CE645 Term Paper Drilled Shafts.pdf

Drilled Shaft Design, Construction Procedure and Construction Inspection

Jay B. Patel, NJIT-Graduate Student CE-645 Rock Mechanics II Professor: Dr. Raghu Dorairaja, P.E. New Jersey Institute of Technology Spring 2014

ABSTRACT

Title: Drilled Shaft Design, Construction Procedure and Construction Inspection

Author: Jay B. Patel, NJIT Graduate Student

The author will demonstrate engineering design process, construction procedure and inspection of drilled shafts.

The author will also discuss the following:

• The Geomaterials Drilled Shaft Bear In.

• How drilled shafts create capacity.

• How drilled shafts are constructed with different methods of construction.

• How to Calculating Shaft Friction resitance in Rock Sockets.

• How to Calculate End Bearing capacity in Rock Sockets.

• Concrete mix design, encountered construction challenges during concrete

placement under water and challenges achieving design compressive

strength of concrete.

• Construction Sequence and Construction Inspection.

• Design Project involving over 400 drilled shafts for a Mixed-‐Use

Development featuring 140 apartments, 22,000 square feet of retail space,

and a four-‐level underground parking garage with 376 spaces.

Table of Contents

Introduction ...................................................................................................................... 4

Description of Drilled Shafts ............................................................................................. 4

The Geomaterials Drilled Shaft Bear In ........................................................................... 5

How Drilled Shaft Create Capacity ................................................................................... 5

Shaft Friction in Rock Sockets ......................................................................................... 6

End Bearing in Rock Sockets ........................................................................................... 7

Types of Shaft Installation ................................................................................................ 8

Dry Construction method ........................................................................................................................ 8

Wet Construction method ....................................................................................................................... 9

Temporary Cased Method ....................................................................................................................... 9

Construction Sequence and Construction Inspection Procedure ..................................... 9

Concrete Mix Design ...................................................................................................... 10

Key Elements For Quality Concrete In Drilled Shafts .................................................... 11

Drilled Shaft Design ....................................................................................................... 11

Appendix A ..................................................................................................................... 13

Jay Patel CE 645- Rock Mechanics II

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Introduction

Drilled shafts are a type of substructure (deep foundation) that are typically

designed and constructed to support a variety of civil infrastructure projects.

Drilled shafts are deep foundation elements capable of carrying high axial

compression, uplift, and lateral loads and thus are ideal for certain unusual loading

conditions such as traffic breaking, wind and seismic forces on bridges and high

rises. Sometimes the loads required are very significant, and engineers seek to

found the base of the drilled shafts into rock where capacities are generally higher.

Drilled shafts can be designed and constructed in large sizes where capacitates

are higher.

The objective of this paper is to examine the elements of design, construction

methods and Construction Inspection factors that affect drilled shaft capacity.

Description of Drilled Shafts

Drilled Shafts are broadly described as a cast-in-place deep foundation whereby

the shaft is stabilized to permit the installation of reinforcing steel and concrete.

Drilled foundations are synonymous with Drilled Shafts, Drilled Piles and Drilled

Caissons. Drilled shafts generally have larger diameters as compared to drilled,

continuous flight, hollow stem auger piles or “Augercast Piles,” but this distinction

is changing as maximum Augercast pile diameters are upwards of 48 inches.

Drilled shafts are perhaps the most robust of deep foundation types, and they are

used for bridges, large buildings and major Civil infrastructure projects due to their

unique ability to resist high axial and lateral loads.

The design capacity of drilled shafts can vary between 50 to 5,000+ tons with

diameters from 12 to 120+ inches. The only limit on drilled shaft diameter is

equipment capability. Typical drilled shafts lengths are 20 to 90+ feet; however,

exceptions as deep as 400+feet.


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The Geomaterials Drilled Shaft Bear In

Shaft bear in what engineers categorize as three material types: 1. Soil, 2.

Weathered Rock (WR) and 3. Rock. Two types of Soils are cohesionless and

cohesive. Cohesionless materials include Sands and Gravels, and non-plastic

Silts. Cohesive materials are clays and sandy/gravelly clays.

This transition between soil and rock is generally vertical in the geologic lithology,

with the degree of weathering decreasing with depth. WR is stronger than soil and

weaker than rock and posses both soil and rocklike properties.

Lastly, rock, a material with no consensus definition in geology or geotechnics, is

defined by engineers in drilled shaft design as a cohesive, cemented geomaterial

that can be identified on the basis of geologic origin. The National Highway

Institute (NHI) in their publication Drilled Shafts: Construction Procedures and

LRFD Design Methods dated May 2010 provided this definition of rock.

How Drilled Shaft Create Capacity

For this discussion, the author will subdivide the explanation into two categories:

shafts bearing in soil/WR and shafts bearing in rock. Shafts that bear in these two

groups of materials behave fundamentally different.

For shafts that bear in soil/WR, engineers sum the side shear resistance based on

differing soil/WR layers and then add the end bearing to determine capacity.

