMiit ijsland for a proposed bedro'pk tunnel undfr
•.'Weymouth iSqre River .to •th.g iBoston -Edison -plant (Stdne & Webster E.ng'.'-^.Corp'.,
19S2,-).. None ,of ;thes,,e rbprings .was ..dfeep'enough to'•reach .tfie proposed tunnel
'ele,v:atipn-,
5,.2,.>t Pvertburd^n . l ^ t e r . i a l s
Jn -'tije ..inter^i-s'land .iDprings the --tw.o main types of •'pverburdeh .'r>e,co,vered were ^
.dense-mar-fne. ,cll:y-and g l a c i a l • - t i l l , -both 'Similar to :those ..described in the
o.U'tfail. ar.ea (Section •§._. V.'l). "The clay .ranges in thickness from 19 to 55 f ee t
w.i-t'h an .average th ickness !-D-f 4iP .feet.. ''Ttie g l a c i a l ' . t i l l ranges in th ickness
from 7 and :59 :feet kveragtn^ 3$.'feet.^. 'The : t i l l .recovered was qu i t e coarse ,
..containing a .large 'gravel and sand component. Boring 88-27 encountered a
g rea te r cl'k^ •.p.qr.fcion.
5.2.,2 L i tho log i^s EncounteFed
5 . 2 . 2 . 1 A r g i l l i t e
As in the borings withi-h -the dii 'f'all and diffuser ar&a, the majority of rock
recove.red ..along the--in.terf»-island-tiinnei alignifient was argillite (Figures 5-7
and 5-8). 'However, variation ifl-ib'S argillite is common betwe.enthe four
borings (AppendivX .B). =I-n borings 88-26 to 88-28'cthe .argillite'is typically
gray.., •raoderateiy hard and unweathered, exhibiting dis.tinctive .bedding. The
bedding iS' thin to laminar ...with a .large percentage of the bedding showing soft
sediment deformation, mostly in .the form of small slump features and rip-up
clasts.. Boring 88-29 encountered over 50 feet of slumped bedding.
The •bednocik exposed-on Rainsford Island, along the proposed tunnel alignment,
is--silso argillite. ..However it :is ;highly sheared, with a prominent near
vertical cleavage, ..A .poorly de.fined cleavage was evident in 88-28 and
.88-27. Outcrops along the scputheastern cove on Rainsford-Island have some
5-26
FIGURE 5-8A - MATERIALS RECOVERED INTER-ISLANO AREA ~ 1988 PROGRAM
TILL (12.Oils).
CLAY (lO.SSli)
DIABASE (3.0«)
3ANDY ARGILLITE (I3.4as;
ARGILLITE (60.8X)
FIGURE 5-8B - ROCK RECOVERED INTER-ISLAND AREA - 1988 PROGRAM DIABASE (3.9ilS)
SANDY ARGILLITE (17.3«)
ARGILLITE (78.6
5-27
large slump blocks (up to 12 ft diameter). Argillite is also evident at
Quarantine Rocks, west of boring 88-28. Here the argillite has undergone some
shearing and complex folding similar to that at Rainsford Island.
5.2.2.2 Sandy Argillite
Sandy argillite is the predominant rock type recovered in Boring 88-27, and
was identified as interbeds in a few areas in Boring 88-28. This sandy
argillite is bedded; however, the beds are poorly defined and are usually 1 to
3 feet thick (medium), similar to the outfall borings. The sandy argillite is
interbedded with the typical argillite and often grades to the argillite.
5.2.2.3 Altered Argillite
Thin layers of altered argillite were found in borings 88-27, and 88-28.
Those in Boring 88-27 are associated with the diabase encountered at the
bottom of the hole, and most likely represent a contact or local hydrothermal
alteration. The altered argillite in Boring 88-28 becomes a pale purple
color, and there are chlorite rich clay layers are found between beddings
planes. The change in argillite color does not by itself indicate alteration,
since Kaye (1984) discusses that argillite is many colors. However, in
combination with the secondary clay the change in color may be due to
alteration. A greenish-gray to green argillite was found in Boring 88-29 with
small layers (less than 1 ft.) altered to a whitish gray, Petrographic
analysis of this material indicates that the rock may have originated as a
tuff/felsite instead of argillite; however, due to the extensive alteration
this material will be classified as altered argillite with some tuffaceous
layers. The core is entirely indurated with calcite and chlorite, which
probably accounts for the green color. Other indications o f alteration in 8 8 -
29 include the large amount of chlorite found along the joint faces.
Typically the hardness of this altered argillite is only very slightly less
than that of the typical argillite; however there are areas where the rock was
soft. Whether this material is truly altered tuff will need closer evaluation
of the petrographic results, including additional thin sections, and analysis
of other argillites discussed in the literature.
5-28
5.2.2.4 Igneous Materials
Diabase, altered ash deposits, and felsite layers are the igneous Uthologies
encountered along the proposed inter-island tunnel alignment. Approximately
4^ of the core recovered contains igneous rock (Figure 5-7). The materials
are similar to those found in the outfall area, although less extensive.
Diabase was only encountered in Boring 88-27 for approximately 40 feet at the
bottom of the hole. This diabase dike or sill is interbedded with argillite
and altered argillite as was observed during the mapping of the Harbor
Islands. The diabase is massive. Near contacts with the argillite it is
aphanitic. However, phenocrysts of plagioclase, are visible in some areas.
Occasional quartz veins cross cut the diabase.
Diabase is also found interlayered with argillite on Hangman Island somewhat
similar to the argillite/diabase relationships seen on the outer Harbor
Islands (STEP, 1988). If the material found in 88-27 is part of a dike or
sill it may be quite extensive and may cross the proposed tunnel alignment.
Felsite and altered ash deposits were found in borings 88-27 and 88-29. These
layers are usually thin (less than 5 feet) and'generally parallel to bedding,
where bedding was visible. In Boring 88-27 felsite layers are found near the
diabase indicating that they may be related. Igneous layers and mineralized
veins, all quite altered, are found throughout Boring 88-29. The igneous
rocks with mineralized veins indicate the presence of hydrothermal fluids
within the bedrock at some time. The presence of pyrite on the Joint
surfaces, and within healed fractures indicates that the hydro-thermal
activity occurred after lithification of the argillite. A thin felsite dike,
was found on the southeast side of Rainsford Island.
5-29
5.2.3 Structural Geology - Inter-Island Tunnel
5.2.3.1 Large Scale Folds and Faults
The inter-island tunnel is closer to land; therefore, it seems reasonable to
extend land faults to the alignment. The compilation geologic map
(Figure 5-2) indicates a complex structure in the southern end of the area.
An unnamed north-northwest trending left lateral fault offsets the Brewster
Syncline, the Hull Anticline, and the Peddocks Island Fault. Long Island
appears to be bounded on either side by northeast-trending faults. A parallel
fault, the Peddocks Island Fault is found in the lower portion of the study
area near Wut Island. Tight localized folds are found on Rainsford Island, as
was a near-vertical cleavage. Both are expected since Rainsford Island is
near the axis of the Brewster Syncline.
The shear velocity analysis of an argillite sample from 88-26 (Mew England
Research, I989 Appendix C) indicates the rock is nearly isotropic. However,
petrographic fabric analysis of a second sample indicates a distinct alignment
of platey clay minerals. Therefore, although the mineral alignment formed
during metamorphism of the proposed argillite, it does not produce an
anisotropy that would affect the rock strength.
Eleven miles of seismic reflection and refraction surveys were run in February
of 1989 (Appendix Q). The survey indicates an extreme bedrock low almost due
west of Peddocks Island. The lowest point being a -181 feet MDC elev. and a
-175 feet MDC elev. along the alignment (Appendix D and Figure 5-7). However,
on the line farther to the west, the low was only a -35 feet MDC elev.
Seismic refraction and reflection surveys from earlier studies (Weston
Geophysical, 1980) indicate a bedrock low of -118 feet MDC elev. near Sunken
Ledge; however this low was not encountered during our survey. The bedrock
velocities range from 11,000 ft/sec to 16,000 ft/sec. In the area south of
Rainsford Island there is a large amount of rock with velocities between
11,000 and 14,000 ft/sec (Appendix Q). This may indicate that the top portion
of the rock is heavily fractured. However Boring 88-28, which is near this
11,000 ft/sec. zone, had high recoveries even in the upper 40 feet of the
rock.
5-30
The area north of Rainsford Island has very little low velocity rock. In the
areas closest to Deer Island seismic information was clouded because the thick
deposit of organic sediments interfered with the seismic signal (Appendix Q).
Oriented core was recovered in each of the four inter-island borings. The
sampling occurred for 30 feet at the depth of the proposed tunnel. Additional
oriented core was recovered at the bottom of borings 88-29 and 88-26 in order
to compare, if possible, the information obtained from the shaft borings on
the islands, and, in the case of 88-26, to compare with data in boring 88-25
along the outfall tunnel. The strikes and dips from these oriented core are
quite variable (Appendix H). There are fewer readings than expected due to
problems with the orienting equipment during the drilling process. Comparison
of the small amount of oriented core data from the four borings is not
possible. Bedding in the four inter-island borings is quite variable.
Typically, the apparent dip ranges from 20 to 30 degrees or 40 to 60
degrees. However, a high percentage of the bedding is contorted by slumping
or soft sediment deformation. Bedding on Rainsford Island is also quite
variable due to the tight folding and evidence that the argillite was folded
after slumping.
5.2.3.2 Joints and Fractures
The rock recovered from the four inter-island borings is of good quality.
Recoveries averaged over 90% and RQDs averaged greater than 85% ( T a b l e s 5-4
and 5-5, Figure 5-7). Joint spacing in these borings is generally wide to
moderately close. Many of the joints are bedding plane separations. There
are very few fracture zones in the inter-island area. The average fracture
zone widths range from 0.2 to 0.8 feet. There was only one healed breccia
zone (Appendix C). However, calcite and quartz veins were common. Joint and
fracture surfaces on Rainsford Island were very closely spaced. However, the
orientation of these joints is quite variable (Table 5-2).
Joint surfaces were variable, many were smooth with a slight gray or olive
green clay coating. Many surfaces also had pyrite. The core in the inter
island area had much more pyrite present than which was sampled in the outfall
area.
5-31
5.3 Geology of the Shafts
Borings have been drilled at the location of each shaft: the outfall shaft
(LDE-51), and an inter-island shaft (LDE-46) on Deer Island, the inter-island
shaft (LDE-58) on Nut Island. Within these borings a much higher percentage
of oriented core was recovered than in the marine borings, with almost two out
of every three core runs oriented.
5.3.1 Geology of the Outfall Shaft on Deer Island
The outfall shaft boring (LDE-51) (Figure 4-9) was located on a drumlin on
the eastern portion of Deer Island. The boring contained 124 feet of fill and
glacial till and 346 feet of argillite (Appendices B and P).
The till is typically composed of a gray plastic clay matrix with fine to
medium sand and subangular fine gravel. This till contains a higher
percentage of clay than the till recovered in the near island outfall borings
and Boring 88-26.
The boring contains both typical fine-grained argillite and sandy argillite.
The sandy argillite is interbedded with the argillite in laminar beds. The
argillite and sandy argillite are gray to dark gray and moderately hard. When
argillite is not interbedded with the sandy argillite it is unweathered;
however, when the argillite is interbedded with the sandy argillite, the beds
are slightly weathered.
Bedding in the boring is very consistent with apparent dips ranging from 15 to
25 degrees. Mo slumping is evident although there are occasional thin
brecciated layers. Oriented core was recovered for a greater percentage of
the boring than in the outfall borings (54 percent of the rock cored was
oriented). Results on the strike and dip of bedding planes also indicate that
bedding is very consistent (Appendix H). Bedding strikes M59W and dips 25ME
on average. Figure 5-10 i s a contour p lo t showing the dip d i r e c t i o n of
bedding.
5-32
FIGURE 5-9
_L
Ul I
% $ . PROJECT NORTH
\ /"/Mm.N^^^:;:: ' 4®'•• 2 ^ P ^
' 'bOo
w£^1
~n'' \ .-^^'^
i • - ' / '
f \y\
2f5=^'-
FRELlMJIl.^Fi i;uP>
BORING LESEND
® LOCATION OF SHAFT BORINGS
LCe-S l : OJTFALL SHAFT
LDE-4G:INTER-ISLAND SHAFT
' I L_
UETCALF & EDDY USSUXUSETTS UlTEt) nESOJRCES AUTHORITV
OEER ISLAM) COCEPTtML DESIGN Of>-
LOCATCN OF SHAFT BORINES
DEER ISLAI«
1 — ' • — : —
_ . . . «HOi<Mrl.<ni
N
Schmidt net, Lower Hemisphere Projection Contours at 15.00% 30.00% 45.00%
FIGURE 5-10
CONTOUR PLOT - BEDDING DIP DIRECTION OUTFALL SHAFT
5-34 ( E T C A L F a E D D Y
For LDE-51 the average recovery is 997» and the average RQD is 9^%
(Appendix B). Joint spacing is typically moderately close to wide, with some
areas of very closely spaced joints. There are a number of fracture zones,
almost all above -37.3 feet MDC elev. (drill depth of 200 ft.; Appendices B
and P). The orientation of the joints (Appendix H) are irregular, compared to
bedding. Most of the fractures seem to be high angle (60-90 degrees). Most
of the strikes generally are east-west, north-south, or approximately N40W.
All of these directions are similar to the regional pattern in the area. The
borehole geophysics (Appendix M) located some of the low angle joints, these
joints (maybe bedding plane separations) are water bearing.
A seismic reflection and refraction survey run in the vicinity of the boring
(the northern drumlin, Appendix E) has velocities through the bedrock ranging
between 14,000 and 16,000 ft/sec. indicating that it is not very fractured, or
weathered. The results of the borehole geophysics survey (Appendix M)
indicate that the velocity of the area surrounding the boring is between
16,500 ft/sec and 17,500 ft/sec.
5.3.2 Geology of the Inter-Island Shaft on Deer Island
The inter-island shaft on Deer Island is located in the southern portion of
the island (Figure 5-9). The boring (LDE-46) encountered 16 feet of fill,
89.5 feet of till, and 19 feet of diabase and 256 feet of argillite
(Appendices B and P).
The till is very similar to that found in the outfall shaft boring, with a
fine clay and silt matrix and some fine to medium sand and subangular fine and
medium gravel. Two fine to medium-grained silty sand layers were found
between 106 and 129 ft; beneath the till interbedded between boulders and
argilliceous gravel just above the bedrock (Appendices B and P).
Diabase was encountered from 133 to 152 feet. The diabase is a greenish gray,
medium crystalline, with occasional calcite veins. The diabase was also
heavily fractured. The diabase is -underlain by gray, fine grained, moderately
hard and unweathered argillite.
5-35
The major portion of the rock retrieved was argillite (256 ft.). The
argillite was gray to light gray, moderately hard and slightly weathered. The
borehole geophysical survey (Appendix M) indicates that there might be some
alteration (baking) at the contact with the diabase.
Bedding in the boring is quite chaotic with most of the boring exhibiting
slump features below -130 feet MDC datum. The bedding above -130 ft. was
laminar with apparent dips between 30 and 40 degrees. Oriented core accounted
for 51^ of the core recovered. Although it is not as consistent as in outfall
boring (LDE-51) the bedding does tend to strike approximately MSOE and dip
38WW (Figure 5-11).
The core retrieved in this boring is of relatively good quality, with an
average recovery of 97^ and an average RQD of 86^ (Appendix B). There are
very few fracture zones and those present range in thickness from 0.2 to
1.3 feet. All fracture zones are above -100 feet MDC elev. Below -100 feet
MDC elev. joints are typically widely spaced, although there are some areas of
closely spaced joints. Many of the joints are bedding plane separations.
There are occasional calcite veins, typically 35 to 45 degrees opposite
bedding. Some of the joints are detectable by the geophysical logging, since
they are water bearing. Mot all water bearing joints were noted on the boring
logs (Appendices M and P).
The seismic reflection and refraction survey in the inter-island shaft area
(Appendix E) indicates bedrock velocities and elevations to be quite
variable. Velocities range from 13,000 to 16,000 ft./sec. There is a bedrock
trough trending southeast, with the low point at - 35 ft. MDC elev. (Figure 8,
Appendix E). The bedrock trough is near an area of low velocities, indicating
that there may be a fault or sheared zone in the vicinity of the inter-island
shaft. The downhole velocities of the material near the boring (LDE-46) are
approximately 14,500 ft/sec (Appendix M). However, the velocity o f the
material in the bottom of the boring is irregular due to the number of water
bearing fractures (Appendix M).
5-36
N
Schmidt net, Lower Hemisphere Projection
FIGURE 5-11
BEDDING DIP DIRECTION -INTER ISLAND DEER ISLAND (LDE 46)
5-37 M F T C A C F a E DO Y
5.3.3 Geology of the Inter-Island Shaft on Nut Island
The inter-island shaft boring (LDE-58) on Mut Island is located in the
northeastern portion of the island (Figure 5-12). The boring contained
26 feet of fill overlying approximately 70 feet of a clay and till and
334 feet of argillite, diabase, and felsite.
The till is a stiff, silty clay with some trace to coarse fine angular gravel
(to 1.5 inches) and coarse to fine sand. The till in the nearby marine boring
88-29 is much coarser, with higher percentages of coarse gravel and cobbles.
The bedrock geology found in this boring is quite complex with diabase,
felsite, and tuffaceous argillite inter-layered with the argillite in the
bottom 133 feet of the boring.
The argillite encountered throughout the boring is gray, fine-grained and
often interbedded with layers of sandy argillite. The argillite is very
slightly weathered to unweathered and is generally moderately hard. The
bedding of the argillite was usually distinct, but highly variable. In some
instances the bedding direction changes 180° in 10 feet (Appendices B and P).
At depths approximately 200 feet below top of rock, the argillite becomes
tuffaceous and is interbedded with typical argillite. At 218 feet below top
of rock is a 37.5 foot diabase sill. The diabase is yellow gray to gray,
medium grained and hard with numerous calcite and quartz veins. There are
also some greenish yellow epidote veins. The contacts with the argillite are
brecciated and irregular. Thin felsite sills and a dike are found in the
bottom 78 ft. of the boring. The apparent thicknesses of these sills ranges
from 0.5 ft. to 8 ft. The felsite is greenish gray and fine grained.
Contacts with the argillite are generally brecciated, as if intruded. The
changes in lithology at the base of this boring were picked up quite well by
the borehole geophysical survey.
The joint pattern in LDE-58 was quite erratic (Appendix H). The Joints are
generally low angle (<40''). The borehole geophysical survey did note some
water bearing areas but these were usually at contact areas where the change
5-38
Inter island Shaft (LDE 58)
FIGURE 5-12 INTER ISLAND SHAFT LOCATION
NUT ISLAND
I 5-39
M E T C A L F a E D O Y
I I I I
in signals was greatest. However, the areas of highest permeability
(Appendix F) at the base of the boring were not retrieved by the borehole
geophysics due to a lack of energy to penetrated the till and rock to that
depth. A drop weight source was due to restrictions imposed by the city.
Geophysical surveys indicate that the rock is good quality with bedrock
velocities of 16,000 to 17,000 ft./sec.
I I I
I I I I I I I I
5-40
I I I I i I I I I i I
i I g I I I
6.0 GEOTECHNICAL PROPERTIES
6.1 MATERIAL PROPERTIES
Data is available from a number of sources on the material properties of the
rock types to be encountered on this project. The testing associated with the
1988 marine drilling campaign consisted of a large number of point load tests
carried out in the field, and a program of laboratory testing consisting of
unconfined compressive strength, triaxial compressive strength, Taber Abrasion
and Shore Scleroscope tests (Appendix C). In addition a small number of
unconfined compressive strength tests on samples from the outfall area were
carried out by the Robbins Company (Appendix C). Finally, some of the papers
on tunneling projects in the Boston area report the results of tests carried
out on Cambridge Argillite, but this data usually only comprises an average
and a range.
It should be remembered that because unfractured cylindrical specimens are
generally required for laboratory testing this usually results in the better
quality rock being selected, and this is particularly true if the solid core
recovery is low. Thus, for example, the values of intact rock strength.
Young's modulus, and to a lesser extent abrasivity may be significantly
overestimated from laboratory testing programs unless the sample selection has
been based on statistically accurate methods. In addition, because of sample
storage and preservation techniques, laboratory testing techniques and the
nature of the testing equipment, the properties determined in the laboratory
can be significantly different from those in the ground. Finally, it is
common for small intact laboratory specimens to exhibit higher strengths than
larger intact specimens (Brady & Brown, 1985) and likewise there will be a
reduction in mass strength in field condition.
6-'
I I I I I I I I I I I I I I I I I I I
6.1.1 Cambridge Argillite
6.1.1.1 Outfall and Diffusers
Approximately 800 point load tests have been carried out on samples of
Cambridge Argillite from the 1988 marine borehole cores in the outfall area.
These indicate IS^Q strengths ranging from 16 to 3352 psi (Table 6-1). The
distribution of results is such that approximately 507. of the results lie in
the range 300 to 700 psi, the average Is^g is 633 psi (Table 6-1). Tests were
carried out parallel and perpendicular to bedding and these showed average
IScQ of 452 and 931 psi respectively. This indicates an anisotropy in
strength parallel and perpendicular to bedding of 2.06. In general higher
strengths were obtained on samples where no bedding could be observed. These
samples represent the more massive siltstones and fine-grained sandstones and
sandy argillite encountered, the average ISJ-Q for these samples was 755 psi.
