GeoEng12334

Widening and deepening adjacent large shafts in rock

E. C. Antonio and N. L. Adams

The influent pumping station (IPS) at Bandra plays a keyrole in the Mumbai Sewerage Scheme. Bandra IPS willbe one of the largest pumping stations of its type in theworld, with a pumping capacity of nearly 22 m3/s. Therock support scheme for the widening and deepening ofthree existing shafts is described, with particular atten-tion being given to the scheme for the narrow zone ofrock between the two pump shafts, termed the ‘narrowneck’. Inherent site conditions and the demands of a newstation on a prescribed station layout have placed un-usual constraints on the customary rock support meth-odology. Several excavation and rock supportalternatives are discussed and the limitations of routinedesign methods are highlighted. The finally agreedscheme was a progressive and carefully planned rocksupport and excavation sequence that considered thepossibility of localised instability during the excavationworks and provided contingencies for several possibleevents. A concrete bridge was constructed to restrainthe existing retaining walls and through-going groutedbolts continuously supported the whole rock section,before, during and after excavation. Inspection andmonitoring was completed before each stage of excava-tion. The rock face and retaining walls remained intactand no serious failures were experienced during excava-tion. A brief discussion of possible rock design alterna-tives, given a virgin site of similar rock, follows theconclusions. Rock excavations are now complete and thecivil works at Bandra IPS are nearing completion.

1. INTRODUCTIONIndia’s largest city is situated on the north-west coast of India,

in the state of Maharashtra. The population of Mumbai

(formerly known as Bombay) has expanded rapidly to about 13

million and is expected to grow significantly over the coming

years. Since the early 1980s, part-financed by the World Bank,

the Brihan Mumbai Corporation (BMC) has embarked on a

series of major engineering projects to collect and dispose of

sewage from the city through a system of tunnels, pipelines,

pumping stations, treatment facilities and long sea outfalls.

With a total of eight pumps, each rated at 3?2 m3/s, the Bandra

influent pumping station (IPS) will be among the largest of its

kind in the world. Work first commenced on the construction of

the Bandra tunnels, pipelines, pumping stations and treatment

facilities in the early 1980s. However, the contracts ran into

difficulties that finally culminated in their cancellation, with

the works only partially complete; new consultants were then

appointed.

This paper briefly describes some of the background which has

influenced the approach taken in assessing the rock support

needs for the excavations left by previous contracts. This is

followed by the assessment of geological conditions, the

assessment of rock support options and finally, the adopted

rock support scheme is described. A brief discussion of the

works and some hypothetical alternatives conclude the paper.

2. THE SITE LAYOUT, ALTERNATIVE DESIGNS ANDTHE IMPLICATIONS FOR SHAFT WIDENINGAt the time of termination of the original contracts, four deep

excavations had been formed and construction of the screen

chamber had been substantially completed to ground level

(Fig. 1). The base slabs in each of the two pump shafts had been

cast, but the collector tunnel system and all the excavations

were flooded close to ground level, at approximately 75?7 m

Port Datum (PD). Based upon the as-drawn profiles of

excavations and the old station design, the new consultant’s

design review revealed the potential for a serious siltation

problem within the collector tunnel system. Furthermore, the

capacities of the wet walls were considered to be incompatible

with the operation of variable speed pumps with the conse-

quence that major modifications to the Bandra IPS design were

proposed. The 1996 construction contracts proposed a new

station design with improved performance but requiring

significant modifications to the pump shaft profiles, as may be

seen in Fig. 2.

By measuring offsets to the rock from a heavy plumb-line, on a

361 m circumferential and vertical grid, the excavations were

mapped. Errors were noted in the position of the shaft centre-

line and in shaft verticality. The rock mass inspections also

revealed a persistent and extensive joint set in the basalt mass.

Fig. 3 is an opened, isometric projection of the surface profile

of pump shaft 2, showing the net distance to the shaft centre-

line at each elevation (i.e. the required clearance minus the as-

found distance). In general the upper shaft areas do not need

any further excavation, being already over-excavated, but the

lower shaft zones require up to 2 m of rock to be removed. The

pattern of rock removal reflects poor blasting control, aggra-

vated by local failures on the persistent joint set noted

previously.

Proceedings of the Institution ofCivil EngineersGeotechnical Engineering 149July 2001 Issue 3Pages 000^000

Paper 12334Received 27/04/2000Accepted 09/10/2000

Keywords:cables & tendons/excavation/rockmechanics

E. C. AntonioConsultant for RockEngineering, AntonioAssociates Ltd, Burnham-on-Sea

N. L. AdamsSenior Resident Engineer,Bandra IPS, Binnie Black &Veatch, Redhill

Geotechnical Engineering 149 Issue 3 Widening and deepening large shafts in rock Antonio 0 Adams 1

In view of the new problems exposed after de-watering, several

options were considered to minimise the extent of excavation

required, particularly in the vulnerable ‘narrow neck’ zone

between the two pump shafts. These included options to reduce

the wet wall lining thickness and to move the pump shafts

apart along an east–west axis to avoid any narrow neck

excavation. However, changes to the complex substructure

would inevitably have led to considerable redesign costs and

delays and after careful consideration the options were dis-

counted.