Figure 1 below provides a schematic describing how capacity is created.

For cohesionless materials, engineers calculate the side shear resistance (shaft to

soil/WR) by determining overburden pressures, converting these pressures into

horizontal pressures via earth pressure coefficients, and then determining the

shearing resistance between soil and the shaft concrete by estimating frictional

interface coefficients. Side shear in cohesive soils are estimated by determining

the undrained shear strength of the clay, determining the appropriate alpha factor

(a reduction coefficient), then multiplying the shear strength and the alpha factor

by the circumferential area of the shaft. End bearing for cohesionless or cohesive


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soils is calculated conventionally per Terzaghi’s original theories on bearing

capacity modified for deep foundation effects.

Drilled shafts with bases formed in rock sockets behave quite differently than

described for soil/IGM above. The primary differences are that the overburden

pressures do not affect the side shear of the concrete in the rock socket, and that

axial support from the soil overburden above the rock socket is ignored. Further

differences include ultimate capacity of the shaft being either from side shear in

the socket, or end bearing, but not the sum of both.

Shaft Friction in Rock Sockets

Drilled shafts are large, highly loaded elements that are difficult (and expensive) to

load test. As such, there is not frequent field testing of drilled shafts to confirm

design assumptions. What engineers are ultimately interested in is the frictional

resistance between the concrete and the shaft rock socket. The engineering

design process results in some basic assumptions or characterizations:

roughness coefficient, jointing of the rock, and soil materials of in filled joints. The

drilling process itself can affect rock socket roughness and obviously the rock type

is a factor. Cleanliness of shaft sockets prior to placement of concrete is also a

concern. There are notable researches performed by several notable authors to

determine socket roughness. O’Neill and Reese suggested that socket roughness

is directly proportional to RQD and qu. The weaker the rock is RQD and qu is, the

lower the socket friction.

Horvath and Kenney suggested socket friction, fSN = 2.5(qu)0.5, is related only to

the rock’s unconfined compressive strength, qu, also an engineer property that is

often measured in the lab and reported on boring/coring log reports.

O’Neill and Reese Equation for socket shaft resistance. Pa = atmospheric pressure, qu = unconfined compressive strength of the rock, aE is the reduction factor.


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End Bearing in Rock Sockets There are several notable authors who have piloted research in calculating end

bearing in rock sockets. Prakoso and Kulhaway determined a relatively simple

empirical equation qBN = Nc* qu.

This time relying on the engineering geology properties of Rock Mass Rating

(RMR) and qu the NHI/Federal Highway Administration’s addresses came up with

the following:

Hoek and Brown have developed their own methodology for determining end

bearing resistance, but this time it relies on the Geologic Strength Index (GSI) as

the socket characterization tool. Hoek believes that GSI is a better method for

determining engineering properties of rock than is the RMR method. According to

Hoek, GSI is based upon an assessment of the lithology, structure and condition

Nc=2.5 is recommended

when qu alone is used to

characterize rock mass, Nc is

not a F.S.

NHI/FHWA Method for base resistance in rock


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of discontinuity surfaces in the rock mass. What is appealing about Hoek’s ideas

is that for the first time, the roughness of the rock mass (a characteristic of

mineralogy) and weathering are factors. The basic premise of Hoek’s GSI theory

is shown graphically below in Figure 4.

Hoek’s equations for end bearing:

Types of Shaft Installation

Dry Construction method Use the dry construction method only at sites where the ground water table and

soil conditions, generally stiff to hard clays or rock above the water table, make it

Decreasing interlocking of rock pieces

Decreasing Surface Quality

100-‐90

10-‐0

A GSI of 100-‐90 would describe a massive, widely jointed rock with rough, fresh surfaces whereas a GSI of 10-‐0 would describe a rock that had indistinguishable blockiness and weathered, slickensided surfaces.

Fig. 4

S, a and mb are Hoek strength parameters based on GSI


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feasible to constrTeruct the shaft in a relatively dry excavation and where the sides

and bottom of the shaft are stable and may be visually inspected by the Engineer

prior to placing the concrete. In applying the dry construction method, drill the shaft

excavation, remove accumulated seepage water and loose material from the

excavation and place the shaft concrete in a relatively dry excavation Use the dry

construction method only when shaft excavations, as demonstrated in a test hole,

have 12 inches or less of seepage water accumulated over a four hour period, the

sides and bottom remain stable without detrimental caving, sloughing, or swelling

for a four hour period, and the loose material and water can be satisfactorily

removed prior to inspection and prior to placing concrete.

Wet Construction method Use the wet construction method or the casing construction method for shafts that

do not meet the requirements for the dry construction method. Provide temporary

surface casings to aid shaft alignment and position and to prevent sloughing

unless the Engineer determines by demonstration that the surface casing is not

required.

Temporary Cased Method Temporary casing is used to stabilize the shaft excavation, and prevent sloughing

or caving of materials, as the hole is advanced with either the Wet or Dry method

of excavation.