A total o f 52 Unconfined Compressive Strength (UCS) tests have been carried
out on samples of argillite during the rock testing program, of which 6 were
on samples containing obvious incipient discontinuities and another 10 on
samples described as sandy argillite. The remainder of the tests were carried
out on samples of unfractured silty argillite, some of which exhibit a slatey
cleavage. The samples with the incipient discontinuities had a strength
averaging 13,700 psi whereas both the sandy and silty argillite, without
incipient discontinuities averaged 15,686 psi. The 10 sandy argillite samples
have an average strength of 17,680 psi. The total range of strength for all
argillite samples is from I,4l8 to 37,672 psi. (Table 6-1; Appendix C).
A further 6 samples were selected for testing by the Robbins Company as part
of an initial assessment of the suitability of utilizing a Tunnel Boring
Machine for the Outfall Tunnel excavation (Appendix C). From these 6 samples,
20 UCS determinations were made and the results ranged from 2,950 psi to
16,670 psi with an average of 9,590 psi. (Appendix C). On the basis of the
available evidence, it is suggested that these results, which are about 40fo
lower than the mean of the other UCS tests, are not representative of the
broad mass of the argillite and that a mean strength of about 15,700 psi is
more appropriate.
6-2
I I I I I I I I I I I I I I i I
TABLE 6-1
MATERIAL PROPERTIES (ROCK) - OUTFALL AND DIFFUSER AREA
UNFRAC. SANDY ALTERED UNFRAC.
ARGILLITE ARGILLITE ARGILLITE ARGILLITE DIABASE DIABASE FELSITE
ALL
PARALLEL
PERPENDICULAR
Is SO (psi):
# TESTS
AVERAGE
AVERAGE
AVERAGE
MINIMUM
MAXIMUM
UCS:
# TESTS
AVERAGE
MINIMUM
MAXIMUM
YOUNG'S MODULUS (psi * 10'"6)
# TESTS
AVERAGE
MINIMUM
MAXIMUM
POISSON'S RATIO:
# TESTS
AVERAGE
MINIMUM
MAXIMUM
TOTAL HARDNESS:
n TESTS AVERAGE
MINIMUM
MAXIMUM
DENSITY (g/cc) :
it TESTS
AVERAGE
7 MINIMUM
MAXIMUM
MOISTURE CONTENT (X):
# TESTS
AVERAGE
MINIMUM
MAXIMUM
803 633 A52 931 16
3352
52 15A56
H18 37672
52 7.14
1.78
10.07
7 0.26
0.21
0.33
23 64.01
37.1
100.3
52
2.73
2.63
2.79
52 0.15
0.01
0.81
(46)
(15686)
(1418)
(37672)
(46)
(7.13)
(1.78)
(10.07)
(0)
(0)
(46)
(2.73)
(2.63)
(2.79)
(46)
(0.15)
(0.01)
(0.81)
56 755
43 1823
10 17680
7168
35725
10 7.75
5.76
9.22
1
0.25
6 74.8
54.9
97.4
10 2.73
2.66
2.79
10 0.1
0.01
0.23
53 175 138 176 23
1025
6 3334
936
5975
6 2.6
0.27
5.12
1 0.09
2 15.9
15.6
16.1
6 2.37
2.07
2.66
6 0.64
0.38
0.86
45 1015
25 2880
11 15917
1763
48475
11 8.79
1.5 13.63
3 0.19
0.28
0.06
3 98.5
69.8
133.8
11
2.86
2.37
3.08
11 0.42
0.03
1.19
(6) (22387)
(10878)
(48475)
(6) (11.56)
(9.56)
(13.63)
(2) (0.26)
(0.24)
(0.28)
(2) (113)
(91.9)
(133.8)
(6) (2.95)
(2.79)
(3.08)
(6) (0.19)
(0.04)
(0.75)
78 697 397 409 52
2114
6 6490
629
12726
6 5.29
0.93
10.85
0
3 59.2
39.4
83.1
6 2.73
2.61
2.85
6 0.48
0.01
1.39
() INDICATES THE VALUES ARE A SUBSET OF THE VALUES IN THE COLUMN TO THE LEFT
6-3
e
I I I I I I I I I I I I I I I I I I I
The results of the testing from the outfall investigation indicate a ratio
between point load strength perpendicular to bedding and unconfined
compressive strength of 16.9:1. The ratio between point load strength
parallel to bedding and UCS is 34.7:1.
A total of five sets of triaxial tests were carried out on adjacent sections
of argillite. The change in fracture strength between unconfined (confining
pressure of 0) and a confining pressure of 400 psi was variable, in one
instance changing 10,000 psi (Boring 88-4). However, the change in fracture
strength between confining pressures of 400 and 800 psi was generally less
than 1,000 psi (Appendix C).
An evaluation of the strength data for the argillite has shown the significant
importance of incipient discontinuities on the recorded value whereas this
does not appear the case with the rock modulus and as such is considered a
more reliable index property. High ratios of the rock modulus to UCS, some in
excess of 1000, relate to specimens which failed along pre-existing planes of
weakness. Samples that failed through intact rock are considered to have
ratios between 250 and 500 and if these results are analyzed separately a mean
strength of 15,686 psi is obtained whereas those tests with a modulus to
strength ratio in excess of 500 gave a mean strength of 9,500 psi.
The Young's Modulus was measured on all the laboratory samples from the 1988
marine boreholes. The tests may underestimate the modulus by up to 3%. This
error is considered to be insignificant when compared to the natural
variability of the rock.
The modulus of all argillite sampled in the outfall including sandy argillite
with incipient fractures measured in these tests ranged from 1.78 x 10° psi to
1.007 X lo' psi with an average of 7.14 x 10° psi (Table 6-1). For the sandy
argillite the results ranged from 5.76 x 10° psi to 9.22 x 10° psi with an
average of 7.75 x 10° psi. (Table 6-1).
Twenty three Taber Abrasion and Shore Scleroscope tests were carried out on
the samples of argillite, six of which were on sandy argillite samples. The
6-4
I I I I I I
I I I I I I I
argillite gave mean values for Taber Abrasion and Shore Hardness of 1.77 and
55.5 respectively, and the sandy argillite gave mean values of 2.31 and 65.8;
indicating the slightly higher abrasivity of the sandy argillite. The total
range of the Taber Abrasion results was 0.75 to 4.08. The range of Shore
Hardness was 38.8 to 74.3 (Appendix C). The Taber Abrasion test was conducted
on specimens smaller than that recommended. However the errors involved are
believed to compensate each other to a degree. Firstly the area available to
the abrasion wheel is reduced leading to a potential reduction in the weight
loss, but secondly the stress acting on the sample is increased because of the
reduced area of contact, therefore increasing penetration rate. The results
are therefore considered to be realistic.
Total hardness values, calculated using the methods of Deere and Miller
(1966), give a range from 37.1 to 100.3 with an average of 60.2. The average
total hardness for the sandy argillite is 74.8 (Table 6-1).
The density and moisture content were determined for each specimen that was
tested (Appendix C). The density range from 2.62 g/cc to 2.79 g/cc with the
average being 2.73 g/cc. The moisture content was quite low ranging from
0.027, to 0.37f., with average being 0.157» (Table 6-1).
6.1.1.2 Inter-Island Area
One hundred forty seven point load tests were performed on the unoriented core
from the four inter-island borings. These indicate strengths ranging from 81
to 2,176 psi (Table 6-2). The average IS^Q was 633 psi (Table 6-2). Tests
were carried out parallel and perpendicular to bedding when bedding was
present (Table 6-2). The nonbedded argillite are assumed to be the more
massive sandy argillite. There were only 3 tests in this material, and they
give an average ISCQ of 1,063 psi. The ratio of Is CQ between parallel and
perpendicular is 1.34. This ratio indicates a slight anisotropy for the rocks
recovered in the inter-island area. Compressional velocity measurements in
argillite from boring 88-26 also indicate a slight anisotropy (Appendix C).
6-5
I I I
TABLE 6-2
MATERIAL PROPERTIES (ROCK) - INTER-ISLAND AREA
Is 50 (psi):
# TESTS
AVERAGE -
AVERAGE -
AVERAGE -
MINIMUM
MAXIMUM
UCS:
# TESTS
AVERAGE
MINIMUM
MAXIMUM
ALL PARALLEL
PERPENDICULAR
YOUNG'S MODULUS (psi *
# TESTS
AVERAGE
MINIMUM
MAXIMUM
POISSON'S RATIO:
# TESTS
AVERAGE
MINIMUM
MAXIMUM
TOTAL HARDNESS:
# TESTS
AVERAGE
MINIMUM
MAXIMUM
DENSITY (g/cc) :
# TESTS
AVERAGE
MINIMUM
MAXIMUM
MOISTURE CONTENT (%):
# TESTS
AVERAGE
MINIMUM
MAXIMUM
1*E-i-6)
ARGILLITE
147 633 553 726
81 2176
6 15673
4305
23835
6 7.3
5.81
8.2
0
1 67.2
6 2.73
2.69
2.79
6 0.09
0.05
0.18
SANDY
ARGILLITE
3 1063
539 1952
4 5615
3426
8667
4 4.32
3.44
6.2
1 0.3
1
39.9
4 2.72
2.7 2.72
4 0.05
0.07
0.76
ALTERED
ARGILLITE
1 15482
1 6.64
0
1
49.4
1 2.81
1 0.2
IGNEOUS
12 1133
81 2194
3 18282
14133
21033
3 8.24
7.05
9.52
2 0.27
0.26
0.27
3
62.2
60.3
65.8
3 2.77
2.73
2.79
3 0.09
0.04
0.17
6-6
I I I I I I
Ten unconfined compression tests were performed on samples of argillite in the
inter-island area, four of these were on samples of sandy argillite. The
average argillite UCS strength for all 10 samples was 11,650 psi; however, for
just the typical argillite samples the UCS fracture strength was 15,673 psi
and the sandy argillite strength was 5,615 psi (Table 6-2). Unlike the
outfall area the UCS of the sandy argillite is less than that of the typical
argillite. This difference may be attributed to the low number of tests
performed.
Unconfined tests near the inter-island alignment have been performed for other
projects (M&E, 1983; Stone and Webster, 1980). These testing programs, and
the tests for the Red Line Extension (Hatheway & Paris, 1979), have an average
UCS strength of 17,950 psi. These results are slightly higher than those from
the outfall area and significantly higher than the value for the inter-island
area.
One suite of confined tests with 0 psi, 400 psi, and 800 psi confining
pressures were performed on an argillite sample in the inter-island area
(Appendix C). This suite of tests on adjacent samples indicates that fracture
strength increase with increased confining pressure, 2000 to 3000 psi per
step.
The results of the inter-island rock testing program indicate a ratio between
UCS and ISCQ perpendicular to bedding is 1:21.6.
Only one total hardness value could be calculated for the argillite and one
for the sandy argillite. They are 67.2 and 39.9, respectively (Table 6-2).
The density and moisture content were determined for each specimen tested
(Appendix C). The densities range from 2.69 to 2.79 g/cc with 2.73 g/cc being
the average density. The moisture contents were also quite low; however, they
were variable between the argillite and sandy argillite with average values of
0.09f, and 0.507, respectively (Table 6-2). The relatively high moisture
content of these samples seems to correlate with the low fracture strength.
6-7
6.1.1.3 The Shafts
From the three shaft borings a total 177 point load tests were performed on
argillite, only one of these was performed on a nonbedded sample. The ISCQ
values range from 26 to 5101 psi with an average of 1,533 psi (Table 6-3).
Point load test were performed parallel and perpendicular to bedding with the
mean IScg ^ ° ^ parallel tests being 1,315 psi and 1,776 psi is the average
value for perpendicular tests (Table 6-3). The effects of rock anisotropy may
not be important in the shafts since excavation is going to be oblique to the
bedding. For the outfall shaft (LDE-51), which has very low bedding,
excavation will be almost perpendicular to bedding.
Twenty two unconfined compression tests were performed on samples of
argillaceous material from the shafts. The average fracture strength was
21,200 psi, with the range being 7,700 and 30,450 psi. These values are
considerably higher than values from the outfall and inter-island areas. The
two low UCS values (less than 10,000 psi) were from the top, fractured portion
of the rock borings LDE-51 and LDE-46.
The results of the testing from the shaft boring investigation indicate a
ratio between ISJ-Q and UCS of 13.8:1 for all samples and 11.9:1 for tests
perpendicular to bedding.
The Young's modulus value of the argillite samples from the shaft borings
range from 6.27 x 10^ to 1.044 x lo" psi, with an average of 8.64 x 10° psi
(Table 6-1).
Variations in fracture strength and Young's modulus were determined from
certain samples by testing adjacent samples. For the samples near the top of
the rock column the Young's modulus values were almost identical, and the
fracture strength varied by 1% (MER, 1989b). The fracture strength for
adjacent samples near the base of the boring varied 167, (MER, 1989b). This
variation indicates what may be encountered during excavation of the shafts in
the argillite.
6-8
I I I I I I I I I I I I I I I I I I I
TABLE 6-3
MATERIAL PROPERTIES (ROCK) - LDE SHAFT BORINGS
Is 50 (psi):
# TESTS
AVERAGE -
AVERAGE -
AVERAGE -
MINIMUM
MAXIMUM
UCS:
# TESTS
AVERAGE
MINIMUM
MAXIMUM
ALL PARALLEL
PERPENDICULAR
YOUNG'S MODULUS (psi * 1*E• 6)
# TESTS
AVERAGE
MINIMUM
MAXIMUM
POISSON'S RATIO:
# TESTS
AVERAGE
MINIMUM
MAXIMUM
DENSITY (g/cc) :
# TESTS
AVERAGE
MINIMUM
MAXIMUM
MOISTURE CONTENT (%):
# TESTS
AVERAGE
MINIMUM
MAXIMUM
ARGILLITE
177 1533
1315
1776
26 5101
22 21200
7700
30450
22 8.64
6.27
10.44
21 0.28
0.1 0.4
22 2.733
2.63
2.764
22 0.21
0.05
1.87
DIABASE
7 2908
1518
5341
4 26480
14900
33740
4 12.76
9.12
16.83
4 0.21
0.11
0.33
4 2.973
2.948
3.04
4 0.08
0.04
0.13
FELSITE
5 1007
56 2334
1 18080
1 10.1
1 0.1
1 2.836
1 0.05
6-9
I I I I I I I I I I I I I I I I I I I
In addition to the laboratory rock testing a borehole geophysical survey was
performed in each of the shaft borings (Appendix M). The downhole velocity
values for the argillite indicate good quality materials with the values being
greater than 12,000 ft/sec but usually less than 18,000 ft/sec.
The density and water content were measured for all laboratory samples. The
density is ranged from 2 . 6 2 to 2.76 g/cc with an average of 2.73 g/cc. The
moisture contents are generally low. The moisture contents range from 0.057"
to 1.87^ with an average of 0.21^. The sample with 1.877" value also has the
lowest fracture strength (Appendix C).
6.1.2 Altered Argillite
6.1.2.1 Outfall and Diffusers
As indicated in Section 5.1.1.3 portions of the argillite recovered in the
outfall area are altered. In some instances the alteration is quite extreme,
with portions of the core recovered being kaolinized and in some instances, a
sticky clay (see boring 88-1, depth 259.4 to 265.5). A total of 53 point load
tests were performed on altered argillite in the outfall area. Most of the
tests were performed on material from borings 88-02 and 88-05. The range of
IScQ was 23 to 1025 psi with an average 175 psi.
The alteration that occurred in boring 88-02 was extreme, with all core
retrieved being altered. If the results from boring 88-02 were omitted from
the analysis there would be a sample size of 18 with a range of 37 to 1025 and
an average of 304 psi. The range of point load strength for borings from
88-02 is 23 to 262 psi with an average IS^Q of 109 psi. The alteration in
boring 88-02 is the most extreme encountered that could be tested. The
alteration that occurred in the rock from other borings was not as extreme,
but much of the core tested has been altered to clay minerals, such as
chlorite. The difference in IS^Q between the two "classes" of alteration
indicates that they both have similar low values which are similar to the low
values seen in the typical argillite (Section 6.1.1.1)
6-10
I A total of six samples were tested in the laboratory to determine unconfined
compression strength (UCS). Of those samples, three were from boring 88-02
and the other three were from boring 88-05 (Appendix C). The range in
fracture strength for these samples was from 936 to 5975 psi, with the average
being 3334 psi (Table 6-1).
Young's modulus was also determined for each laboratory sample. The range was
from 5.12 x 10 to 2.7 x 10^ psi. For the modulus the differences between the
"classes" becomes apparent with the values from boring 88-02 ranging from
2.7 X IO-' to 4.3 X IO-' psi while the modulus values from 88-05 range from
4.47 X 10^ to 5.12 X 10^; an order of magnitude difference.
Taber Abrasion and Shore hardness values were determined for two samples from
boring 88-02 giving total hardness values of 15.6 and 16.1 (Table 6-1;
Appendix C).
The strength seems to be closely linked to moisture content and density. The
moisture content for the samples tested (Appendix C) range from 0.387, to 0.867,
with the average being 0.647, (Table 6-1). The maximum moisture content
sampled in the typical argillite from the outfall areas was 0.377,.
It seems that strength decreases with increased alteration as would be
expected. Samples, which were not very altered, had ISI-Q values similar to
that of typical argillite. Strength decreases rapidly as alteration to'
kaolinite advances.
6.1.2,2 Inter-Island Tunnel
Altered argillite was found in small layers in the four inter-island borings,
usually near igneous intrusions (Section 5.2.2.3 and Appendix B). Mo point
load tests were performed on the altered material. One UCS test was
performed. The sample has a fracture strength of 15,482 psi which is only
slightly lower than the average strength of the typical argillite in the
inter-island area (Table 6-2).
6-11
Shore scleroscope and Taber abrasion tests were performed on the sample of
altered argillite, giving values of 54.8 and 1.17 respectively. The total
hardness for this sample is 49.4 which is harder than the altered argillite
tested in the outfall, and is within the range of total hardness values for
typical argillite in the inter-island area (Table 6-2).
The moisture content is 0.27, which is lower than the altered argillite in the
outfall tunnel.
It appears that the altered argillite sample tested in the inter-island area
is not as altered as the material sampled in the outfall area.
6,1.3 Igneous Materials
6.1.3.1 Outfall and Diffusers
Two classes of igneous materials were encountered in the outfall area:
felsite and diabase. The felsite class includes tuffs and ashflows.
Seventy-eight point load tests were performed on felsite materials in the
outfall area. Many of these tests were performed on samples from boring 88-01
which is a slightly altered bedded ash/tuff (M&E, 1989). The average IS^Q for
all of the felsite samples tested, both parallel and perpendicular to
"bedding", is 697 psi. The 39 tests performed on.samples without apparent
"bedding" is higher than the tests on the "bedded" materials (Table 6-1).
This is consistent with the relationships seen in the argillaceous samples.
Six felsite samples were tested in the laboratory (Appendix C). The fracture
strengths ranged from 629 to 12,726 psi, with an average fracture strength of
6,490 psi (Table 6-1).
Shore scleroscope and Taber abrasion tests were carried out on 4 and 3 samples
respectively. The Taber abrasion values range from 3-08 to 1.00 and the shore
hardness values range from 37.1 to 59-7. This gives 3 total hardness values
of 83.1, 39.4, and 55.0.
6-12
Moisture content values range from 0.01? to 1.39? with the samples of highest
moisture content having the lowest fracture strength. Some of the felsite
samples have been slightly weathered, which is not always noticeable in hand
specimen, but probably accounts for the samples of high moisture content.
Forty-five point load tests were performed on diabase samples. The samples
tested ranged from altered to fresh. The ISCQ values range from 25 to
2,880 psi, with an average value of 1,015 psi (Table 6-1). Eighteen of the
point load tests were performed on altered diabase samples. When these tests
are separated out the average ISCQ for the non-altered diabase increases to
1413 psi and the average value for the altered diabase is 417 psi.
Eleven samples of diabase were tested in the laboratory. Six of the samples
were fresh and massive, three were altered and/or weathered, and the remaining
two were heavily fractured. The range of fracture strength for all eleven
tests is 1,763 to 48,475 psi with an average of 15,917 psi (Table 6-1). The
range in fracture strength for the six massive diabase samples is 10,878 to
48,475 psi with an average of 23,505 psi. The two fractured diabase samples
have UCS values of 9788 and 6497 psi (Appendix C). The altered and weathered
diabase samples have much lower UCS values, as would be expected, with an
average value of 5924 psi, which is slightly less than the average fracture
strength value of the felsite material.
Modulus values for the more massive diabase range from 9.56 x 10 psi to
1,36 X lo" psi, with an average of I.I6 x lo" psi (Table 6-1). The fractured,
altered, and weathered diabase samples have modulus values which range from
1.5 X 10^ to 8.2 X 10^ psi with an average 5.46 x 10^ psi. The weathered
diabase sample has the lowest modulus value; however, the altered diabase
samples are similar to the fractured sample values.
Total hardness values have been calculated for three diabase samples from the
outfall area (Table 6-1; Appendix C). Two were from massive diabase samples
and one from an altered diabase. The massive samples indicate total hardness
values ranging from 91.9 to 133.8. The altered diabase sample has a total
hardness value almost 507, lower than that of the massive diabase with a value
of 69.8.
6-13
I I I I I I I I I I
The ratio between UCS and IS^Q for the massive diabase is 1:16.6 while for the
altered and fractured diabase is 1:16.3 which is very similar.
The density and moisture content were determined for all laboratory samples
(Appendix C). The range for density for all of the diabase samples was 2.37
to 3.08 g/cc (Table 6-1). The fractured, altered, and weathered diabase
samples usually had lower densities than the more massive diabase samples.
The moisture content values ranged from 0.03 to 1.19/5. Again, the more
fractured and altered samples had the higher moisture contents. The massive
diabase had values average O.I97, (Table 6-1).
6.1.3.2 Inter-Island Area
Twelve point load tests were performed on igneous materials in the
inter-island area. The materials included diabase and ash layers; however
most were performed on the diabase from 88-27. The range of Is^g is from 81
to 2194 psi, with an average of 1133 psi (Table 6-2).
Three igneous samples were tested in the laboratory, two of the samples were
diabase. The UCS values were high, ranging from 14,133 to 21,033 psi with an
average of 18,282 psi. This would give a ratio of UCS to IS^Q of 1:16.14,
which is similar to the ratios developed for the diabase in the outfall area.
Taber abrasion and Shore hardness values were determined for each of the
igneous samples. The values range from 1.59 to 2.25 for the Taber Hardness
and 39.5 to 73.4 for the Shore Hardness. This gives total hardness values
ranging from 65.8 to 60.3, which are lower than the total hardness values for
the massive diabase in the outfall area even though UCS values fall within the
range of those for the outfall area (Tables 6-1 and 6-2).
The Young's modulus determined for the three samples range from 7.05 x 10 to
9.52 X 10° (Table 6-2). These values are below the values determined for the
massive diabase in the outfall area, this may be a result of the small number
of tests performed in the outfall and inter-island areas.
6-14
Density values for the igneous samples are quite high ranging from 2.73 to
2.79 g/cc. The moisture contents are low, ranging from 0.04 to 0.177»,
indicating little or no fracturing and/or alteration in the samples.
6.1.3.3 The Shafts
Felsite and diabase were encountered in two of the three shaft borings (LDE-46
and LDE-58).
Five point load tests and one unconfined test were performed on felsite
samples. The average IS^Q fo"" the felsite is 1007 psi (Table 6-3). This
average value is higher than the 697 psi value for the outfall area. This
difference may be due to the small number of tests performed. The UCS test
gives a fracture strength of 18,080 psi. This value is higher than the UCS
values for the felsite in the outfall tunnel. The ratio between Is^g and UCS,
for the small number of samples, is 18:1, which is higher than the ratios for
diabase along the tunnel alignments.
Seven point load tests on diabase samples were performed on materials from the
shaft borings indicating an average ISCQ of 2908 psi (Table 6-3). The tests
give a range of fracture strength of 14,900 to 33,740 psi with an average of'
26,480 psi. This average value is slightly higher than the average IScg value
for the outfall area.
The four UCS values were performed on materials from LDE-46 and LDE-58, two
tests from each boring. The tests were done on samples within 6 feet of each
other. The results indicate the range in variability within the rock mass
(Appendix C). The diabase from boring LDE-46 (diabase found at the top of the
rock column) had fracture strength values ranging from 14,900 and
33,740 psi. The diabase found in boring LDE-58 (near the bottom of the rock
column cores) ranges from 30,600 to 26,680 psi (Appendix C). This variation
is quite extreme and must be taken into account for excavation. The cause of
the variation is due to the presence of an incipient fracture in the samples
with the lower fracture strength. Similar variabilities may also be
encountered in the outfall and inter-island tunnel excavations.
6-15
Modulus values for the diabase in the shafts is also quite high, ranging from
9.12 X 10° to 1.68 X lo" psi (Table 6-3)- The modulus values did not vary
considerably between the two adjacent samples.
As would be expected the density of the diabase samples are quite high ranging
from 2.95 to 3.04 g/cc (Table 6-3). The moisture contents are relatively low,
ranging from 0.04 to 0.137» (Table 6-3).
In addition to the laboratory rock testing, borehole geophysical surveys were
run in each boring (Appendix M). The results of these surveys indicate that
there are differences in properties, with density logs and gamma logs being
able to detect the changes in lithology from argillite to diabase and to
felsite. In boring LDE-58, the sonic tool and VSP indicate that the felsite
layers have a lower velocity than the argillite and diabase. The diabase
seems to have a slightly higher velocity than the argillite.
6.m Till
Till will be encountered during excavation in the diffuser area and at each
shaft location.
6.1.4.1 Diffusers
Mine borings were drilled in the diffuser area during the I988 Marine program
(88-14, 88-3, 88-18, 88-15, 88-9, 88-9A, 88-10, 88-11, 88-12, and 88-4). The
average thickness of the till in the diffuser area was 36.4 ft. A description
of the till is given in Section 5.1.1 and in Appendix B. A down-hole sliding
hammer (175 lb. wt. with a free fall of 5 ft.) was used to collect the split
spoon samples. The blows were usually high (38 to 50) indicating a dense
material. In boring 88-15 a SPT sample at the base of till required 150 blows
to drive the sampler 6 inches. One till sample was tested at the laboratory
(Appendix L). The liquid limit was 347, and the plastic limit was 197,.
6-16
6.1.4.2 Shafts
The till is essentially cohesive, consisting of sand and gravel in a clay and
silt matrix (Appendices B and P). Approximately 50^ of each sample tested
passed through a 200 mesh sieve (Appendix L). Attenberg limits were
determined for 3 till samples (Appendix L). The liquid limits range from 19
to 30 percent and the plastic limits range from 13 to 18 percent
(Appendix C). The water content, as percent weight, range from 5.3 to
18.0 percent. At Deer Island a markedly weaker upper layer was indicated by
SPT tests extending to an elevation of 19 feet (MDC elev.) at the outfall
shaft location and 59 feet (MDC elev.) at the inter-island shaft location.
"N" values are typically 30 to 50 in the upper layer and greater than 100 in
the lower layer although values as low as 80 were more common at the inter
island shaft location. At Mut Island the "M" values are consistently above
100 from the top of the till, but this is at an elevation of 85 feet (MDC.
elev).
Pressuremeter tests were performed in the auxiliary shaft holes. Four tests
were performed in till (Appendix M). Preliminary pressuremeter moduli range
from 75.1 to 150.6 tsf in LDE-51A and 231.4 to 512.1 for LDE-46A. The results
of this testing indicate that the cohesive matrix controls the overall
deformation as this deposit and that the high SPT blowcounts encountered in
the till are a result of the coarse cohesionless soil particles (cobbles and
gravel).
6.1.5 Supra-Glacial Soil - Shaft Borings
The material overlying the till in the Mut Island borehole consists of
interbedded sands and clays. Two SPT tests carried out in this material gave
"N" values of 24 and 39 which indicates that the materials is stiff or medium
dense. A pressuremeter test in this material gave a preliminary pressuremeter
modulus of 145.8 tsf.
6-17
I I i 1 I I f I I I t I I I I I I I I
6.1.6 Sub Glacial Soil - Shaft Borings
The material underlying the till at LDE-46 (the Deer Island inter-island
shaft, figure 5-9), consists of a layered boulder and sand sequence
(Appendix B). The sand layer within the sequences appears to be under
pressure, and became a running sand during drilling. SPT values in this layer
are high at approximately 100. A grain size analysis indicates primarily fine
to medium sand with only 3% of the material from this area passes through the
200 mesh screen (Appendix L).
This layer has been found in other borings extending offshore to the south and
southwest of this boring.
6.1.7 Marine Clay
The marine clay will be encountered in the vicinity of the diffusers.
Laboratory testing is available on samples from the diffuser area
(Appendix L). These tests consist of water contents, Atterberg Limits, quick
undrained and consolidated undrained strength tests and consolidation tests.
In addition, a few SPT tests were carried out in the boreholes; however, the
majority of the soil samples were retrieved using a sliding down-hole
hammer. The blows recorded while using a sliding hammer in this vicinity were
quite low usually 0 to 10 for a 6" increment. In borings using SPT tests, the
equipment sank under its own weight. SPT values from other borings in the
outfall and inter-island areas range from weight of rods (0) to 24 (M&E,
1989). The low M values indicate the weak nature of these deposits (i.e.,
N<1) although overwater a considerably greater number, and therefore weight,
of rods is required to perform the tests than is used onland.
Typically, the clay had liquid limits from the laboratory tests range from 30
to 59^ and the plastic limit from 16 to 287,. The moisture content is usually
a few percent lower than the liquid limit except in one sample where the water
content is 15^ higher than the liquid limit (Appendix L). This unusual result
occurred because one organic lens with liquid and plastic limits of 92 and 38
respectively was found within the undisturbed sample. Water contents taken at
one tenth of a foot spacing verified the variation of this material.
6-18
I I I I I I I I I I I I I I I I I I I
The undrained shear strengths from both the unconfined and consolidated
undrained triaxial tests range from 0.55 to 0.81 tsf, but with one result of
0.23 tsf, which is the only test that does not show strain-softening behavior
(Appendix L). It is believed that this strength result is unrepresentative as
a result of sample disturbance. The remaining results do not indicate any
increase in strength with depth although this may be a reflection of the small
number of tests. The strain-softening behavior suggests a degree of over-
consolidation. Since the water content is close to the liquid limit it is
expected that the material will show high sensitivity and low remolded
strengths.
The three consolidation tests show preconsolidation pressures of 1.2 tsf,
2.0 tsf, and 2.2 tsf giving over consolidated ratios of 2, 4.7 and 3.5
respectively (Appendix L). The second and third results are not unreasonable
for marine clay at these depths. These two tests gave Cv values of o p ' ^ 2
approximately 3 to 5 x 10"'- cm /sec and an m^ values of 5 x 10 - ft /ton for a
stress increase from 0.6 tsf to 1 tsf. Although the values for Cv appear
reasonable, the values for mv appear low and a more reasonable value for use
in design would be 1 to 2 x 10"^ ft/ton.
6.1.8 Recent Deposits
6.1.8.1 Diffuser Area
These relatively thin unconsolidated sediment deposits which overlie the
marine clay in the vicinity of the diffusers have not been studied by in situ
or laboratory testing, but are believed to be generally unconsolidated soft
organic clay and silt or granular materials with low densities and strength.
6.1.8.2 Shafts
As discussed in section 5.3 artificial fill was encountered in each of the
shaft borings. SPT values in the fill range from 10 to 66 but vary between
boring (Appendices B and P).
•19
I I I I I I I I I I 1 I I I I I I I I
Below the fill but above the till in LDE-46 are recent silt and clay deposits
(Appendix P). The preliminary results from a pressuremeter test performed in
this deposit indicate a pressuremeter modulus of 109.8 tsf.
6.2 DISCONTINUITIES
Discontinuities in the form of either joints, bedding plane breaks, cleavage
or faults generally reduce the strength and stiffness properties of the
ground, often to a significant extent. However, from study of the borehole
cores it would appear that many of the discontinuities are healed either by
calcite or quartz and these would be expected to have higher strengths and
stiffnesses than unhealed joints. Also in the better quality rock, where RQDs
are typically in excess of 90f,, the unhealed joints appear to be fresh and
tight. In terms of stiffness these joints are unlikely to exert a major
influence on the overall behavior of the ground with the mass stiffness being
close to the intact stiffness. On the other hand the compressive strength
will be reduced, particularly under low stress conditions, because of the loss
in cohesion and the tensile strength will be reduced to zero. In sections
where the rock is poorer, such as in weathered zones, shear or fault zones or
where discontinuities are open or infilled with clay, both the stiffness and
strength can be significantly reduced.
Serafim and Pereira (1983) proposed a relationship linking Young's Modulus, E,
with the Geomechanics Classification parameter, RMR, as given below:
g _ ^Q(RMR-10/40)
where E is in GPa (multiply by 0.145 x 10° to obtain psi). This equation is
plotted in Figure 6-1.
This would indicate that if the RMR reduces from 83 to 34, typical of the good
and poor rock conditions to be encountered along the tunnel alignment, the
modulus would reduce from approximately 9.7 x 10° psi to 6.0 x w'^ psi, a
reduction of 957a. It is apparent that this equation gives a reasonable range
of values for the argillite if cognizance is taken of the absolute upper bound
value established from laboratory testing of intact specimens.
6-20
180
170
160
150
140
a>
K)
0 (L O
• ^ ^
V) 3 3
•0 0 3 M VI
0 3 Jf o
"0 a:
I ou
120
1 10
100
90
80
70
60
50
40
30
20
10
Figure 6.1 Young 's Modulus v, RMR
0 ~ " ^ ^ ^ ^ ^ ' y ^ n f I I I I I I I I I I I I I I I I T I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I
10 20 30 40 50 60 70 80
CSIR rock mechan ics c lass i f i ca t ion . RMR
90 100
I I I I I I I I I I I I I I I I 1 I I
In zones of poorer, more fractured rock the strength is also significantly
reduced and in many situations this may be of greater importance than the
change in stiffness. Hoek and Brown (1988) suggested relationships linking
the intact and mass material parameters "m" and "s" and the RMR value. Two
relationships were proposed, one for tunnel boring machines (TBM) and one for
other methods of excavation because it was found that TBMs disturbed the rock
mass around the tunnel considerably less than methods such as drill and blast.
Both the argillite and the diabase contain incipient or healed discontinuities
which form planes of weakness that result in a reduction of strength below
that of the intact material. However, these discontinuities are not included
when calculating RQD or fracture spacing and therefore do not influence the
value of RMR. Therefore it is considered appropriate to include those test
specimens that failed along such discontinuities when calculating the
representative unconfined compressive strength to use in the Hoek-Brown
failure criterion.
The relationship between unconfined compressive strength and RMR for the
Cambridge Argillite and the diabase derived from the proposals of Hoek and
Brown (1988) is shown in Figure 6-2. This shows that the apparent strength of
the rock reduces rapidly as the degree of fracturing increases. However, even
in fault and shear zones where the RMR reduces to around 30, the unconfined
strength is in excess of 330 psi and therefore because of the generally low
stress regime it is not considered that the stress will exceed the rock mass
strength in either the tunnels or the shafts on this project.
The intention of Hoek and Brown (1988) in relating the "m" and "s" values to
RMR was to determine the changes in mass strength due to changes in degree of
fracturing. In situations where the intact properties of the ground change
such as in the altered argillite it is important to account for this by
reducing the value of the intact values of "m" and "s^" and not the value of
RMR. It is probable that in fault zones where the intact material strength
may be slightly changed it is appropriate to use only the reduction in RMR and
not that of the intact material properties.
6-22
I
24
^ M a
• 0 • C 0 M 3 0 X 4 * *
r •*• 0>
c 9 u
t/1
tn o D
22
20
18
16
14
12
10
8
6
Figure 6.2 Fracture St rength v. RMR
•
I I I I I I I I I 1 1 1' 1 1 1 1 1 1
y
• l l i l l l l l l '
/
/ / y y /
/
1 /
/ /
/ /
/ /
/
/
I • • 1 r 1 1 • 1
30 40 50 60 70 80
CSIR rock mechan ics c lass i f i ca t ion , RMR
90 100
I I I I I I I I I I I I I I I I I I I
The presence of discontinuities in the rock has two other effects on the
ground behavior which in this case are considered to be of more importance
than the reduction in the material properties, namely; (i) the development of
unstable blocks in the crown and sidewalls of the tunnels and (ii) the control
on the overall rock mass permeability (see Section 6.3).
Because of their mode of formation, discontinuities generally have preferred
orientations and can be grouped into sets. Typically, but by no means
exclusively, these sets are orientated in three mutually orthogonal directions
and in sedimentary rocks one of these sets is generally orientated parallel to
bedding planes. Depending on the relationship between the size of the
excavation and the spacing of the joints, these sets can combine to define
potentially unstable blocks of rock. These blocks can be dealt with in a
number of ways. They can either be allowed to fall, or be physically removed,
provided this does not jeopardize the overall stability of the tunnel.
Alternatively, active or passive support can be installed to stabilize them.
Support is preferred in the majority of cases as it reduces the risks of gross
failure, reduces the relaxation of the rock mass, increases safety and reduces
the amount of overbreak.
The orientation and spacing of the discontinuity sets that have been
identified are discussed in Sections 5.1.3, 5.2 and 5.3 for the relevant parts
of the project. The summary plot of the data suggests a random distribution
of joint orientations as recorded on the oriented core. At present no clear
pattern of joint orientations has emerged. However, with the increased data
available from the 1989 marine investigation it may well prove possible to
divide the tunnel alignments into sections exhibiting essentially similar
joint patterns.
The orientation data from the exposures on the Outer Harbor islands
(Appendix I) indicate that there should be few joints striking parallel to the
outfall tunnel axis and also few joints dipping out of the face at a shallow
angle although some dipping steeply out of the face should be expected. These
results tend to agree with the oriented core data from the outfall shaft. The
bedding on the islands and at the outfall shaft is typically dipping at
6-24
I I I I I I I I f I I I I I I I I I I
between 20 and 30° which is considered too shallow an angle to form release
planes on which unstable blocks could slide. However, elsewhere steeper
bedding dips are commonly observed and in these locations sliding on bedding
is likely. The general form of the folding is such that the strike of the
bedding will typically trend at approximately 30° to be axis of the outfall
tunnel.
6.3 PERMEABILITY
Groundwater is one of the major uncertainties on this project and may have a
substantial influence on the ultimate project cost. In the bedrock, the
permeability of the intact material is extremely low and the groundwater flow
will be completely controlled by the discontinuity network. The discrete
nature of the discontinuities requires judgement with the interpretation of
packer tests and subsequently in detailed estimates of likely groundwater
inflows since it is usual to assume that the ground can be represented by an
equivalent porous medium. From theoretical considerations it has been found
that the amount of flow passing through a discontinuity is proportional to the
cube of the width of that discontinuity opening and thus in those sections of
the ground where the discontinuity apertures are small, the inflows should be
minimal. Unfortunately it is not possible to measure discontinuity apertures
directly in borehole cores and thus the variation in permeability cannot be
obtained. However it was observed that where the RQDs exceeded 907, the
discontinuities were generally tight. Very few discontinuities at or near
tunnel horizon possessed any staining or showed indication of groundwater
flow.
The majority of the information on the permeability of the rocks to be
encountered on the inter-island and outfall tunnels is derived from a total of
133 double packer tests carried out during the 1988 marine investigation, one
of which could not be interpreted. The detailed results are presented in the
1988 marine drilling report (M&E, 1989). Initially during the campaign only
selected sections of the boreholes were tested based on the degree of
fracturing of the borehole cores. Subsequently continuous sections of
borehole were tested and this allowed a more complete picture of the
6-25
I I I I I I I I I 1 I I I I I I I I I
permeability to be obtained. Typically, tests were carried out with three
increments and two decrements of pressure, with the pressures chosen to ensure
that the vertical effective stress at the test horizon was not exceeded. Some
of the tests are unusual and suspect in that the interpreted permeability at
low pressures is negligible, but increases markedly as the pressure is
increased. The precise cause is uncertain although it is presumed that either
leakage past the packers or artificial opening of preexisting discontinuities
is responsible. Where this effect has occurred the assumption has been that
the permeability at the low pressures is more representative of the
permeability of the ground at that location. The interpreted permeability for
each packer test is presented in Appendix F. In an attempt to overcome, or at
least monitor, leakage past the packer and thereby improve the quality o f the
insitu permeability data double packer equipment with monitoring above, within
and below the test zone was undertaken on a continuous basis in the bedrock
sections o f each a f the shaft boreholes i n a d d i t i o n bo the standard double
packer tests (Appendix F). However on some occasions no test could be carried
out because it did not prove possible to achieve adequate seating of the
packers.
The information on tunnel inflows is rather limited in the literature,
however, some details are presented for the Main Drainage Tunnel and the
Maiden Tunnel. Mo information is given on the condition of the tunnel at the
time of measurement of the inflow in particular what proportion if any of the
permanent lining had been installed. The Maiden Tunnel gave the highest
inflow of around 0.193 gals/min/ft. length of tunnel for an approximately
14.5 foot diameter tunnel, whereas the Main Drainage Tunnel had maximum
inflows of the order of 0.090 gals/min/ft. length of tunnel for a 12 foot
diameter tunnel. Both of these values are the maximum observed inflow over
the full length of the tunnel.
6.3.1 Outfall and Diffusers
A typical cross-section of the outfall tunnel has been analyzed to calculate
the steady-state water inflows. The analysis assumed that the till is
impermeable, but that recharge occurred both horizontally and from below
6-26
through the Cambridge Argillite with the boundaries placed at an equivalent
distance of 0.5 miles. The calculated inflows were found not to be sensitive
to the distance of this boundary. The tunnel was assumed to be 27 feet in
diameter, located 300 feet below sea level and at 100 feet below the base of
the till. The steady-state inflow in gals/min/ft length of tunnel was
calculated
a s :
Q = 5400 x K
where K is in cm/sec
Thus for the typical condition of K = 5 x 10 - cm/sec an inflow of
0.27 gals/min/ft is obtained. This is in broad agreement with the value
quoted for the Maiden Tunnel but is somewhat higher than that in the Main
Drainage Tunnel. However short sections of tunnel could have inflows of up to
5 gals/min/ft could occur.