Construction Sequence and Construction Inspection Procedure

Construction sequence includes

• Drilling concurrent with Drilling observation/ Drilling inspection

• Bottom cleanliness and Shaft wall Inspection

• Concrete Placement


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Concrete Mix Design Drilled Shaft concrete should have high slump and is relatively fluid concrete that

should be placed with the tremie that would eliminate the possibility of segregation

when working with shallow ground water. A tremie is a long pipe typically 4 inches

to 12 inches in diameter; tremie pipes takes concrete from pump to the bottom of

the excavation. During concrete placement, the tremie tip elevation should be

maintained below the surface of rising concrete typically about 5 to 10 feet below

the top of rising concrete. However in dry conditions free-fall concrete placement

can be used although it is restricted by some State agencies.

The concrete mix design for drilled shafts should produce a sufficient slump

(typically between 6 and 9 inches) to ensure that lateral fluid concrete pressure will

develop against the excavation walls, the concrete must flow smoothly through the

reinforcing cage under its own buoyant weight without “piling up” near the tremie.

A mixture with desired workability will not result in more than a few inches of

difference in height between the top of the concrete surface near the tremie and

concrete on the outside of the reinforcement as shown in figure below. Further,

the concrete should maintain a slump no less than 4 inches (slump loss limit) for

several hours. This typically allows enough time to remove the tremie and any

temporary casing while the concrete is still fluid enough to replace the volume of

the tremie or casing and minimize suction forces (net negative lateral pressure)

during extraction. However, recent studies suggest that a final slump in the range

of 3.5 to 4 inches (or less) at the time of temporary casing extraction can

drastically reduce the side shear capacity of the shaft (Garbin, 2003).


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Key Elements For Quality Concrete In Drilled Shafts The majority of construction problems which compromise the quality of drilled

shafts come from a failure to adequately consider one or more of the following

categories:

• Workability of concrete

• Maintaining workability for the duration of the pour

• Compatibility of congested rebar and concrete mix

• Cohesive paste to avoid segregation and bleeding

• Control the sand content of the slurry and the stability of the hole during

excavation and concrete placement

Drilled Shaft Design The author was fortunate enough to be one of several principal geotechnical

engineers involved with the design and construction inspection of Mixed-Use

Development featuring 140 apartments, 22,000 square feet of retail space, and a


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four-level underground parking garage with 376 spaces. The project included

completion of a secant-pile foundation wall (400 drilled shafts) due to site

geometry and shallow groundwater issues. Design calculations and specifications

are included in Appendix A.


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Appendix A Drilled Shaft Design Calculations for a 22,000 square feet Mixed use

Development

References

(1) “Drilled Shafts: Construction Procedures and LRFD Design Methods,” NHI Course 132014, FHWA-NHI-10-016, FHWA GEC 010, U.S. Dept. of Transportation, Federal Highway Administration, May 2010

(2) “Analysis and Design of Drilled Shaft Foundations Socketed into Rock”, prepared by Cornell University for the Electric Power Research Institute, EPRI EL-5918, August 1988, by Authors Carter and Kulhaway

(3) Power point presentation, “Determination of Unit Tip Resistance for Drilled Shafts in Fractured Rock using the Global Rock Mass Strength”, by Truzman, Corley and Lipka (undated)

(4) Mullins, A.G. 2010, Thermal Integrity Profiling of Drilled Shafts, DFI Journal, Dec 2010.

(5) Piscsalko, G. and Cotton, D. 2011, Non-Destructive Testing Methods for Drilled Shaft and ACIP Piles, Proceedings from Deep Foundation Institute 36th Annual Conference.

(6) Brown, D., (2004). “ Zen and the Art of Drilled Shaft Construction,” GeoSupport 2004, ASCE, GSP 124, pp. 19-33.

(7) Brown, D. A. and Camp, W.M., 2002. “Lateral Load Testing Program for the cooper River Bridge, Charleston, SC” Geotechnical Special Publication No. 116, ASCE, pp. 95-109.

(8) Yao, S. and Gerwick, B. (2004). “Underwater Concrete,” Concrete Int’l, Feb., pp. 77-82.

(9) Federal Highway Administration, 1997. Drilled and Grouted Micropiles,

State-of-Practice Review. Report No. FHWA-RD-96-016/019, United

States Department of Transportation, July 1997. Four Volumes.

(10) Bruce, D. A., 1992. “Recent Progress in American Pin Pile

Technology,” Proceedings, ASCE Conference, Grouting, Soil

Improvement, and Geosynthetics, New Orleans, Louisiana, Feb. 25-28.

(11) NAVFAC, 1986. Soil Mechanics, Soil Dynamics, Foundations and

Earth Structures, and Deep Stabilization, and Special Geotechnical

Construction, Design Manual DM-7.02 Department of the Navy, Naval

Facility Engineering Command, Alexandria, Virginia, September.

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CE645 Term Paper Drilled Shafts.pdf