Typically inflows into rock tunnels are associated with a few more open
discontinuities with the majority of the discontinuities providing only minor
seepage. It is considered that only if inflows exceed 1 x 10 -" cm/sec, i.e.,
inflows in excess of 5 gals/min/ft, over a considerable section would major
difficulties associated with water be encountered and ground treatment be
required.
It should be noted that at the face the inflows for a given permeability will
exceed those quoted above because of the three-dimensional flow pattern
associated with the face and the non-steady state flow that occurs as the
water pressures around the tunnel are drawndown.
The fact that the tunnel will be excavated beneath the sea, need not result in
large inflows of water since low permeability materials overlie the bedrock.
The groundwater flow in the bedrock is controlled by the discontinuities and
therefore the storage will be small. However since the storage is limited,
the packer tests should also stabilize rapidly to a condition closely
6-27
I I I
approximating the steady state. Thus, it is considered that the
permeabilities measured are appropriate for tunnel inflow analysis. However
it is expected that because of the relatively long distance from recharge
boundaries, there will be rapid reduction in ground water inflows from the
high values initially associated with the change in storage to a steady state
condition in which water is drawn in from these recharge boundaries.
Quantification of this effect is complicated owing to of the large number of
unknowns such as the storage capacity of the rock, but is likely to be up to
one order of magnitude.
The surficial materials will be penetrated the diffusers. In the diffusers
the presence of high permeability material should not greatly influence their
construction.
6.3.2 Inter-Island Tunnel
In the four boreholes along the inter-island alignment 17 of the 29 double
packer tests indicated permeabilities equal to or in excess of 1 x 10
cm/sec, and 13 of these tests indicated permeability values greater than or
equal to 1 x 10" cm/sec. The range of permeability values in the zone of the c
proposed tunnel horizon (-175 to -255 feet MDC Datum) is less than 1 x 10~°
cm/sec to 3 X 10" cm/sec.
6.3.3 Shaft Data.
The shafts will be excavated through the soil column and the rock to an
elevation of approximately -290 feet MDC Datum for the outfall and -240 and
-225 for the inter island shafts at Deer Island and Mut Island respectively.
Falling head permeability tests were performed in auxiliary holes near each
shaft boring, the results of these tests are given in Appendix 0. The values
of Kn in the till range from 0 to 9-32 x 10"^ (Appendix M). The
permeabilities in the sub-glacial deposits (Section 6.1.5) range from 7.08 x
•10"^ to 4.06 X 10"^ cm/sec. Two falling head tests were performed in the fill
at LDE-58A giving an average Kn of 3.58 x 10"^ cm/sec.
6-28
1 I I I I I I I I I I I I I I I I I I
Fifty-four double packer tests were performed in the three shaft borings.
Average permeabilities determined from this data range from 2.63 x 10"' to
2.72 X 10~3 for LDE-59 <1.0 - 10"" to 3.43 x 10"^ for LDE - X51 and 0 2.54 x
10"* to 1.04 X 10"5 for LDE-46 (Colder, 1989; Appendix F). In LDE-58 some of
the high permeability areas are below the proposed depth of the shaft.
The borehole geophysical survey, (Appendix M) indicates zones of water flow
for each of the borings, and also indicates some zones which are water poor.
The hydraulic conductivity of individual fractures were determined from the
data and the values range from 50 to 1,500 ft/year.
6.4 IN SITU STRESS
The pre-existing state of stress is an important input parameter for
estimation of the deformations and extent of any zones of failure around
tunnels and other underground excavations. Consideration should be given to a
program of in-situ stress measurement at an early stage of the construction
period to check the accuracy of any assumptions made in the design.
A number of techniques have been attempted to determine the state of stress in
the rock near the surface in the area. For the most part these attempts have
been inconclusive or unsuccessful. Stress release has been observed along
readouts and by offsets of glacially polished surfaces in various spots
throughout Mew England. The offsets have been attributed to glacial loading
and unloading, effects of thermal change and hydration (see Appendix K).
For the outfall tunnel the total vertical stress, a^, at a depth of 360 feet
below sea level will be approximately 220 psi representing 100 feet of water,
60 feet of overburden and 200 feet of rock. It is usual in rock for the mean
horizontal stress to exceed the vertical stress (Brown and Hoek, 1988)
particularly the stronger and older strata. This would intuitively be
predicted in this area in view of the stress history of these rocks in terms
of their burial and exhumation and also the compressional nature of the Boston
Basin. For these reasons it is anticipated that the ratio of mean total
horizontal stress to total vertical stress, k, will be greater than one and
possibly as high as two.
6-29
I I I I I
I I I
Three packer tests in the shaft borings (two in LDE 51 and one in LDE-46, both
on Deer Island) performed while utilizing sophisticated equipment to monitor
downhole pressures appeared to open existing fractures in the rock
(Appendix F). The tests at Deer Island were at a depth o f 350 to 370 feet and
370 to 390 feet. Both tests appeared to re-open a fracture at approximately
300 psi. A test at Mut Island was at a depth of 380-399 feet and appeared to
reopen the fracture between 310 and 320 psi. These tests indicate that the
minimum horizontal stresses would be in the order of 300 psi at a depth of 350
to 400 ft below the ground surface. This would indicate that the ratio of
mean total horizontal stress to total vertical stress are within the ranges
stated above.
The maximum circumferential stress around a circular tunnel is given
approximately by (3K - Da^. For the condition of K = 2 and o^ = 300 psi, a
maximum stress of 1500 psi is obtained. This is substantially lower than the
strength of the Cambridge Argillite or the intrusive diabase, even allowing
for the adjustment due to the presence of the discontinuities. In addition
the circumferential stress will reduce rapidly with distance into the rock at
the same time as the radial stress increases. It is therefore considered that
overstressing of the ground will not be a problem except possibly in highly
fractured fault zones or kaolinized o r chloritized zones. It is also
considered that the alteration process will probably preclude the possibility
that a significant stress anisotropy could exist in either the horizontal or
the vertical planes. Under this stress regime this high stress will also be
present in the crown of the tunnel which will aid the stability of all but the
most unstable blocks.
6.5 MASS CHARACTERIZATION
It is anticipated that a wide variety of rock conditions will be encountered
along the two tunnel alignments due to variations in the rock type, degree of
weathering or alteration and the degree of fracturing. It is necessary to
quantify the effect this change has on the behavior of the rock during
excavation. On previous projects a number of rock classification schemes have
been developed of which two have become well established. These are the MGI
6-30
I I I I I I I I I I I I I I I I I I I
classification system proposed by Barton et al. (1974) and the CSIR
Geomechanics classification system developed by Bieniawski (1973). These
schemes take account of properties such as strength, degree of fracturing,
weathering, and groundwater conditions. To use these classification systems,
it is necessary to subdivide the ground into rock units displaying similar
characteristics. Initial proposals for rock units applicable to this project
are given in Table 6-4. The associated Q-values and RMR values and the
typical geological conditions are given in Table 6-5.
Based upon the available information proposed values for the Hoek and Brown
"m" and "s" parameter appropriate for preliminary designs are presented in
Table 6-6. These strength criteria should only be used where the
discontinuity spacing is small in relation to the size of the zone of
"failure." Where this is not the case the rock behavior surrounding the
tunnel should be modelled as discrete separated by discontinuities rather than
as a weakened continuimi model as suggested by the Hoek-Brown equation.
Review and up-date of these "m" and "s" values will be necessary when data
from the 1989 Maine boring campaign becomes available.
6-31
I I I I I I I I I I I I I I I I I I I
TABLE 6-4. ROCK CLASSIFICATION UNITS
Q
RMR
Q-rating
RQD
Jn
Jr
Ja
Jw
SRF
RMR-rating
Strength
RQD
Spacing
Condition
Water
Direction
>10
>65
90-100
4-9
2
1
1
1
15
20
>15
20
10
-5*
>10
>65
90-100
9
2-1.5
1
1
1
10
20
>15
20
8-10
-5*
4-10
56-65
75-90
12
2-1.5
1
1-0.8
1
7-10
17-19
7-15
20
5-10
-5«
1-4
44-56
50-75
12
1.5
1
0.66
1
10
7
7
20
4-10
-5*
<1
<44
<50
12
1.5
1
0.66
1
10
5
5
12
2-7
-5*
<0.1
<25
<50
15
0.5-1.5
1-4
0.5
5-10
0.4
5
5
6
0-7
-5*
* -10 to -12 when orientation of discontinuities unfavorable
6-32
I I I I I I I I I I I I I I I I I I I
TABLE 6-5. GEOLOGICAL CONDITIONS
Rock Unit
1. Fresh unmicrofractured diabase and other igneous intrusions. Very strong to extremely strong Argillite, Average fracture spacing greater than 1 ft.
2. Microfractured diabase. Typical fresh Argillite and ash. Average fracture spacing greater than 1 ft. tight joints.
3. Same rock types. Tight fractures at spacing of 8 inches to 1 ft., or laminated bedding in crown of tunnel dipping at less than 20°.
4. Same rock types. Tight fractures at average spacing of 6 inches to 8 inches, or slightly open fractures at wider spacing.
5. Same rock types. Fractures with clay infilling or slightly open. Highly altered Argillite with fracture spacing at less than 1 ft. Thin bands of completely altered Argillite, ash or diabase in otherwise good rock. Discrete thin clay-filled faults. Shatter zones.
6. Substantial fault or shear zones. Zones of completely altered Argillite, ash or diabase.
6-33
I I I I I I I I I I I I I I I I I I I
TABLE 6-6, HOEK-BROWN STRENGTH PARAMETERS
Rock Unit
1
2
3
4
5
6
SC = SC =
15, 23,
,000 ,000
psi psi
RMR
>65
>65
56-65
44-56
<44
<25
Argillite diabase
Undis
m/mi
<0.29
>0.29
0.24
0.17
<0.14
<0.069
iturbed
5
<0.021
>0.021
0.012
0.0039
<0.0020
<0.00024
Dis
m/mi
<0.082
>0.082
0.057
0.039
<0.018
<0.0047
turbed
5
<0.003
>0.003
0.0013
0.00024
0.000088
0.0000037
6-34
I I I I I I I I I I I I I I I I I I I
7.0 SEISMICITY
This section describes a summary of geologic and seismologic analyses
performed in the context of developing and supporting appropriate seismic
design criteria for the Deer Island Secondary Treatment Facility (DISTF). All
analyses performed to develop seismic design criteria are provided in a final
report included as Appendix K. Seismic design criteria, including design
response spectra and design acceleration time histories are required for
surface structures to be located on Deer Island (approximate coordinates of
42.35°M; 70.954°W) and for the inter-island and outfall tunnel components
extending south to Nut Island and offshore to the following coordinates
(NE: 42.433°N, 70.77°W; SE: 42.372°M, 70.77°W).
Statistical and probabilistic methods have become the standard tool of modern
sciences and engineering technologies. Applied to seismic design, the
technique has proven to be useful and versatile as it offers flexibility in
modeling the available data and defining quantitatively the various
uncertainties. By examining alternative models and variations in seismic
hazard results, the method can be used to define the relative importance of
various assumptions. This sensitivity analysis approach permits the
recommendation of appropriately conservative seismic design criteria through
an integrated assessment of available geologic and seismologic data.
The level of seismicity is relatively higher in eastern Massachusetts than in
other regions of Mew England. Although knowledge about causes of the observed
seismic activity and neotectonic movements is still limited, earthquake
recurrence frequencies and earthquake-induced ground motions can be
constrained from the seismological data available for the region. In this
context of limited data and knowledge, the probabilistic approach for
selection of an appropriate design earthquake for the Deer Island Secondary
Treatment Facility is invoked to examine consequences on seismic hazard levels
of alternative hypotheses on future earthquake occurrences.
7-1
I I I I I I I
I I
The following bounds have been used for compilation of geologic and
seismologic data: 40°M to 45°M and 68°W to 74°W. This study region has been
adopted such that all seismogenic regions located within radii of about 200 km
from the project site are included in the seismic hazard computations. The
potential for damaging ground motions is minimal for earthquakes located at
distances greater than 200 km; thus, several important earthquake sources in
the northeast are not discussed in this seismic hazard assessment due to their
distances from the site,
7.1 Probabilistic Seismic Hazard Methodology
The probabilistic seismic hazard methodology employed in this assessment of
the DISTF project was introduced by Cornell (1968). It was formally encoded
by McGuire (1976) into a standard FORTRAM computer program, entitled EQRISK
(USGS OF Report 76-67). The methodology was further refined during an
extensive application to the Eastern United States (EUS) (east of 105°w) by
the Electric Power Research Institute (1986), a two year project in which
Weston Geophysical participated as the expert panel for the northeast region
of the United States.
The term "seismic hazard" is defined as the annual frequency of exceeding a
particular seismic ground motion amplitude, such as peak ground acceleration
or velocity, at a specified location. Computations of seismic hazard require
the integration of geological and seismological information formulated in
terms of three mathematical models.
These include:
In Step 1, seismic source zones are delineated using all geological and seismological information available for the region. Geometries of the seismic sources, and implicitly distances to the site, as well as the nature of the sources, faults or clusters of seismic activity, are expressed in several alternative models.
In Step 2, rates of seismic activity or earthquake recurrence curves for each one of the sources are established by performing statistical analysis.
7-2
I I I I I I I I I I I I I I I
I I I
• In Step 3, an attenuation model, or models, are selected as the most appropriate predictor of earthquake ground motions for the region.
• Finally, in a last stage, the computations of the hazard are performed. Results can be expressed in various formats, either as a probability level that a certain acceleration will not be exceeded over a specified time interval, or as the annual frequency that a particular acceleration level may be exceeded.
7.2 Geology
7.2.1 Regional Geology
The Deer Island Secondary Treatment Plant and related facilities are located
on the Atlantic Coastal Plain province within the Appalachian orogen. The
linear Appalachian erogenic mountain belt, extending the length of eastern
United States and southeastern Canada, marks the boundary of the Paleozoic
collision of the North American and African/Eurasion continents. A complex
collision and welding of these major plates and several intervening
microplates occurred during the Taconian (455-445 mya) Acadian (400-355 mya)
and Alleghenian (300-250 mya) orogenies.
The Mew England region of the Appalachian erogenic province, including
offshore areas of the Gulf of Maine, is subdivided into distinct linear
tectonic belts or terranes, characterized and typically separated by
fundamental northeast to north-trending crustal structures (Figure 7.1). The
lithotectonic belts are comprised of deeply eroded remnants of Precambrian
cratonic basement. Paleozoic continental margin sediments, island arc
volcanics, back arc basin sediments, and microcontinental blocks which were
progressively and multiply transported, deformed and ultimately welded onto
the Precambrian North American craton.
The lithotectonic belts comprising the southeastern Mew England region of this
investigation include the Bronson Hill Anticlinorium, Kearsarge-Central Maine
Synclinorium, Coastal (Maine) Belt, Nashoba Thrust Complex and Avalonian
Platform (Figure 7-1).
7-3
I REGIONAL BEDROCK GEOLOGY
STRATIFIED SEQUENCES PLATFORM EUGEOSYNCLINAL
PLUTONIC ROCKS
f _ I QUATERNARY 1° ' ' ^1 TERTIARY
CRETACEOUS
FELSIC
I S t j L-1'y.v-l Z i o & S M JURASSIC
PERMO-TRIASSIC
j-M :\ JURO-TRIASSIC
[X.c:-:| CARBONIFEROUS
[; D : | DEVONIAN
SILURO-
A!OS:<1 MIDDLE TO LATE i l i l ^ PALEOZOIC
^ ° ' I DEVONIAN
I CAMBRO-1 ORDOVICIAN
ULTRAMAFIC ROCKS | \ >•
EARLY PALEOZOIC K ^ S
FAULTS
H.H.F. HOMCY HU.I. FAUCT L . C F tAKC CHAR FAULT C . - K F , CLINTON>MCweURY FAULT e.8.F. 8LO0OY BLUFF FAULT N.R.F, NONESUCH BIVtR FAULT N.F. NOnUMBCCA FAULT
INFERRED
L.F. LUBEC FAULT e.F. BCLLCISLC FAULT r . r . FRCKRICTON FAULT CF . CATAMARAN FAULT R . e . - K F MOCKY BROOK-MILLSTfieAM FAULT L.L.T.F LOCAN& LINC THRUST FAULT
I. GEOLOGIC DATA FROM SCABROOK FSAR ( 19821, FYFFE (19821, WILLIAMS (1978), OAVIES( I977>, CAOY (19691, OOYLE ETAL.( I967I , OOLL ET AL.( 19611. ANO BILLINGS (1955). AUSTIN (1979)
2. STRATIGRAPHIC REVISIONS IN EASTERN MASSACHUSETTS ARE NOT YET COMPLETE ( L E N K E T A L , 1 9 e 2 , ZEN E T A L . 1982, ZARTMAN ANO NAYLOR. IN PRESS 1.
_ l L 1 ,L_
Seismic Design Recommendation for the
Mass. Water Resources Authority Deer island Secondary Treatment Facility
for Metcalf & Eddy. Inc.
Regional Geologic and Tectonic Elements Map
Weston Geophysical Fig. 7.1
5/89
7.2.1.1 Structure
The New England region of the Appalachian orogen has recorded a complex series
of structures which have often been subjected to multiple deformations during
collision, translation and extension. Early ductile, and later brittle
faults, including compressional thrust and reverse faults, strike-slip and
oblique-slip faults, and tensional normal faults, are widely recognized at
both local and regional scales. Given the apparent localized occurrence of
historical seismic activity, it is believed that the prevailing east northeast
directed principal horizontal stress accumulates to produce earthquakes only
on favorably oriented faults. In addition such faults may have localized
asperities, fault intersections or crustal inhomogeneities which serve to
impede aseismic creep or adjustment along the structures. Description of
significant faults or fault zones which are recognized within distinct seismic
source zones in the northeast is available in Section 4.1,2 of Appendix K.
7.2.1.2 Geophysics
Gravity and aeromagnetic data provide information for interpretation of the
subsurface configuration of lithologic and structural features. In the
northern Appalachian fold belt, broad regional trends of both gravity and
magnetic patterns are strongly controlled by the predominant
northeast-trending grain of the lithotectonic belts and structures.
7.2.1.2.1 Regional Gravity Field
The regional gravity pattern corresponds well with the overall north-south to
northeast trend of the Appalachian lithotectonic belts (Figure 7-2). A
distinct gravity gradient trending along the western border of the Mew England
states has been interpreted as marking a substantial Precambrian crustal
boundary between miogeosynclinal deposition to the west and eugeosynclinal
deposition, subsequently deformed and uplifted to the east (Diraent, 1968). In
general, gravity values are high over the Gulf of Maine and over structural
highs. A generally lower field is apparent over structural lows
(Kearsarge-Central Maine Synclinorium) where crustal subsidence and deposition
7-5
REGIONAL BEDROCK GEOLOGY
STRATIFIED SEQUENCES
PLATFORM EUGEOSYNCLINAL
PLUTONIC ROCKS
FELSIC MAFIC
QUATERNARY TERTIARY CRETACEOUS
f l y Fp5rq3 CRETACEOI uSg WiWfd JURASSIC »-_F PPDMn.TQ i°3
CRETACEOUS f . , .v . i , | :;^l--.v.-;.--1 JURASSIC itftrM f' PERMO-TRIASSIC
j - t JURO-TRIASSIC
c:-; CARBONIFEROUS
0 . DEVONIAN
SILURO-
I MIDDLE TO LATE P5P3T I PALEOZOIC
^•>' I DEVONIAN
I CAMSRO-I ORDOVICIAN
ULTRAMAFIC ROCKS | \ f-
3 EARLY PALEOZOIC W g ' i
FAULTS HIGH-
H M F L.C.f. C . - H F e.a.F. N.H.F. N.F.
2 BASEMENT
— « — i 1
ANGLE THRUST
MOt*EV N»LU FAULT L*»X CHAR FMJLT CtlNTON-MCweURV FAULT BLOOOV BLUFF FAULT NONESUCH RrvCR FAULT NORUMSEGA FAULT
" *wr ERRED'
L.F. LUSCC FAULT B.F. BEtLClSLE FAULT r . r . FACOCRtCTON FAULT CF. CATAMARAN FAULT R e . - M . F ROCKV BROOK-M«.LSTREAM FAULT L L . T F LOGANS LINC THRUST FAULT
I. GEOLOGIC DATA FROM SEABROOK F S A R ( I 9 B 2 1 . FYFFE (1982). WILLIAMS ( I 9 7 a l . 0AV1ES(I9771. CAOY (1969), OOYLE ET iL.(19671.
DOLL ET AL.( I96I) , ANO BILLINGS ( I 9S5 I .
2. STRATIGRAPHIC REVISIONS IN EASTERN MASSACHUSETTS ARE NOT YET COMPLETE (LENK E T A L , 1982, ZEN E T A L , 1982, ZARTMAN AND NAYLOR. IN PRESS ).
TOTAL BOUGUER ANOMALY MAP CONTOUR INTERVAL: 5 MGALS
I I LESS THAN -50MGALS GREATER THAN 30MGALS
KANE.MP, YELLIN. M J., BELL. K.G., AND ZEITZ, I., 1972 GRAVITY ANO MAGNETIC EVIDENCE OF UTHOLOGY ANO STRUCTURE IN THE GULF OF MAINE REGI(X4, U S GEOLOGICAL SURVEY PROFESSIONAL PAPER 7 2 6 - B , 22p .
HILORETH.C.T (COMPCER), 1976. GRAVITY MAP OF ONSHORE-OFFSHORE NORTHEAST UNITED STATES ANO SOUTHEAST CANADA, REGIONAL MAP NO. 1. SECONDED., NEW ENGLAND SEISMOTECTONIC STUDY. WESTON OBSERVATORY, WESTON , MA, U S, NUCLEAR REGULATORY COMMISSION CONTRACT AT(19-241 - 0291 .
Seismic Design Recommendation for the
Mass. Water Resources Authority Deer Island Secondary Treatment Facility
for Metcalf & Eddy, Inc.
Regional Bouguer Gravity Anomaly Map
Weston Geophysical Fig. 7.2
5/89
I I I I I I I 0 n n I I I
occurred during formation of the Appalachian orogen. Local circular
anomalies, high and low, correspond to syn- and post-metamorphic mafic and
felsic intrusives.
7.2.1.2.2 Regional Aeromagnetic Field
Not surprisingly, the regional aeromagnetic trends are very similar to the
gravity anomaly map, however, a more detailed pattern generally results from
shallower causative lithologic and structural features (Figure 7-3).
Strong, northeast-trending, linear anomalies associated with the Nashoba
Thrust complex result from magnetite-rich metavolcanic rocks faulted against
rocks of low magnetic signature. This fault complex can be traced
northeastward offshore for over 20 miles, on a N67E trend (Simpson et al.,
1979; Birch, 1983).
In addition to the Nashoba Belt, other terranes and belts recognized on-shore
appear to extend into the Gulf of Maine, based on characteristic aeromagnetic
signatures. A high-relief anomaly pattern close to the shore, indicative of
shallow burial of the crystalline basement, becomes less distinct offshore
where thicker post-Jurassic continental margin sediments have accumulated.
7.2.1.3 Discussion
The preceeding descriptions developed more fully in Appendix K, provide a
summarization of the geologic and structural framework of the northeastern
United States (NEUS). This information provides a basis for analyzing the
pattern of historical seismic activity in order to delineate spatial
associations, and potential cause and effect relationships. Certain
generalizations and apparent relationships are evident, some better
constrained than others. Possible causative geologic features include a zone
of crustal weakness related to Mesozoic intrusives, faulting associated with
Triassic-Jurassic rift basins, variable crustal subsidence in coastal
embayments, and intersections of brittle faults with older ductile fault
trends. Variable fault plane solution orientations suggest that a
7-7
I
^ 7
72° „ ^
•f
REGIONAL BEDROCK GEOLOGY
STRATIFIED SEQUENCES
PLATFORM EUGEOSYNCLINAL
QUATERNARY
PLUTONIC ROCKS
FELSIC
TERTIARY CRETACEOUS
gSL.|.-.,„,V.i CRETACEOUS uSoEaSa JURASSC t i i
i-5| JURASSC PERMO-TRIASSIC
J t JURO-TRIASSIC
j.':':e:-:| CARBONIFEROUS
1'. D ; | cevoNiAN
[ 1 ^
1 CO 1 | c o .
1 SILURO-DEVONIAN
CAMBRO-OROOVICIAN
Y-'Jhx-A MIDDLE 10 LATt V-»<K-,\ l;"-''---'l PALEOZOIC t<,«v>.|
ULTRAMAFIC ROCKS | *, <• ]
Viyi-fh EARLY PALEOZOIC WMf:i
-sL I^^P^I BASEMGNT
FAULTS
H F HONEY HILL FAULT C F, LAKE CHAfl FAULT - N F , CLtNTON-NEweUHT FAULT B.F. B L O O O Y BLUFF FAULT fi.F. NONESUCH R»V£R FAULT
NORUMBCGA FAULT
L.F. LU6EC FAULT e.F BELLeiSLC FAULT f . f fREDERlCTON FAULT C. F. CATAMARAN FAULT R B -M.F ROCKT bROOK-MILLSTREAM FAULr U L .TF L O C A N ' S LINE THRUST FAULT
I, GEOLOGIC DATA FROM SEABROOK FSARHSeZ) . FrFFE (1962). WILLIAMS ( (9761 . DAVlESt 1977), CAOY {(969». OOYLE ET AL.( 1967).
OOLL ETAL.(1961}. AND eiLDNGS (1955).
2 STRATIGRAPHIC REVtSfONS IN EASTERN MASSACHUSETTS ARE NOT YET COMPLETE ( LENK ET AL.. 1982, ZEN ET A L , t9e2, ZARTMAN ANO NAYLOR, IN PRESS ).
REGIONAL AEROMAGNETIC MAP CONTOyR INTERVAL = ZOO GAMMAS
I '. I LESS THAN.-200 GAMMAS GREATER THAN 4COGAMMAS
ZE ITZ . I., HAWORTM.RT. WILLIAMS. H., ANO OANIELS. O, L., (COMPILERS). 1980. MAGNETIC ANOMALY MAP OF THE APPALACHIAN OROGEN, MEMORIAL UNIVERSITY OF NEW FOUNOLANO, MAP NO 2.
Seismic Design Recommendation for the
Mass. Water Resources Authority Deer Island Secondary Treatment Facility
for Metcalf & Eddy, Inc.
Regional Aeromagnetic Map
Weston' Geophysical Fig. 7.3
5/89
lithologically and structurally heterogeneous crust responds to the prevailing
northeast-southwest to east-west stress regime originating at depth. In
contrast to plate boundary environments, where fault motions are somewhat
predictable, the reaction of faults in an intraplate environment to the
regional stress regime is not readily apparent.
Given the lack of any unique structure - earthquake correlations in the NEUS,
the preferred method of analyzing historical seismicity to model future
activity involves defining seismic source zones based on interpretation of
geological and geophysical data. Because of the complexity of the structural
fabric, equivocal nature of structure-seismicity relationships and brevity of
the earthquake record, alternative source zonations are necessary. The
proposed source zonations attempt to constrain seismic activity patterns to
definable geologic structural terrains; mapped, interpreted and inferred.
7.2.2 Local Geology
7.2.2.1 Stratigraphy
The proposed secondary treatment facilities and interconnecting tunnels will
be located on compact glacial tills and within underlying mildly deformed
rocks of the Boston Basin (Figure 7-4). The basin contains 17,000 feet of
interbedded sedimentary and rhyolitic and andesitic volcanic rocks from a wide
variety of depositional environments. Rapid transitions between alluvial,
fluvial, lacustrine, lagoonal and marine shelf deposits are indicative of an
active block-faulted tectonic environment during deposition.
Borings drilled along the tunnel alignments and in the diffuser area
penetrated marine sediments and till overlying an irregular bedrock surface.
Bedrock core recovered from borings c o n s i s t s primarily of argillite of the
Cambridge Formation with local igneous intrusive sills and dikes.
Post-lithifieation structures include joints, coated with clay and calcite,
and brittle faults ranging from single planer surfaces to wider fault zones
with local brecciation, and gouge formation. Quartz and calcite
mineralization and slickensided surfaces also occur. The majority of the core
7-9
I I
42° 30'
42° 15'
EXPLANATION
MILFORD DEDHAM ZONE
SEDIMENTARY AND VOLCANIC ROCKS AND METAMORPHIC EQUIVALENTS
DZI Lynn Volcanic Complex - Rhyollte, agglomerate and tuff.
€Zc Cambridge Argill ite -gray argill ite and minor quartzite.
CZf Roxbury Conglomerate, sandstone, siltstone, argillite and melaphyre.
<Zrb Melaphyre in Roxbury Conglomerate
-Cbw Braintree Argil l i te and Weymouth Formation
Zm Mattapan Volcanic Complex - Rhyolite, melaphyre, agglomerate, and tuff.
Zw Westboro Formation - Quartzite, schist, cak-silicate quartzite and amphibolite.
INTRUSIVE ROCKS
Dpgr Peabody Granite-Alkalic granite containing ferro hornblende.
SOqgr Quincy granite - Alkalic granite.
Ongb Nahant Gabbro - Labradorite - pyroxene gabbro, hornblende gabbro and hornblende aior i te.
Zdngr Gray granite to granodiorite
Zdgr Dedham Granite - Light grayish-pink to greenish-gray, equigranular t o slightly porphyrit ic, variably altered grani te.
Zdi Diori te - Medium-grained hornblende diori te metamorphosed in part to amphibol i te and hornblende gneiss.
Zdigb Diorite and gabbro
Contact
High -angle reverse fault - Bar and ball on upthrown side
Fault for which sense of movement is unknown or undifferentiated
Interpreted faults for which unequivocal f ie ld demonstration is lacking are shown by dashed lines
1—T r ^ T " "1 5 K M
5Mf l .ES _ l
Source: Zen et. al., 1983
Seismic Design Recommendation for the
Mass, Water Resources Authority Deer Island Secondary Treatment Facility
for Metcalf & Eddy. Inc.
Site Area Geologic Map
Weston Geophysical Fig. 7.4
5/89
appears fresh, with minor weathering features restricted to the bedrock
surface. Bedrock from several cores, however, showed local kaolinization
which was variably pervasive and not apparently related to increased bedrock
fracturing.
7.2.2.2 Structure
The internal geometry of the Boston Basin is controlled by east
northeast-trending folds and faults which parallel the basin boundary faults
to the north and south (Figure 7.4). Approximately nine faults cut the basin
into long narrow fault blocks up to 15 km in length (Kaye, 1982). The
longitudinal faults are interpreted to be high-angle reverse. These faults
show minor cataclastic deformation indicating ductile movement or healing by
relithifieation early in the history of the basin. Cross-faults oriented
northwest and north-south show slickensides with strike-slip components (Kaye,
1982). Some of the cross-faults are characterized by large shear zones, often
with associated diabase dikes.
Faults, fracture zones, joints and dikes have been mapped extensively in
previously bored tunnels beneath the Boston area, and are extrapolated
offshore utilizing seismic reflection data supported by bedrock mapping
conducted on the Outer Harbor islands. Additional discussion of bedrock
structures is presented in Sections 4 and 5 and Appendix K of this report.
7.2.2.3 Surficial Deposits
The physical nature and vertical extent of surficial deposits is important for
the consideration of ground motion attenuation for facilities not founded on
bedrock. A wide variety of artificial and natural surficial deposits occur in
the site area, including fill, sludge, till, glaciofluvial sands and gravels,
glaciolacustrine sands, silts and clays, and marine clays (Boston "blue
clay"). Natural deposits have been eroded, transported and redeposited by
post-glacial fluvial and marine processes into existing near surface
configurations in alluvial marsh and swamp, and tidal marsh and beach
environments. The majority of these deposits will not be a significant factor
7-11
to project facilities, as the tunnels will be in bedrock and surface
facilities will be constructed on till or driven piles. Details of the soil
conditions are available in the boring logs (Appendix B) and discussion in
Appendix K.
7.2.2,4 Local Geophysical Investigations
Detailed gravity and aeromagnetic maps available for eastern Massachusetts and
the Boston Harbor and Massachusetts Bay offshore areas were examined and
compared to mapped geologic information. The predominant features delineated
by the aeromagnetic and gravity anomaly maps are the Cape Ann plutons and the
Nahant gabbro. Prominent circular aeromagnetic anomalies associated with
these intrusive bodies are in contrast to a broad featureless pattern
characteristic of the Boston Basin rocks, particularly offshore.
In addition to interpretation of existing geophysical data, marine seismic
reflection and marine/land seismic refraction surveys were conducted along the
proposed tunnel alignments and on Deer Island. The offshore data were
interpreted to produce bottom and bedrock surface contour maps, as well as
cross-sections showing various components within the unconsolidated materials
overlying the bedrock surface. The bedrock surface, as previously described,
is irregular. The contour pattern suggests linear valleys and ridges which
have been interpreted to be, in part, structurally controlled (Weston
Geophysical, 1988).
7.3 Stress Regime
Modern stress field configuration in the northern Appalachian region has
commonly been inferred from strain relief measurements, geologic stress
indicators, and earthquake focal plane solutions. A more definitive method of
stress measurement, hydrofracture of a rock core boring, has been successfully
employed, however, few measurements utilizing this technique have been
undertaken in Wew England. Detailed discussion pf stress field investigations
is provided in Section 5.0 of Appendix K.
7-12
The northeast to southwest tectonic plate vector of the North American
lithospheric plate is the primary source at depth in the crust for the
prevailing northeast-southwest to east-west compressive stress field reported
in the northeastern United States. Zoback and Zoback (1980, 1981) and Yang
and Aggarwal (1981) have noted that the stress direction in the mid-continent
parallels the absolute plate motion of North America away from- the -
mid-Atlantic ridge. In contrast the coastal New England region, east of the
Appalachians shows a more random shallow stress field pattern. Several
potential causes for this effect have been suggested, all of which ultimately
originate with the non-uniform character of the Appalachian erogenic belt.
The interaction of distinct lithotectonic blocks, bounded by a series of
regional-scale faults produced by a long history of compressional and
tensional regimes, would likely modify any uniformly imposed plate tectonic
forces.
7.4 Seismic Activity
7.4.1 Earthquake Data Base
Weston Geophysical's earthquake data base has been used in this study to
prepare the seismicity maps and to calculate the recurrence parameters
necessary for assessing seismic hazard for the DISTF. The computerized file
of earthquake information has been developed over a period spanning more than
two decades, under a strict quality assurance program required for any
technical work related to the safe design of nuclear power plants. Instead of
blindly relying on one or two historical catalogs and a single source of
bulletins for the more recent events, this catalog was developed by using
comparative parallel listings to identify typographical errors and duplicate
entries and to remedy the interdependency of secondary sources. It also
incorporates periodical revisions of epicentral locations and magnitudes as
they become available in technical journals or through ad hoc research on
specific earthquakes, performed by Weston's seismologists.
7-13
I I I I I I I I I I
7.4.1.1 Completeness and Reliability of the Earthquake Data Base
The completeness and reliability of any earthquake catalog is a function of
the distribution and density of both human population and seismographic
monitoring equipment. Prior to the instrumental era in seismology, which
began about 50 years ago in eastern New England, all information about
earthquake activity has been inferred from various forms of written
documentation on earthquakes' effects on people and structures. The
completeness, or percent of 'actual' earthquake activity within a region that
has been documented in catalogs, is thus greatest for regions that were
settled earliest. Therefore, the record of earthquake activity for the site
region in eastern Massachusetts is perhaps the most complete for any place in
New England due to the early settlement in the early I600's and population
expansion in this area around Boston.
In addition, seismographic monitoring of earthquake activity got an early
start in the site region due to deployment of a seismographic observatory at
Weston, Massachusetts in 1929. Subsequently, other stations were added and
regional seismographic networks were installed and expanded in the Northeast
from the 1960's to the present. Currently, the Northeast United States
Seismographic Network, which has operated since the mid-1970's, is capable of
locating all earthquakes in New England of magnitude 2.0 or larger.
An understanding of the completeness and reliability of the earthquake catalog
for the study region is essential for estimation of earthquake recurrence
frequencies and, therefore, for seismic hazard levels which are derived from
the frequency of earthquake activity among several additional considerations.
7.4.2 Regional Seismicity
Earthquake activity for the region surrounding the site is shown on
Figure 7-5. This seismicity map includes all catalogued events for the time
period from 1600 through 1986. Appendix K provides additional discussion on
symbols used on the seismicity map.
7-14
o o s s
o o
CO
+ 46" 0.00' + 46" 0.00
3
X
ICO
I o
In lo IN
i or UJ
I + ^ 0 ° o.oo'
44° 0 0 0 '
+ 42° 0 0 0 '
LEGEND MACNirUDE W i O t S TRQH 3 . 0 ro 7 . 0
r iME WINDOW BEGINS 1500 ENDS 19fi7
MAGNITUDE
a
•
SCALE 1 =3000000 50 100 150 200 KILOMETERS
+ 40° 0.00'
Seismic Design Recommendation for the
Mass. Water Resources Authority Deer Island Secondary Treatment Facility
for Metcalf & Eddy, Inc.
Regional Seismicity
Weston Geophysical Fig. 7.5
5/89
The site is located in a region that has been exposed to repeated seismic
activity over the past several hundred years. The largest events located in
the study region include the November 9, 1727 Newbury, Massachusetts
earthquake and the November 18, 1755 Cape Ann earthquake. The 1727 event has
an epicentral location about 50 km north-northeast of the DISTF site and the
1755 Cape Ann event has a location about 60 km northeast of the site.
Uncertainty in locations of these historical events can range to 25 km or
more; however, the available documentation including numerous accounts of
effects of these earthquakes at many coastal locations, supports a conclusion
that these events were located just offshore of northeastern Massachusetts
(Weston Geophysical, 1976). The earlier of these two events is attributed a
maximum Modified Mercalli epicentral intensity of VII; the latter is assigned
a maximum intensity of VIII. In addition, on the basis of the large areal
extent over which the 1755 event was felt, i.e., from Maryland to Nova Scotia,
a magnitude o f about 6.0 is estimated f o r this event (Street and Lacroix,
1979). Maximum reported damage effects in the vicinity of the DISTF site
resulting from these earthquakes have been assessed as an intensity of VII.
This level had been reported for Gloucester, Essex, and Ipswich, near the
epicenter, as well as at several other towns, including Boston, in eastern
Massachusetts. Other towns in southeastern New Hampshire, and southern Maine
were similarly affected. Damage included numerous toppled chimneys, stone
walls, and other masonry structures.
7.4.3 Local Seismicity
During the past two decades, seismographic monitoring of earthquake activity
in the NEUS has been greatly enhanced as a result of organization of the
NUSSN. Implementation of this regional network has permitted detection and
accurate location of all earthquakes ranging in magnitude to as low as about
2.0. Recent, more accurately located seismic activity is illustrated in
Appendix K-A along with epicenters of historical events. This recent activity
observed since 1970 exhibits several clusters surrounding the site. The most
prominent cluster is located in central and southern New Hampshire; another
more active area is located in the area of Westford and Chelmsford,
Massachusetts; the final cluster is located northeast of the site and extends
offshore from' southern Maine.
7-16
7.5 Seismic Zonations
A prerequisite for determination of probabilistic estimates of seismic ground
motion hazard is delineation of active faults, fault zones or clusters of
seismic activity (that are indicative of the presence of active fault zones)
in the region of the site. Throughout the northeast, positive correlations
have yet to be made between an earthquake occurrence and displacement on a
causative active fault. Earthquakes are believed to result from reactivation
of buried faults whose movements produce no, or not easily recognized, ground
surface movements. Seismic hazard evaluations in the northeast and throughout
much of the central and eastern U.S., where few active faults have been
identified, are cormonly performed using seismic source zones that are
delineated to encompass regions of enhanced seismic activity. This method is
also employed in this study.
Alternative seismic source zonations are examined in this study to evaluate
the impact of opposing assumptions on future seismic activity in the vicinity
of the DISTF project. Based on a first assumption, clusters of seismic
activity, evident even over a relatively brief 200-300 year period, are deemed
to be indicative of zones of crustal weakness characterized by faulting that
will continue to be potential seismically active during the next 50 to 100
years of engineering interest. The clusters of seismic activity should
therefore be attributed a higher likelihood of producing the next major
seismic event relative to the background regions between these clusters.
A series of regional seismotectonic provinces is employed to define the
seismicity of the site region under this first assumption that future activity
is more likely to recur in zones of historic activity. Three alternative
seismic zonation models that use various provinces are examined in this hazard
assessment.
An additional alternative seismic source is defined as the entire study region
shown on Figure 7-5. The basis for this zone is the common underlying
Northern Appalachian regional geology. An assumption used in delineation of
this seismic zone, or any other seismic zone, is that earthquake activity is a
7-17
random process, therefore all points within the zone have an equal likelihood
of experiencing earthquakes of any size. Under this assumption, the clusters
of historical seismicity evident on Figure 7-5, can be the result of a limited
time window of about 200 to 300 years for which earthquake data are
available. Given sufficient time of several hundred to several thousand years
a random seismic pattern would emerge throughout the site region. In the
meantime, the next major event could occur at any point within, or outside,
the observed clusters of historical seismic activity. Neither this assumption
of spatial randomness, nor the prior one of spatial stationarity can be
currently validated, thus seismic hazard is computed for the DISTF site using
these alternative assumptions.
7.5.1 Regional Tectonic Provinces - Seismic Zonation Model 1
The clusters of historical and recent seismic activity in the Northeast have
been interpreted into an association with several regional seismotectonic
provinces (Weston Geophysical, 1986; EPRI NP-4726, Vol. 5). These provinces
include the following. An illustration of the provinces is shown in
Appendix K.
Zone 013 - White Mountains Intrusives Domain
Zone 014 - Maine-New Brunswick Zone
Zone 015 - Avalon Terrane Seismic Zone
Zone 016 - Southeast Mew England Platform
Zone 017 - Northeast Massachusetts Thrust Fault Complex
Zone 021 - Nexus of Intersecting Structural Features
Zone 039 - Marragansett Basin
Bases for definition of these tectonic provinces are given in Appendix K.
7.5.2 Alternative Zonation - Model 2
An alternative seismic source zonation to the series of seismotectonic
provinces listed in the subsection 7.5.1 is a broad region around the site
illustrated previously on Figure 7-5. As stated earlier the basis for this
7-18
alternative zonation is the assumption that future activity will occur as a
random process, spatially, but with a frequency consistent with that observed
in the historical and recent earthquake record for the region. This
assumption therefore opposes the previous assumption that underlies the
formulation of the regional seismotectonic province, i.e., that future
activity will most likely be constrained, spatially and with regard to
frequency, to the historically more seismically active regions.
7.5.3 Alternative Zonation - Model 3
Seismic source zones for Model 3 accommodate proposed late northwest-trending
and transform faults perpendicular to the Atlantic spreading ridge axis that
originated during early Jurrasic-Cretaceous sea floor spreading. Onshore
extrapolation of these oceanic fracture zones, utilizing geophysical evidence
of faulting in the offshore Triassic-Jurassic rift basins and offsets of the
east coast magnetic anomaly, coincides with local onshore evidence of late
brittle deformation and intrusive activity. This particular alignment of
source zones extends the offshore evidence of structures, southeast of Cape
Cod, north northwestward past Cape Ann into the area of Mesozoic plutons in
southeastern New Hampshire.
Three subdivisions (White Mountains, NE Mass, Southeast New England) reflect
distinct clusters of seismic activity which may result from the complex
intersection of northwest-trending structures with the northeast-trending
lithotectonic belts or blocks. Areas of increased seismic activity have been
suggested to occur where brittle faults are impeded or deflected by crustal
inhomogeneities such as intrusive bodies or crustal blocks such as the Nashoba
Belt.
7.5.4 Alternative Zonation - Model 4
Seismic source zones developed for Model 4 are primarily drawn to enclose
clusters of historical seismic activity. The outlines correspond locally to
mappable structures and lithologic boundaries, but cross others. This
suggests that presently unrecognized, or poorly constrained structures, likely
7-19
at depth within the crust, are primarily responsible for seismicity, or
interact in a complex manner with mappable Appalachian lithotectonic trends.
The White Mountain arcuate zone of seismicity, in southeastern Mew Hampshire
and eastern Massachusetts, is bounded by the axis of the Central New Hampshire
Anticlinorium on the north and west. To the south, the southwestward curve of
the zone transects the northwest-trending arc of the Nashoba Thrust Belt and
encompasses the Boston Basin. The interaction of the early north to
northeast-trending Appalachian lithotectonic structures with Mesozoic brittle
faults and intrusives may control this pattern of seismic activity.
7.6 Earthquake Recurrence Frequency
7.6.1 Frequency vs Magnitude Model
The annual frequency of earthquake activity in all seismic source zones is
derived using the Gutenberg and Richter (1944) empirical magnitude-frequency
relationship. Derivation of the frequency of earthquake activity in seismic
sources is thoroughly discussed in Appendix K. Results of all earthquake
statistical analyses are provided in this appendix.
7.6.2 Maximum Magnitude Estimates
Estimation of the maximum magnitude that could potentially occur at any given
point in an intraplate tectonic environment such as in the Northeastern U. S.
presents one of the more perplexing problems in the fields of seismology and
earthquake engineering. Review of maximum activity reported in similar
tectonic environments throughout the world would suggest the potential for
maximum earthquakes to attain magnitudes in the range of 6.0 and 7.0 mi . For
the purpose of establishing seismic ground motion hazard levels for the site
of the DISTF, maximum magnitudes for all seismotectonic sources were estimated
to be in the range of 6 1/4 to 6 1/2 m.. These maximum magnitudes reflect
earthquake sizes 1/2 magnitude unit, or greater, than the largest known
historical events catalogued over the past 200 to 300 years in the study area.
7-20
7.7 Regional Ground Motion Attenuation
7.7.1 Horizontal Earthquake Motions at Top of Bedrock Surfaces
Design of proposed treatment facilities and related outfall and inter-island
conveyance tunnels requires specification of design response spectra at
bedrock surfaces. To provide seismic design response spectra at bedrock
surfaces, recently published attenuation models were reviewed to determine
their applicability to these seismicity analysis for the MWRA Deer Island
Project. These publications include: Boore and Atkinson (1987) and Tore and
McGuire (1987). Both of the referenced studies employ a stochastic
methodology for predicting response spectra (for S% of critical damping) at
bedrock surfaces for eastern North American earthquakes. In brief, the
stochastic method provides predictions of earthquake ground motions at bedrock
surfaces by assuming that ground motions can be modeled as bandlimited,
finite-duration white Gaussian noise. Random vibration theory is applied to
derive expected values of ground motion amplitudes. The technique includes
parameters to define the basic spectral composition of seismic energy
emanating from the earthquake focus as well as parameters to define energy
loss (attenuation) due to geometrical spreading and frequency-dependent
absorption due to passage of elastic waves through the earth's crust.
Expanded discussions on this random-vibration technique for prediction of
earthquake motions are provided in the referenced publications.
The two referenced attenuation models were used as a guide for selection of a
model to be used in the seismic hazard computations for the DISTF structures
founded on bedrock. Parameters of the selected bedrock attenuation model are
given in Appendix K.
7.7.2 Horizontal Ground Surface Motions - Firm Foundation Conditions
Additional requirements of the probabilistic seismic hazard methodology are
descriptions of attenuation of earthquake-induced strong ground motion to be
applied for soil foundations. These attenuation functions should reasonably
predict peak ground motion amplitudes specific to the site region and to
7-21
account for varying local foundation conditions at the sites being studied.
One set of models chosen for this study was developed by Hasegawa et. al .
(1981). The Hasegawa (I98I) studies resulted in definition of attenuation
models for the peak ground acceleration (PGA) and peak ground velocity (PGV)
parameters. These relationships have been used to produce seismic risk maps
for eastern Canada and their predicted ground motions compare favorably with
several recent instrumental measurements of EUS earthquakes. Due to their
mode of development, these models are best applied to predict ground motions
at sites underlain by firm foundation materials, such as rock covered with a
veneer of dense till.
7.8 Probabilistic Seismic Hazard Results
Due to the large area covered by the DISTF and its proximity to a region of
significant historical seismic activity, seismic hazard was computed at
several points around the perimeter of the project and at the center of key
project sites. Following are coordinates employed in the hazard computations.
Site No.
1 2 3 4 5 6
Latitude (M)
42.35 42.375 42.40 42.333 42.372 42.27
Long (W) .
70.954 70.90 70.833 70.77 70.77 70.95
Locale
Deer Island Outfall SW Outfall Center Outfall NE Corner Outfall SE Corner Nut Island
7.8.1 Seismic Hazard Results - Bedrock Surfaces
The seismic hazard methodology previously discussed, including the various
geologic/seismotectonic models, earthquake recurrence frequencies, and
spectral attenuation models for rock surface was applied to the 6 sites
identified above. The resulting probabilistic seismic hazard results at these
sites are discussed in this sub-section. Recall, probabilistic seismic hazard
provides an estimate, based on various geological and seismological input, of
the frequency at which a range of earthquake-induced ground motion amplitudes
will be exceeded at the construction points of interest. Hazard results are
7-22
used primarily as a justification for recommending seismic design criteria for
critical facilities if an engineering decision has been made to implement
seismic designs capable of withstanding remote earthquake events characterized
by particular return periods. In the present case, the engineering decision
has been made to characterize the Operating Design Earthquake (ODE) with a
mean return period of several hundred years, and the Maximum Design Earthquake
(MDE) with a mean return period of several thousand years. The bedrock
surface seismic design response spectra will therefore be recommended based on
the following seismic hazard results computed for sound bedrock foundation
conditions. Probabilistic seismic hazard results at the 6 selected project
sites for seismic zonation Model 1 are shown on Figure 7-6. These results are
probabilistically-derived bedrock response spectra (for % of critical
damping) corresponding to annual exceedance probabilities of 10"^, 10"-^,
10"^, or equivalently with earthquake return periods of 100, 1000, and 10,000
years, respectively. Results for all sites are similar for short return
periods of about 100 years implying comparable exposure at these sites to
lower amplitude ground motions generated by small local events or by more
distant moderate events. Results, however, differ somewhat for higher
amplitude, more remote ground motions. For example, probabilistic spectral
amplitudes (at 10 annual exceedance frequency) are 25% greater at the ME
corner of the project relative to the Mut Island site. This effect results
from differences in proximity of these sites to the zone offshore of
Northeastern Massachusetts that has produced significant historical earthquake
activity.
7.9 Seismic Design Recommendations
Due to the scope of the DISTF project, structures will have their foundations
on earth materials ranging from sound bedrock to dense tills and soft fills
and marine sediments. Seismic design response spectra are supplied for the
various geologic conditions present at project construction sites.
7-23
i 4 - F t 5 - f i 9 I5 - -J7- -25
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r^<i^
-1 0 10 10
P e r i o d ( s e c o n d ) 1 I A 2 0 1 - D l 11.^2001 . D l 1 1 A 2 0 0 0 ) . 0 1 Ml.^201 .SW 1 H 2 0 0 1 .SW 1 1 ^ 2 0 0 0 1 . S W 1I .A201 -CN 1 1 A 2 0 0 I . C N 11 A20001 .CN 1 1 A 2 0 1 . N E 1 1 A 2 0 0 1 . N E 1 1 A 2 0 0 0 I . N E 1 1 A 2 0 1 - S E 11.A2001 .SE H A 2 0 0 0 1 .SE 1 1 A 2 0 1 . N l 1 1 A 2 0 0 1 . N l 1 I A 2 0 0 0 1 . N l
Seismic Design Recommendation for the
Mass. Water Resources Authority Deer Island Secondary Treatment Facility
for Metcalf & Eddy, Inc.
Probabilistic Rock Surface Spectra Seismicity Model 1
Weston Geophysical Fig. 7.6
5/89
7.9.1 Bedrock Design Response Spectra for the Operating Design Earthquake
(ODE) and the Maximum Design Earthquake (MDE)
As previously introduced, an engineering decision was made at the conceptual
design phase of this MWRA project to implement appropriately conservative
earthquake resistant designs for all elements of the DISTF facilities. To
this end, the Operating Design Earthquake (ODE) was to be characterized by a
mean return period on the order of several hundred years, and the Maximum
Design Earthquake (MDE) by a mean return period on the order of several
thousand years.
The probabilistic seismic hazard assessment described in previous sections,
and the results of which are summarized on Figure 7-6, provides the basis for
recommending design response spectra that are consistent with the mean return
period characterizations adopted for the ODE and MDE. Recommended bedrock
surface design response spectra for the ODE and MDE are shown on Figure 7-7 in
comparison to average probabilistic spectra (represented by seismicity Model 1
results) at annual exceedance probabilities of 10" , 10"- , and 10"^. At low
to intermediate frequencies (e.g., 1 hz to 10 hz) the MDE response spectrum
has been constructed to provide the desired mean recurrence interval of
several thousand years (actual range is 2000 to 4000 years in the frequency
range of 1 to 10 hz). At higher frequencies, (e.g. 10 hz - 50 hz), the MDE
spectrum lies near the 10"^ probabilistic spectrum, thus is characterized by a
lower mean earthquake return period of about 1000 years. 'It is commonly
recognized that, although high acceleration motions are likely to occur at
high frequencies (greater than 10 hz) particularly at rock sites for eastern
US earthquakes, these high-frequency, high-acceleration ground motions tend
not to produce structural damages. This relative unimportance of the
high-frequency motions in contributing to structural damages is supported by
various observations of recent earthquakes.
Further, this relative unimportance of the high frequency motions is
interpreted to result from the fact that the high frequency components of the
ground motion have a relatively short duration and produce extremely small
7-25
-o c o o V
CO \ n
u c
S>5 LO
o o
«> • > I o
-o 3 u (0
a.
-2 10
H1A201.NE h.lA2001 .NE
ODE.050
-1 10
0 10 10
Period (second)
Recommended Seismic Design Spectra (or the Operating Design Earthquake and ihe Maximum Design E a r t h q u a k e for applicalion at Bedrock SurTaces
Seismic Design Recommendation for the
Mass. Water Resources Authority Deer Island Secondary Treatment Facility
for Metcalf & Eddy. Inc.
Recommended Seismic Design Response Spectra
for Bedrock Surfaces
Weston Geophysical Fig. 7.7
5/89
particle displacements. Given this background, the slightly less conservative
return period of about 1000 years was employed to construct the high frequency
portion of the MDE response spectrum. It is noted that high-frequency,
high-acceleration motions can have engineering impacts on certain equipment,
such as electrical relay devices.
Typical engineering practice has previously been to establish the Operating
Design Earthquake as producing one-half of the seismic loading of the MDE.
This approach was applied and the resulting spectrum was checked to determine
if the desired return period characterization was achieved by this standard
procedure. The resulting ODE spectrum is modestly more conservative at lower
frequencies ( 1 - 1 0 hz) than the specified return period goal of several
hundred years. The actual return period for the ODE is near 600 to 700 years
(2 hz to 5 hz frequency band). At higher frequencies the recommended ODE has
appropriate mean return periods in the range of 100 to several hundred
years. Parameters of the recommended Bedrock surface response spectra {5%
damping) for the MDE and ODE are discussed further in Appendix K.
Seismic design criteria including response spectra and acceleration time
histories are provided in a format to enable dynamic analyses of structures to
be built at the DISTF. In addition, for the case that simplified analyses,
such as pseudo-static methods, are deemed to be appropriate for determining
seismically-induced forces to be used in engineering design, seismic criteria
are provided in terms peak ground accelerations and peak ground velocities.
There seismic design criteria, whose development are completely discussed in
Appendix K and provided below.
7-27
Design Ground Motion Values Bedrock Motions
I
Case
MDE
ODE
Structure Natural Frequency
(hz.)
greater than 20 10-20
less than 10
greater than 20 10-20
less than 10
Peak Ground Accelerations
(g.)
0.25 0.18 0.14
0.125 0.09 0.07
Peak Ground Velocity (in/sec)
3.9 3.9 3.9
2.0 2.0 2.0
7.9.2 Design Response Spectra for Soil Sites
In addition to structures being founded in, or on bedrock, such as the outfall
tunnels, the DISTF will include structures to be built on soil foundations
present at the Deer and Nut Island sites. Based on recent geologic studies
including geophysical surveys and test borings, soil and underlying rock
profiles have been developed for island site locations. These site profiles
are shown on Figures 7-8 and 7-9 for the Deer Island and Mut Island sites,
respectively. Seismic design response spectra for soil sites were developed
using an approach that included probabilistic hazard computations employing
attenuation models suited for predicting ground motions at firm soil sites
(see Appendix K), as well as formal computations of earthquake responses of
the actual soil columns that exist at the Deer and Nut Island sites. In
addition, recently published (EPRI, Risk Engineering, 1989) soil response
amplification factors developed for a range of soil thicknesses and
compositions were factored into the final recommendation of design response
spectra to be used at soil foundations.
Figure 7-10 illustrates a summary of the probabilistic analyses applicable for
generalized firm soil conditions. These probabilistic hazard results
illustrate soil ground motions at annual exceedance probabilities of 10 ,
10"-^, and 10" . Compared to these probabilistic hazard results are formal
computations of earthquake responses of the geologic columns present at Deer
and Mut Islands. Dotted curves on Figure 7-10 represent computations of soil
7-28
North
A
CM
o
200 -1
^ 150 -
to Ol
a
so «
rt I
u. CO
<D
rt X ffCEtev. 139")
g 100-1
I I i i
50 -
94 Vp = 7000 ft/sec Vs = 2500-2800 ft/sec
133
130
TILL
c
South
A'
Vp = 2000-2500 ft/sec
Fill
?4" 32.5'
Sandy Clay & Silty Sand
Vp = 4600-5000 ft/sec
Proposed Secondary Clarifiers Base Slab Elevation 116' SEALEVEL 105.65 —
Clay
82.5' _ Vp = 6500-7000 ft/sec
• 7 7 ^
'Gray Argillite Bedrock?
-50 - "
LDE-51 Boring Location
Line 3 Seismic Refraction Crossline
37 Standard Penetration Test Value
- 7 ^ ^
BEDROCKC
• " 7"''/\*V—
Vp = 16,000± ft/sec Vs = 9000 ft/sec
TILL
Vp = 6000-6500 ft/sec Vs = 1800 ft/sec
8 37
69 80 Vp = 7000 ft/sec Vs = 2500-2800 ft/sec
I I L 100 200 FT.
J Seismic Design Recommendation
for the Mass. Water Resources Authority
Deer Island Secondary Treatment Facility for
Metcalf & Eddy, Inc.
Geologic Cross-section Deer Island
Weston Geophysical Fig. 7.8
5/89
BORING L D E - 5 8
TIDE INDUCED FLUCTUATION
130-
1 2 0 -
1 1 0 -
1 0 0 -
lu 90-
!< a
O 80
2 O
I UJ 70 • LU
60
5 0 -
4 0 -
30 - •
GRAVEL - SAND MIXTURE
FILL
Base Siib~EievatJon~
SiLTY CLAY
CLAYEY SAND W/ SANDY CLAY
CLAYEY GRAVEL
GRAVELLY CLAYEY TILL
TOP OF ROCK
SPT BLOWS SEISIV1IC VELOCITIES
10
15
32
3 9
150+
6 7
102
129
85
Vp = 5500 f t /sec
Vs = 750-900 f t /sec
Vp = 6000-6500 f t /sec
Vg = 1800 f t /sec
Vp = 7000 f t /sec
Vg = 2500 f t /sec
Vp = 16,000-17,000 ft /sec
Vg = 9000 f t /sec
SOURCE: HAGER-RICHTER GEOSCIENCE BORING LOG FOR LDE-58
1 Seismic Design Recommendation for the
Mass. Water Resources Authority Deer Island Secondary Treatment Facility
for 1 Metcalf & Eddy, Inc.
Representative Soil Profile
Nut Island
Weston Geophysical Fig. 7.9
5 / 8 9
^o* . 0 *
I2-APR-89 2 3 - 0 2 J 7
C o u
CO \
in V
JZ u c
u o
u > I o
3
10
10
-2 10 10
P e r i o d ( s e c o n d ) HASOI.NE HASOOl.NE HASOOOI.NE .MDESOILE-OSO MDESOiLL.050 SHKNUrV.050 SHKDR60V-050 SHKORlOOV.050 SHK0RCLYV.O50
P r o b a b i l i s t i c R e s p o n s e S p e c t r a ( o r F i r m ( s o i l ) s i t e s i v s -c o m p u t e d s o i l c o l u m n a m p l i r i c a i i o n s
10
Seismic Design Recommendation for the
Mass. Water Resources Authority Deer Island Secondary Treatment Facility
for Metcalf & Eddy, Inc.
P r o b a b i l i s t i c S p e c t r a vs .
C o m p u t e d So i l A m p i i f i c a t i o n s
Weston Geophysical Fig. 7.10
5/89
amplifications using a standard methodology ('SHAKE' program). In brief,
excitation of soil layers at the island sites due to occurrence of the MDE
bedrock motion was calculated using soil and rock properties determined during
the site investigations. Solid lines represent published soil column
amplifications for till-like strata of thickness ranging from 30' to 80'
(EPRI, Risk Engineering, 1989). The dashed curve represents the soil
amplification computed for the area at Deer Island underlain by a sequence of
clays, e.g., in the area of boring 376 (Figure 7-8). (See Appendix K for
further details on the soil amplification studies performed to define response
spectra to be applied at soil sites.)
Earthquake response studies of soils present at the island sites indicate the
potential for most significant amplifications of earthquake ground motion at
periods in the range of 0.20 to 0.30 seconds for areas underlain by tills, and
about 0.70 for the area underlain by clay. Due to the high frequency
composition of the MDE bedrock accelerations, supported by several recent
recordings of Eastern U. S. earthquakes, and properties of soil overburden
materials, soil amplifications are also predicted at higher frequencies of 10
to 25 hz. Recommended design response spectra for the MDE and ODE to be
applied at soil surfaces take into consideration the soil amplifications
determined by the earthquake response analyses. In addition, the recommended
response spectra observe the intended seismic safety criteria that the MDE and
ODE be associated with remote earthquakes characterized by mean return periods
of several thousand and several hundred years, respectively. Recommended
horizontal component response spectra for the MDE and ODE to be applied at
soil sites are illustrated on Figure 7-11. Parameters of the design response
spectra are provided in Appendix K. In addition, acceleration time histories
corresponding to response spectra at the bedrock surface and at the top of
ground surfaces (at slab elevations) at Deer and Nut Islands are provided in
Appendix K.
As for the case of structures being founded on bedrock, seismic design
criteria are provided in a format that can be applied in dynamic structural
analyses. In lieu of performing dynamic analyses using the recommended time
histories and response spectra, alternative seismic design criteria are
7-32
rP*
10 i3-APR-89 01'04.J8
C o u V CO \
in V
o c
u o
I o
3 U (0 CL.
r o-
- 0'
- 0'
P e r i o d ( s e c o n d ) HASOI.NE HASOOl NE HASOOOI.NE MDES01L.050 ODESOIL.OSO
R e c o m m e n d e d R e s p o n s e S p e c t r a f o r t h e M a x i m u m D e s i g n E a r t h q u a k e a n d O p e r a l i n g D e s i g n E a r t h q u a k e t o b e a p p l i e d a t s o i l g r o u n d s u r T a c e s
I
Seismic Design Recommendation for the
Mass. Water Resources Authority Deer Island Secondary Treatment Facility
for Metcalf & Eddy, Inc.
MDE and ODE Response S p e c t r a fo r So i l S i tes
Weston Geophysical Fig. 7.11
5/89
provided which can be applied in simplified engineering analyses that require
as input peak ground accelerations and peak ground velocities. These
parameters, developed in Appendix K, are summarized below.
Case
MDE
ODE
Structure Natural Frequency
(hz.)
greater than 20 15-20 10-15
less than 10
greater than 20 15-20 10-15
less than 10
Peak Ground Acceleration
(g.)
0.35 0.32 0.28 0.25
0.175 0.16 0.14 0.125
Peak Ground Velocity (in/sec)
5.1 5.1 5.1 5.1
2.5 2.5 2.5 2.5
7-34
I I 8.0 ENGINEERING RECOMMENDATIONS
8.1 Tunnels
8.1.1 Alignment
8.1.1.1 Outfall Tunnel
The alignment of the outfall tunnel is mainly determined by the location of
the outfall shaft and the diffusers. Nevertheless some lateral variation in
horizontal alignment is possible and may be justified to avoid areas of
potential geological hazard.
The geophysical survey carried out along the outfall tunnel alignment
indicated that a zone of low velocity in the bedrock existed in the middle of
the investigation area, which is believed to be related to either increased
fracturing or alteration o f the bedrock or possibly both. Thus an alignment
in the north or south of the area of investigation would appear to be
advisable.
The presence of a large sill or probably sills in the vicinity of "The Graves"
will mean that an alignment in the south of the area will require the tunnel
to pass through the sills at an oblique angle. However these sills are also
orientated such that they will probably pass through the western edge of the
diffuser area and therefore on most, if not all, alignments they can be
expected to be encountered. Nevertheless, on northerly alignments it would be
possible to alter the direction of the tunnel in the vicinity of the sills so
that they cross them at a more perpendicular angle thus reducing the length of
tunnel in the stronger rock.
On this basis it would appear that northerly alignments are preferable.
However, uncertainty exists over the exact boundaries of the Nahant Gabbro
complex and whether igneous rocks extend to the south of Mahant, although the
magnetic anomaly map indicates that it is unlikely that the complex extends
far beyond its surface outcrop on Nahant. More importantly, the top of
8-1
bedrock is substantially higher along the southern margin of the area of
investigation and the greater rock cover should be beneficial to the tunneling
operations. It is therefore recommended that the alignment be located along
this southern margin and that the 1989 investigations be concentrated along
this preferred alignment.
Considering now the vertical alignment of the tunnel, the top of bedrock is
associated with a zone of increased fracturing up to 50 feet deep, which will
result in increased permeability and reduced ground stability. It would be
preferable to locate the alignment below this zone with at least one diameter
leeway. Thus the tunnel should lie a minimum of 80 feet below top of
bedrock. The greatest depth of top of rock along the alignment occurs at
approximately Chainage 90+00 (Figure 5-3) at an elevation of -110 feet MDC
elev. Thus the tunnel should be deeper than -190 feet MDC elev. at this
point.
8.1.1.2 Inter-Island Tunnel
The selection of the horizontal alignment for the inter-island tunnel is
mostly influenced by the required locations of the two terminal shafts.
Freedom does exist, however, to make adjustments to the alignment in order to
address geological constraints. Since the site investigation is at an early
stage with limited existing geophysical information and only four completed
boreholes, a straight line alignment between the two shafts is presently
recommended. The distance between the two shafts is 24,937 feet.
Recent information from a seismic reflection and refraction survey performed
in February 1989 has confirmed the existence of a deep depression in the
surface of the rock between Mut Island and Rainsford Island due west of
Peddocks Island. The exact extent and depth of this depression will be
defined in the 1989 geophysical and marine drilling campaigns and the
horizontal alignment of the tunnel may be modified in detailed design to skirt
the hollow and maintain adequate rock cover to the tunnel crown. Early
indications suggest that this could be accomplished by deviating the tunnel
approximately 1,500 feet to the west using horizontal curves 1,000 feet radius
and adding some 300 feet in length to the tunnel.
8-2
Further amendments may be made to the horizontal alignment in design to avoid
any other unfavorable geology on the proposed alignment revealed by the
upcoming 1989 geophysical and marine drilling campaigns. Such geology could
comprise zones of severe faulting, substantial diabase dikes or kaolinized
zones.
With regard to the vertical alignment, two criteria are considered to
govern: the provision of a nominal slope in the tunnel away from the
excavation face and the recommended maintenance of approximately 70 feet of
rock cover above the crown of the tunnel.
The 70 feet of rock cover is recommended as offering a safe margin bearing in
mind the limited geotechnical investigation available at this time. This
amount of cover is probably tending to the conservative side. However, this
is attainable with only the relatively low incremental cost increase of the
deeper shafts which is considered less significant than the reduction of risk
during excavation of a submarine tunnel through variable and inevitably
difficult to predict rock conditions.
8.1.2 Ground Conditions
The proportions of the various rock types encountered in the 1988 marine
investigation along the outfall tunnel alignment are shown in Figure 5-1B.
This indicates that a total of 88f. of the rock was argillite of which 5.5/5 was
altered to a greater or lesser degree. However, the majority of this altered
argillite occurred in two boreholes, 88-2 and 88-5, both of which lie some way
to the north of the preferred alignment. If these two boreholes are omitted
for the analysis, the percentages of the different rock types becomes:
Rock Type
Argillite Sandy argillite Altered argillite Diabase Intrusives Tuff
Percentage
76.2 12.1
1.3 5.9 1.0 3.5
8-3
At present this is considered to be the best estimate of the proportions of
the individual rock types to be encountered along the outfall tunnel, and is
also believed to be a reasonable representation of rock types along the
presently planned inter-island tunnel alignment. The results of the much more
comprehensive 1989 marine drilling will give a much better picture of the
inter-island tunnel geology.
The other major geological influences relate faulting and intrusions. Two
sills are present in the Outer Harbor (Kaye, unpublished) and both of these
may intersect the outfall tunnel alignment. Assuming a dip of the sills of
30° and an angle of 30° between the strike of the sill and the tunnel
alignment, and using a best estimate thickness of I8O feet and 300 feet they
will be encountered in the tunnel for distances of 720 feet and 1200 feet
respectively. This would form 47. of the total outfall tunnel length.
Major east to northeast trending faults are thought to occur in the
vicinity. If, as has been suggested previously, the Brewster Fault is
associated with the low velocity zone (Figure 5-2) and this phenomenon is
brought about by the presence of alteration, then altered rock may be
anticipated in both the Outfall and inter-island tunnels. The thickness o f
this zone is not known at present and may well not be known until excavation
begins, although the width of the low velocity zone near Deer Island is much
narrower than elsewhere, and no low velocity zone associated with the Squantum
Fault was observed during the limited investigation for the inter-island
tunnel. Other major faults exist cutting across the inter-island tunnel
alignment (Figure 5-2). The faults and fracture zones encountered in the
boreholes were generally narrow and frequently recemented (Table 5-3). Water
inflows would nevertheless be expected to be higher in the vicinity of these
faults as was encountered in the Maiden Tunnel adjacent to the Northern Border
fault. It is conceivable that these other major faults which c r o s s the inter
island tunnel alignment may also be associated with alteration.
There is some uncertainty regarding the extent of water inflows associated
with the igneous intrusions. On the present evidence it is considered that
8-4
the igneous material will not prove to have a significantly different
permeability to the general mass of the argillite. However because of the
folded nature of the Boston basin rocks and the probability that this folding
occurred after the igneous rocks were intruded, it is possible that shearing
could have occurred between the intrusions and the country rock (generally
argillite) resulting in more permeable contact zones. This is most likely to
be true of the two major sills in the Outer Harbor and further investigation
is recommended during the I989 marine investigation to provide more
information on this point.
8.1.3 Excavation Method
Three methods of excavation have been identified for construction of the
tunnels; Tunnel Boring Machine (TBM), Point-attack boom machines, and "drill
and blast" techniques. Each of these forms of excavation have their
advantages and disadvantages in terms of speed, safety, suitability to the
ground conditions and flexibility to changes in those ground conditions.
8.1.3.1 Tunnel Boring Machine
In suitable conditions Tunnel Boring Machines provide the fastest method of
excavation available. Their main advantages are:
• That they can excavate rapidly to the final profile of the tunnel, restricting overbreak and disturbance of the surrounding rock,
They can be provided with automated tunnel lining installation equipment which allows excavation to proceed almost continuously,
• They require a smaller number of people to operate reducing costs and increasing safety.
Tunnel Boring Machines however have a number of disadvantages. They are
expensive and therefore require long drives of a constant cross-section to be
economic. They are designed for particular ground conditions and should the
actual ground conditions be different they can easily prove to be unsuitable
and uneconomic to operate. It is not possible to provide ground support
8-5
immediately after excavation so that the rock must be initially stable after
excavation. Should the ground conditions become unsuited, it is very
difficult to advance the tunnel in headings without disassembling the machine;
and should ground treatment be required ahead of the face, the presence of the
machine restricts access.
8.1.3.2 Point Attack, Boom Type Machines
Machines of this type, often known as Partial Face machines, or Roadheaders,
have a small cutterhead on the end of a boom, and an operator moves the head
across the face, removing rock from the point of contact. The boom can either
be mounted on its own self-propeiied chassis, or can be installed within a
tunnel shield.
The advantages are as follows:
• The same machine can be used to excavate a range of cross-sections and excavation shapes
The machines are usually available "off the shelf" and long lead times are avoided.
• Access can be made to the face to provide support or undertake grouting.
The machine can be withdrawn or replaced if conditions change and drill and blast excavation can be used.
The disadvantages, when compared with a full face TBM, include;
• The instantaneous rate of progress is much lower than a TBM, and depends more on rock fracture patterns and on the skill o f the operator.
• They are not generally economic in rocks with strengths above about 7,000 psi, depending on the discontinuities. .
• Chassis versions rely mainly on deadweight for reaction, and the machines tend to have difficulty with irregular rock strengths across the face. They are less suited to circular tunnels and working conditions in the invert tend to become difficult in wet conditions.
8-6
• Because the working parts are more highly stressed than TBMs, the breakdown rate tends to be greater.
The slower rate of advance, despite the shorter lead time, probably precludes
the use of this type of machine from the long length of single face drive on
the outfall tunnel.
8.1.3.3 Drill and Blast Techniques
"Drill and blast" has been the most commonly used method of excavation for the
tunnels in the Boston area and is known to be effective in the prevailing
conditions. The method has great flexibility and can be used in virtually all
rock conditions. The main disadvantages are the slow rate of advance due to
the sequential process of drill, charge, blast, muck and support; the damage
that can be caused to the surrounding rock by the blasting process; and the
requirements for large numbers of specialist personnel.
8.1.3.4 Preferred Options
In view of the length of the outfall tunnel, the time restraints and the
prevailing rock strengths a TBM is considered the most appropriate choice.
None of the boreholes along the preferred alignment gave any indication of
substantial sections of ground which would cause difficulty to a TBM. Short
sections of altered argillite or fault zones may be encountered in which the
TBM could experience problems with slip of its gripper pads in the weak
material, but this is expected to be minimal. Localized saline water inflows
of at least 10 gpm/ft of tunnel are also anticipated, and the TBM should be
designed to cope with flows of this volume.
Approximately 30^ of the unconfined compressive strength tests on unaltered
argillite gave values in excess of 20,000 psi and less than Q% gave strength
greater than 25,000 psi. Three results were higher than 30,000 psi. For the
diabase two results exceeded 25,000 psi, but only one was greater than
30,000 psi (Appendix C). It is therefore likely that less than 5% of the
8-7
tunnel length will be in rock of sufficient strength to significantly reduce
the rate of penetration.
Further information on the use of a TBM on the outfall tunnel drive is
contained in the Conceptual Design Report for DP6. It is believed that
average rates of advance of the order of 90 to 95 ft/day should be able to be
maintained.
In the diffuser area the tunnel will be required to progressively reduce in
cross-sectional area to maintain equal flow to each diffuser riser. It is
estimated that at approximately 3000 ft along the diffuser tunnel it is no
longer economic to backfill the tunnel to obtain the reduced cross-sectional
area, but is preferable to reduce the cross-sectional area being excavated.
It will therefore be necessary to remove the TBM at this point and continue
with some other form of excavation. It appears from the boreholes that in
this particular area, the RQDs are lower than average, and this increased
degree of fracturing may favor the use of a point attack machine for the final
3300 ft. Drill and blast techniques could also be used, or the contractor may
even consider installing a smaller diameter TBM.
The TBM erection chamber and marshalling tunnel at the base of the shaft are
probably best suited to drill and blast excavation, although point attack
machines could be used.
From the limited investigation data available at present it appears that the
ground conditions along the inter-island tunnel alignment are also best suited
to TBM excavation. Some uncertainty exists over the higher permeability
results which have been measured along this alignment, as well as the three
major faults which may be intersected. It is conceivable that in these
locations the combination of high groundwater i n f l o w s and poor ground
conditions may necessitate the installation of significant amounts of support
close to or at the face. Forward grouting could improve these conditions.
Point attack machines, or drill and blast methods could also be used on this
alignment and the possibility of excavation from two faces or four (if an
8-8
intermediate shaft on Long Island is used) could result in an advance rate
comparable to that of a single TBM. Nevertheless, difficulties with the
disposal of muck, the cost of an extra shaft, and the cost of additional labor
would favor the use of a TBM. The method of excavation will require further
consideration once the proposed 1989 investigations have been undertaken and
more information is available.
8.1.^ Support
8.1.4.1 Pr imary Support
Because of the blocky nature of the Cambridge Argillite, significant lengths
of both tunnels will require some degree of primary support, although in the
majority of cases this will be required to support localized unstable
individual or multiple blocks in the roof and sidewalls of the tunnels rather
than to provide a confining pressure to potentially overstressed sections of
ground. The degree of support depends to a large extent on the nature of the
discontinuities, in particular their orientation, aperture, infilling,
frequency and persistence, but, in general the degree of support will increase
as the RQD reduces.
Primary support should be installed close to the face to restrict the movement
of the ground so that excessive loosening of the rock mass does not occur, but
at the same time, the strength of the ground is allowed to become fully
mobilized. It is thus important that the TBM should incorporate provision for
support installation close to the face and so that large sections of ground
are not left unsupported. The use of a "comb shield" extending back from the
cutter-head housing is advised to allow for installation of rock bolts before
the support provided by the TBM is completely removed.
In most ground conditions, rock bolting is the preferred primary support
method incorporating mesh where necessary and with steel channeling spanning
between the rock bolts in the poorer ground conditions. The bolts should
ideally be installed within the "comb shield" of the TBM and therefore will
not need to be tensioned. The additional ground movements which result as the
8-9
support provided by the face is progressively removed should be adequate to
induce tension within the bolts. However, if these bolts are installed
further than 2 diameters behind the face a nominal tension should be
applied. The mesh should also be erected within the "shield" of the TBM and
as the TBM advances and the bolts come clear of the "shield", the nuts on the
rock bolts should then be tightened to bring the mesh into contact with the
tunnel wall. It is recommended that bolts should be the primary support
method whenever possible. In sections of completely altered argillite
alternative support may be required if the rock is too weak for the bolts to
obtain a reliable anchorage.
With TBM excavation overbreak should in general be minimal. Steel sets may
have to be installed in the poorest ground conditions, where overbreak is also
likely. If steel sets are used, it will be necessary to ensure that they are
firmly blocked against the tunnel walls using closely spaced wedges, or
preferably a continuous mortar pack.
To provide support between the steel sets, lumber lagging boards are
frequently used. Lagging suffers from the disadvantage that it cannot be
placed in close contact with the rock and therefore allows some ground
relaxation to occur. It is recommended that mesh should be used in the
majority of cases and that this should be installed outside the steel sets
with short "pins" used where n e c e s s a r y in between the sets to tie the mesh to
the wall.
The primary support required in each of the rock units defined in Table 6-5
are listed as follows:
8-10
SUPPORT REQUIREMENT
Rock Unit
1. Localized rock dowels if installed near face or tensioned bolts if installed more than two diameters behind face. Slow advance by TBM.
2. Localized rock dowels if installed near face or tensioned bolts if installed more than two diameters behind face. Ideal tunneling case.
3. Patterned untensioned rock dowels required with spacing of 5 to 6 feet in crown and locally in side walls.
4. Patterned untensioned rock dowels required with spacing of 3 feet in crown and sidewalls. Use of weld mesh advisable.
5. Patterned tensioned rock bolts on 3 feet spacing with weldmesh in crown and sidewalls and preferably with use of shotcrete. Alternatively steel arch ribs with lagging.
6. Steel arch ribs with lagging or weldmesh with later application of shotcrete. Alternatively, except in areas of completely altered argillite, patterned tensioned rock bolts on 3 feet spacing with weldmesh and 4 inches of shotcrete with or without fibers.
A version of this classification rationalized for construction is presented in
the Conceptual Design Reports.
For those sections where excavation is carried out by means other than TBM, it
is recommended that shotcrete is used as integral part of the support in rock
units 5 and 6 and locally in unit 4. Typically 2 inches of shotcrete will be
required in unit 5 and between 6 and 8 inches in unit 6. Bolts and mesh
should be installed after the first layer of shotcrete has been placed. As an
alternative to the mesh, steel fibers can be used in the shotcrete to improve
tensile and shear strength properties.
8.1.4.2 Secondary Lining
The s e c o n d a r y lining will almost certainly be formed of cast-in-place concrete
and to facilitate placing, the lining will be between 12 and 15 inches
thick. These linings will be capable of supporting all reasonable ground
loads that could be imposed upon them during the design life of the tunnel.
8-11
They also have a factor of safety against failure under the conditions of full
hydrostatic water loading acting on the external surface with zero water
pressure inside. The hydrostatic water pressure, which is approximately
150 psi at tunnel elevation, will only develop if the lining is effectively
impermeable in relation to the ground.
A further design case that will need to be addressed is that of an initially
unstressed secondary lining being loaded internally by the effluent as the
tunnel is brought into operation. This will result in tensile stresses
developing in the lining which will attain a maximum at the inside surface.
The magnitude of this stress will depend on the geometry of the lining and the
relative modulus of the ground and the lining. when the modulus of the ground
equals that of the lining, which will generally be the case for materials on
the boundary between rock units 2 and 3, the tensile stress equals that of the
effluent pressure inside the tunnel (approximately 150 psi). This tensile
stress increases markedly as the modulus of the ground reduces. It is likely
that in certain situations the tensile strength of the concrete will be
exceeded leading to cracking of the lining. Once cracks have developed the
situation rapidly stabilizes as the water pressure outside the lining
increases to that internally. It is not believed that these cracks will
prejudice the integrity of the lining. The use of reinforcing within the
lining is not advised as this increases the stiffness of the lining therefore
increasing the magnitude of the tensile stresses that develop.
In the Dorchester Tunnel (Ashenden, 1982) cracking of the cast in place
concrete tunnel liner was caused by opening of horizontal bedding planes in
the laminated argillite. An extensive program of grouting was later required
to seal the tunnel. It is expected that the requirements to restrict the
amount of ground relaxation by proper use of primary support will avoid this
situation on the current project.
Typical average inflow rates into both tunnels are considered to be around
0.1 gpm/ft taking account of the apparently higher permeabilities on the
inter-island tunnel and the larger diameter of the outfall tunnel. However a
value as high as 0.3 gpm/ft is not considered unreasonable on the basis of the
8-12
permeability test results. This would yield total inflows of about
900,000 gallons/hour for the outfall tunnel and 500,000 gallons/hour for the
inter-island tunnel. Ground treatment ahead of the face, in areas of
significant inflow, will reduce this figure considerably, but it is obviously
advisable to install the permanent lining as the tunnel is being excavated
where possible.
8.1.5 Probing Ahead and Ground Treatment
8.1.5.1 Probing Ahead
With almost all tunnels some uncertainty of the ground conditions along the
alignment will exist at the time that construction starts, irrespective of the
amount of investigation undertaken. The degree of uncertainty will depend on
the nature of the investigations and the complexity of the geology. For these
two tunnels it is considered essential to probe ahead of the face when
tunneling under the sea.
Three philosophies may be considered regarding the use of probing ahead:
(1) Continuous probing for the full tunnel drive. For tunnels driven by TBM, probing is usually undertaken during the scheduled maintenance periods. The probe hole should allow for an overlap.
(ii) Probing ahead undertaken in pre-selected areas of geological uncertainty.
(iii) Probing ahead undertaken when conditions deteriorate in the face, such as increased water inflows or reduced fracture spacing. The criteria to determine when probing is required should be determined before start of construction and then updated as experience on the real ground conditions is gained.
Method (i) involves the greatest amount and cost of probing, but increases the
confidence and safety and reduces risk during tunneling. Method (ii) assumes
that all the poor sections of ground are able to be identified from the
results of the site investigations, but it removes from the site personnel the
responsibility of deciding where probing is required. Method (iii)
8-13
potentially results in the least probing, but requires experienced and
observant personnel on site to identify in advance the tell-tale signs of
poorer ground ahead of the face. This is more difficult in a TBM driven
tunnel where the 15 feet nearest to the face is obscured by the machine.
For the inter-island tunnel and the effluent outfall tunnel it is strongly
recommended that continuous probing is adopted (method (i)).
Probing usually involves rotary percussive drilling from which a limited
amount of information can be obtained on the lithology of the material from
the r e t u r n s , the strength of the material from the rate of penetration of the
drill, and the permeability of the material from the inflow down the drill
hole. Instrumentation of the drill rig in terms of penetration rate, torque
and thrust provides additional information. A simple means of calibrating the
quantity of water inflow is necessary, and facilities can be made available
for the use of packer testing equipment to identify whether high inflows are
resulting from localized zones or from the whole borehole length. This
information can be useful when considering the requirements for ground
treatment. In-hole geophysical investigations can also be carried out in the
probe holes if required.
Coring of probe holes, while of geological interest, is of limited value in
tunnel construction, unless undertaken as part of an extensive investigation
in an area of particular difficulty. The time taken to drill, extract and
analyze c o r e s can seriously delay machine progress, and the additional
information does not usually assist decisions on forward grouting or other
factors affecting machine driving.
8.1.5.2 Ground Treatment
Certain sections of tunnel may be sufficiently permeable to require ground
treatment. It is expected that this will be true of sections in which the
permeability is 1 x 10"- cm/sec or greater. It is obviously important that
these sections are identified by the probing ahead before they are encountered
in the tunnel. Grouting in ground of this permeability will be carried out
8-14
using cement based grouts using single or multiple injections over a volume to
encompass the whole tunnel outline. The requirements for additional grouting
can be made on the basis of previous grout tests or on the water flow from
probe holes. It is therefore essential that the TBMs are designed to allow
ground treatment to be readily undertaken.
8.2 SHAFTS
Because of the strength and clay content of the till it is expected that this
material is generally capable of standing unsupported for short periods. The
presence of fissuring will determine the allowable area of unsupported
excavation, but it appears that normal shaft excavation techniques will be
successful in this material. Nevertheless, it will be necessary to provide
some form of coating, such as shotcrete, to the exposed till surface to
prevent drying-out and ravelling.
At the outfall shaft location, till will be encountered at ground level, but
at the other two shaft locations there is overlying material, which is
expected to be more permeable and require immediate support on excavation. At
Nut Island either sheet piling or diaphragm walling will be required extending
at least into the top of the till. At the Deer Island inter-island shaft the
top of the till is located near to mean sea level and it may prove possible to
remove the overlying fill by an open excavation prior to construction of the
shaft. Backfilling could then be carried out around a constructed collar if
necessary. If open excavation is not possible at this location, then sheet-
piling to the top of till could be used.
The material underlying the till at Deer Island inter-island shaft location is
granular and its density is such that when dry it would be capable of standing
unsupported in vertical faces. In view of the relatively high permeability of
the material it will, however, be unstable if a flow of water to the shaft
occurs. It is not considered practical to form a dewatering scheme for
material of this transmissivity at this depth, therefore some form of ground
treatment or cut-off wall will be required. Ground treatment would be in the
form of grouting either from the ground surface during construction of the
•8-15
upper part of the shaft, or from within the shaft as the base of the till is
approached. The former method has the advantage of allowing excavation of the
shaft to continue during the grouting operation, but suffers from the
disadvantage that considerably greater depths of drilling are required.
Alternatively a diaphragm wall could be constructed for the full depth from
ground level into the top of rock. The presence of boulders within the till
and the underlying material is unlikely to cause particular problems to the
diaphragm walling process, but the strength of the till could result in slow
excavation rates.
When further details of the permeability of the near surface zone of rock are
available it will be possible to assess the need for grouting to reduce
potential water inflows. If slurry walls are used extending through the till,
grouting can be added by the installation of tubes within the wall during
construction through which drilling of grout holes can be undertaken. If
grouting is used to treat the material underlying the till at the Deer Island
inter-island shaft, this can readily be extended into the bedrock, but at
other locations a separate grouting exercise may be required.
For a total depth of shaft below bedrock surface of 300 feet and average
permeability of 1 x 10" cm/sec, water inflows through the rock should be
approximately 130 gals/hr. However a 20 foot zone of fractured rock of
permeability 1 x 10*" cm/sec, as was observed in the Nut Island borehole and
is probable at the till rock contact in the other shafts, would itself yield
around 4000 gals/hr and will probably require grouting.
It is considered that the superficial materials overlying the bedrock will be
capable of being excavated by normal mechanical means, whereas drill and blast
techniques will be necessary in the bedrock.
Rock stability will essentially be controlled by the discontinuities. Until
reliable discontinuity orientation data is available it is not possible to
assess the requirements for support although it is believed that rock bolts,
shotcrete and mesh support will be adequate. The indication is, however, that
many steeply inclined discontinuities are present, including bedding fractures
8-16
in the two inter-island shaft boreholes and this will lead to significant
numbers of potentially unstable wedges. If the discontinuities form sets then
it may be possible to provide patterned rock bolt support over the sector of
the shaft in which potentially unstable wedges a r e p r e d i c t e d , w i th o n l y s p o t
bolting elsewhere. In the more fractured zones Identified earlier it will
probably be necessary to provide patterned bolting around the complete
circumference of the shaft.
8.3 DIFFUSERS
An additional investigation is required in the d i f f u s e r a r e a s t o allow
detailed design of the diffusers to take place. This will be carried out
during detailed design.
Each of the diffusers will consist of an approximately 3 feet diameter, fully
lined, vertical riser sunk to tunnel horizon on which the diffuser cap will be
founded.
It is almost certain that the caps and upper parts of the risers will have to
be designed to resist the large lateral loads which may be applied by fishing
gear or ships anchors and anchor cables. There is also a possibility that a
cap may be struck by a falling anchor.
These loads may be resisted by:
• A. large diameter casing concentric with the risers driven to sufficient depth to obtain lateral resistance, and mucked out and filled with reinforced concrete/grout.
• A rock armor or similar apron to prevent the offending object catching on the riser caps
• Both a large casing and an apron
Current and earthquake loads on the riser will be trivial compared with the
accidental loads but will have an important effect upon the apron as it will
be necessary to consider:
8-17
• The risk that the apron will sink in an earthquake
• The depth and extent of the scourholes around the caps.
In the relatively flat areas where the d i f f u s e r s w i l l be constructed the sea
bed consists of sand, mud and mixed mud and gravel. Sea bed deposits of this
kind are almost invariably mobile and this expectation is confirmed by recent
side scan sonar surveys, video films and suspended solids measurements. The
sands and muds of this kind are known as marine deposit and have been
deposited during the most recent marine transportation so that they are less
than 8,000 years old and have never been exposed to the air or consolidated;
they are therefore loose and easily eroded.
The depth of the marine deposit at any point can be determined quite
confidently by continuous, high resolution seismic profiling.
In many places the whole of the marine deposit may be fluidized by the
fluctuating (wave generated) shears and pressures in storms. Where this
occurs the analogue seismic records are completely white (i.e. the material is
"transparent" to acoustic energy). However both the recent Weston (Sparker)
and Woods Hole (Boomer) records both show marked layering in the lower parts
of the marine deposit and it can therefore be concluded that the material is
stable under all natural conditions.
The transmission lines of the geophysical profiles obscure the 6 feet of
material immediately below the sea bed and sand waves up to 6 feet long are
visible on the record so the depth of naturally unstable material will be
between 1 and 6 feet: this depth can and should be estimated with greater
certainty from a 20 kHz seismic record.
The disturbance to the regime caused by the diffuser caps and the buoyant flux
caused by the effluent discharge will scour the bed to below the naturally
stable level and this (further) depth should be estimated to give "lowest sea
bed level" at the caps. For this calculation particle size analysis and the
8-18
cohesion (if any) of the upper 10 to 15 feet of the marine deposit should be
determinpd. determined
The remainder of the marine deposit will provide lateral support to the cap
and riser top. To allow this support to be calculated the density, angle of
internal friction and cohesion of the layers more than 10 feet below the sea
bed should be measured in the 1989 marine campaign.
Most of the bedrock should be relatively easy to drill and the holes will
stand without casing or cementing except:
• If altered argillite is encountered.
• If the holes pass through fracture zones
• Subvertical diabase dikes are encountered.
The boreholes and marine geophysics should give an indication of the risk that
these ground conditions will occur in an area but will not be able to predict
whether they will be present or absent in a particular riser hole. These
risks need to be assessed as:
Some casing might be required to stabilize the holes in the most decomposed argillite
Cementing may be necessary in fracture zone
A stiff drill with well fitting stabilizers should be used to minimize the deviation where holes pass, obliquely, through diabase dikes.
If the cap and upper casing are founded on the clay it will consolidate
transferring load from the base of the upper casing and the grout/concrete
filling to the liner tube. The consolidation of the clay under increase of
vertical load of up to around 1 ton/ft should therefore be known and the
appropriate laboratory tests should therefore be made from samples taken from
the selected diffuser site.
8-19
No particular geotechnical problems are anticipated with drilling the riser
holes in the recent marine sediments and the clay. Some difficulties may
arise in the till due to the p r e s e n c e of large boulders, but the diffusers
will be located to minimize the thickness of till to be penetrated. The risk
of encountering erratic boulders on the glaciated surface of the clay will
remain.
It is possible that the disturbance caused by jack up legs being forced into
the ma r ine d e p o s i t and c l a y and their subsequent withdrawal and the collapse
of the cavity may disturb completed risers and caps. The need to avoid this
risk is likely to restrict the way jack-up barges can be oriented relative to
the holes and may restrict the types of barge which can be used. Geotechnical
data at the selected diffuser site are therefore required to enable the actual
penetration of the legs to be calculated.
8.4 INSTRUMENTATION
As discussed above, our knowledge of the ground conditions will be somewhat
limited at the start of construction despite the number of investigations that
will have been carried out by that time. This is as true of the ground
p r o p e r t i e s a s fo r geo logy itself, because of the reliance placed on the
results of small scale laboratory tests. It is therefore recommended that
instrumentation is installed in the shafts to measure properties such as the
rock mass modulus and the insitu stress conditions to check the design
values. This instrumentation would be installed at probably two separate
elevations corresponding to typical ground conditions. The instrumentation
would consist of multiple point extensometers, convergence monitoring and
possibly stress cells installed at the rock-lining interface. Both the
extensometers and the stress cells should be provided with remote readout
units because of the difficulties of access as the shaft is excavated further.
Because of the great flexibility of bolts and shotcrete as a method of
support, it is common f o r instrumentation arrays to be installed in areas
where this support is used to determine the exact support requirements and to
allow savings to. be made on the original design or to identify when and where
8-20
additional bolts or shotcrete are required to prevent excessive deformations
occurring. It is suggested therefore that some further instrumentation arrays
are installed in areas of rock units 5 and 6.
8-21
>
o
>
I I I
APPENDIX A
REFERENCES CITED
APPENDIX A REFERENCES CITED
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Bailey, Richard H., 1984, A precambrian continent Margin Sequence (slope deposits and olistrostromes) Boston North Quadrangle, MA in Hanson, L.S., ed.. Geology of the Coastal Lowlands Boston, MA to Kennebunk, ME, MEIGC Guidebook, Dept. of Geology, Salem State College, Salem, MA.
Bailey, Richard H., 1984, Cambrian Rocks of East Point, Mahant, in Hanson, L.S., ed., Geology of the Coastal Lowlands Boston, MA to Kennebunk, ME, MEIGC Guidebook, Dept. of Geology, Salem State College, Salem, MA.
Bailey, Richard H., 1976, Geology of the Squantum "tillite", in Cameron, B., ed.. Geology of Southeastern Wew England, MEIGC Guidebook, Science Press, Princeton, M.J.
Barosh, Patrick, 1984, Regional Geology and Tectonic History of Southeastern Mew England, in Hanson, L.S. ed., Geology of the Coastal Lowlands Boston, MA to Kennebunk, ME. MEIGC Guidebook, Dept. of Geology, Salem State College, Salem, MA.
Bieniawski, Z.T., 1973, Engineering Classification of Jointed Rock Masses. Trans. S. Afr. Instrn. Civ. Engrs., Vol. 15, p.334-344.
Billings, M.P., 1976, Geology of the Boston Basin, in Lyons, P.C, and Brownlow, A.H., eds., Studies in New England Geology: Geol. Society of America, Memoir 146, p 5-30.
Billings, M.P. (compiler), 1955, Geologic Map of Mew Hampshire: Mew Hampshire Planning and Development Commission. Scale 1:250,000.
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Billings, M.P., and Tierney, F.L., 1964, Geology of the City Tunnel Extension, Greater Boston, Massachusetts, Journal of the Boston Society of Civil Engineers., Vol. 57, pp 111-154.
Birch, F.S., 1983, Preliminary geological interpretation of a new magnetic map of the inner continental shelf of Mew Hampshire; Geological Society of America, Programs with Abstracts V15, Mo. 3, p.196.
Brady, B.H.G. and Brown, E.T., 1985, Rock Mechanics for Underground Mining; London: George Allen & Unwin.
Brown, E.T., & E. Hoek, 1988, Discussion on Paper 20431 by R. Ucar entitled "Determination of Shear Failure Envelope in Rock Masses". J. Geotech. Eng. Div., ASCE, vol. 106, GT9, pp. 371-373-
Cady, W.M., 1969, Regional tectonic synthesis of Northwestern Mew England and adjacent Quebec: Geological Society of America Memoir 120, l8l p.
Camp Dresser & McKee, 1988, Seconda ry Treatment Facilities Plan, submitted to the MWRA.
Cardoza, Kelly Durfee, 1987, Geochemistry of the Middlesex Fells Volcanics (Mafic Member) and Brighton Volcanics, Boston-Avalon Zone, Eastern Massachusetts, MS thesis, Boston College, Mewton, MA.
Cornell, C.A., 1968, Engineering Seismic Risk Analysis. Bulletin of the Seismological Society of America, vol. 58, Mo. 5, p. 1583-1606.
Cullen, T.R.; Young. Jr., L.W.; and Kevllle, F.J.; 1982, Tunneling through the Cambridge Argillite, in Farquhar, O.C, ed., Geotechnology in Massachusetts, Univ. of Mass., Amherst, pp. 197-212.
Davies, J.L., compiler, 1977, Geological map of Northern New Brunswick: New Brunswick Dept. of Natural Resources Map MR-3 scale 1:250,000.
Dill, Robin B., 1986, Excavation of the Porter Square Station Rock Chamber, Bulletin of the Association of Engineering Geologists, Vol. XXIII, No. 4, pp. 479-486.
Doll, C.C, CadyW.M., Thompson, J.B., and Billings, M.P., 1961, Centennial geologic map of Vermont: Vermont Geological Survey, Scale 1:250,000.
Doyle, R.G., compiler, 1964, Preliminary geological map of Maine: Maine Geological Survey, Scale 1:500,000.
Dugan, J.P., 1982, Pressure Grouting to Repair the Dorchester Water Tunnel, Boston, in Farquhar, O.C, ed., Geotechnology in Massachusetts, Univ. of Mass, Amherst, pp. 221-228.
EPRI, 1986, Seismic Hazard Methodology for the Central and Eastern United States, NP-4726, Volume 5, Tectonic Interpretations by Weston Geophysical.
FitzGerald, Duncan, 1984, Coastal Geology of Winthrop, MA, in Hanson, L.S., ed,. Geology of the Coastal Lowlands Boston, MA to Kennebuck, ME, NEIGC Guidebook, Dept. of Geology, Salem State College, Salem, MA
Hatheway, Allen W. and Paris, William C , 1979, Geologic Conditions and Considerations for Underground Construction in Rock, Boston, Massachusetts, in Hatheway, A.W., ed., Engineering Geology in New England ASCE Preprint No. 3602.'
Hoek, E. and Brown, E.T., 1980, Empirical Strength Criterion for Rock Masses; J. Geotech.' Eng. Div., ASCE, Vol. 106, G-T9, p. 1013-1035.
Hoek, E. and Brown, E.T., 1988, The Hoek-Brown Failure Criterion - a 1988 update. Proc. 15th Canadian Rock Mechanics Symposium; 5 pp.
Kaye, CA., 1984 Boston Basin Restudied; in Hanson, L.S., ed.. Geology of the Coastal Lowlands Boston, MA to Kennebunk, ME, NEIGC Guidebook, Dept. of Geology, Salem State College, Salem, MA
Kaye, C.A., 1980, Bedrock Geologic Maps of the Boston North, Boston South, and Newton Quadrangles, Massachusetts; U.S. Geol, Survey Misc. Field Studies Map MF-1242, scale 1:24,000.
Kaye, CA., 1979 Engineering Geologic Framework of the Boston Basin, in Hatheway, A.W,, ed,, Engineering Geology in New England. ASCE Preprint Mo. 3602
Kaye, C.A., 1967, Kaolinization of Bedrock of the Boston, Massachusetts Area; U.S. Geological Survey, Professional Paper 575-C pp. C165-C172.
Kaye, CA., undated, the Brewster Island Sill - Swarm of Layered Olivine Diabase; Abstract from unpublished Notes, compiled by Metcalf & Eddy for the United States Geological Survey.
Lenk, Cecilia, Strother, Paul K., Kaye, C.A., and Barghoorn, Elso S., 1982, Precambrian Age of the Boston Basin: New Evidence from Microfossils, SCIENCE, Vol. 216, pp. 619-620.
Maguire, R.K., 1976, Fortran computer program for seismic risk analysis; United States Geological Survey, Open File Report OF-76-67. 68p.
Metcalf & eddy, 1983, Mut Island Facilities Planning Project, Site Options Study.
Metcalf & Eddy, 1989, Boston Harbor Geological and Geotechnical References, for the MWRA, 2 volumes,
Metcalf & Eddy, 1989, 1988 Marine Drilling, Summary Report, for the MWRA, 2 volumes.
New England Research Inc., 1989A, Rock Properties, Secondary Treatment Plant, Deer Island (Inter-Island and Outfall Tunnel areas).
New England Research Inc., 1989B, Rock Properties Secondary Treatment Plant, Deer Island and Nut Island (Shaft Borings).
Rahm, D.A., 1962, Geology of the Main Drainage Tunnel, Boston, Massachusetts, Journal of the Boston Society of Civil Engineers, v. 49, pp. 310-368
Richardson, S.M., 1977, Geology of the Dorchester Tunnel, Greater Boston, Massachusetts; Journal of the Boston Society of Civil Engineers, v. 63, pp. 247-269.
Serafim, J.L., and Pereira, J.P., 1983, Considerations of the Geomechanical Classification of Bienawski; Proc. Int. Symp. on Engineering Geology and Underground Construction, Lisbon, Vol. 1, Pt. II, p.33-42.
Sheridan, A.W., 1988, Geology of the Milford Plutonic Suite; Unpublished M.S. Thesis Dept. of Geology and Geophysics, Boston College, Chestnut Hill, MA, 56 p.
Smith, CJ., 1985, Late Proterozoic Avalonian Magnetism North of Boston, Eastern Massachusetts; Unpublished M.S. Thesis, Dept. of Geology and Geophysics, Boston College, Chestnut Hill, MA.
Street, R. and LaCroix, A.V., 1979, An Empirical Study of New England Seismicity: 1727-1927, Bulletin of Seismological Society of America, vol. 69, Mo. 1, p 159-175.
Tierney, F.L., Billings, M.P., and Cassidy, M.M., 1968, Geology of the City Tunnel, Greater Boston, Massachusetts, Journal of the Boston Society Civil Engineers, v. 55, pp. 60-96.
Williams, Harold, compiler, 1978, Tectonic lithofacies map of the Appalahlan Orogen: Memorial University of Newfoundland Map No. 1, Scale 1:1,000,000
Weston Geophysical Corp., 1980, Offshore Geophysical Survey, Edgar Station Study, Boston Edison Company, Weymouth, MA; for Stone & Webster Engineering Corporation.
Weston Geophysical Corp., 1981, Seismic Survey, Metropolitan District Commission, Nut Island Facilities Planning Project, Quincy, MA; prepared for Metcalf & Eddy, Inc.
Weston Geophysical Corp., 1988, Coarse Grid Marine Geophysical Surveys, Deer Island Secondary Treatment Facility Boston, Massachusetts of Massachusetts Water Resources Authority; for Camp Dresser & McKee, Inc.
Weston Geophysical Corp., 1976, Geologic Investigations for Boston Edison Company, Pilgrim Unit 2, Docket Wo.-50-471, BE567603.
Wolfe, CW., 1976, Geology of Squaw Head, Squantum, MA; in Cameron, B., ed. Geology of Southeastern New England, NEIGC Guidebook, Science Press, Princeton, N.J.
Zartman, R.E. and Maylor, R.S., 1984, Structural implications of some radiometric ages of igneous rocks in Southwestern Mew England, Geological Society of America Bulletin, vol. 95, p. 522-539
Zen, E-an, ed., 1983, Bedrock Geologic Map of Massachusetts: U.S. Geol. Survey, scale 1:250,000.