Any major failure of cofferdams or other rock section would be

catastrophic on a site where all available space was already in

use or had been allocated to heavy, fixed, plant. Furthermore,

with a coastal site that is subject to annual monsoon

conditions, the cofferdams are vital structures, retaining 4?5 m

Site boundary

Collector tunnel

(to be renovated)

Existing tunnels

to be filled

Inlet

shaft

Screen

chamber

Stubtunnel

Mahim

Creek

Mahim Bay

New link

tunnel SLD crane

Hoist

Surge

shaft

Pump

shaft

No. 2

Pump

shaft

No. 1

Tower

crane

Site Road

Site Road

Electricity

sub-station

(existing)

Workshops (existing)

Generator

buildings

and offices

(existing)

0 10 20 30 40 50 60 70 80 90 100

Scale: m

Fig. 1. Schematic site layout at Bandra IPS

Mean sea level 74·46 m

Mean high water 76·37 m79·00 m

66·00 m

61·50 m

46·80 m

44·30 m

32·50 m

Basalt

Tuff breccia

Pump shaft 1 Pump shaft 2

Existing concretebase slabs

(to be broken out)

Existing linktunnels

(to be filled)

Existing excavation33·0 m dia. 34·7 m deep

Enlarged excavation up to37·0 m dia. 46·5 m deep

Cofferdam

C L

Cofferdam

New linktunnel

C L

Fill material

0 5 10 15 20 25 30 35 40 45 50

Scale: m

Fig. 2. Schematic east^west section of the two pump shafts and the narrow neck showing the extent of excavation

2 Geotechnical Engineering 149 Issue 3 Widening and deepening large shafts in rock Antonio 0 Adams

of overburden used to reclaim the site from Mahim Bay. A rock

support design review was therefore started to satisfy three

areas of concern

(a) the current stability of all the excavations

(b) the excavation and support methodology in the narrow

neck portion between the pump shafts (with particular

attention given to maintaining the integrity of the existing

cofferdams)

(c) to assess the extent and scale of measures necessary to

safeguard the excavations before, during and after exca-

vation, until the concrete lining is cast.

3. THE GEOLOGICAL SETTINGOF THE BANDRA IPSSITEThe west coast geology of India is dominated by volcanic rocks

of the Deccan Plateau,1 a huge area of some 500 000 square

kilometres. However, in Mumbai, the geological sequence is

uniquely modified2 to include for Inter-Trappean rocks includ-

ing volcanic tuff, breccia and other volcanic rocks; and the 108dip of the Mumbai lava flows contrasts with the horizontal

aspect in most other areas. The local geology at Bandra IPS is

a simple sequence of fill (comprised of basalt blocks and soil)

over the principal unit of compact basalt, which extends

down to 40 m below ground level, underlain by a tufaceous

breccia.

The basalt is a good-quality, compact variety, strong or very

strong and found as a regularly jointed mass with only slight

weathering near surface. One steeply dipping and persistent

joint set is found with a spacing of between 0?5 and 3 m,

extending over tens of metres, trending north–south. The joints

divide the whole basalt mass including the narrow neck zone

between the two pump shafts. Below the basalt, the tufaceous

breccia is relatively unjointed and homogeneous. For rock mass

characterisation, routine rock mass assessments were made

using two systems proposed by Barton et al.3 and Bieniawski.4

Ground water inflows were low and although continuous

seepages in the south-east and north-west quadrants were

noted, the shaft excavations remained relatively dry after initial

de-watering. The particular relevance of the location of these

seepages may be explained in terms of the local de-stressing in

the set of persistent joints, as discussed later.

4. ASSESSING GENERAL ROCK SUPPORTNEEDSWith large and closely spaced openings, the orientation and

scale of the IPS excavations are sufficiently large to influence

in situ field stresses, even at relatively shallow depths. It is

considered prudent that such influences are recognised and

considered, since some aspects of rock mass condition can be

explained on the basis of stress relief deformations that the

rock mass may have already suffered during the initial phase of

excavation. The evidence from recently constructed tunnels in

Mumbai suggests that a high north–south horizontal stress does

exist.5 Therefore, aligned on an east–west axis, the two pump

shafts present a very wide opening across the orientation of

high horizontal stress and this increases the likelihood of past

failures around the periphery of the shafts and within the

narrow neck zone.

Although it is not known how the original excavations pro-

gressed, the poor quality of surface finishing and poor horizon-

tal and vertical alignment of the faces suggest that there had

been rock failures during excavation. The final line and level of

the shafts are largely a chance position, the result of poor

Lower shaft surface requiresup to 2 m trimming but withsome sections already 1 mover-cut

Upper shaft surfaceneeds no furtherexcavation

2

1

0

–2

–1

Ove

r ex

cava

ted

or tr

imm

ing

requ

ired:

m

78 68 66 64 62 60 58 56 54 52 50 4876 74 72 70

Approximate shaft elevation: m PD

Approximate fa

ce bearin

g

E

NEN

NW

W

SWS

SE

Position of link tunnel

1–2 m

0–1 m

–1–0 m

–2–1 m

Fig. 3. Isometric plot of rock surface trimming in pump shaft 2


blasting practices and strongly controlled by rock structure. In

this situation, further excavation works must take into account

the likelihood that the current faces are disturbed.

The stability of a strong and dense basalt is dominated by the

behaviour of joints within the rock mass. The presence of the

steep, north–south trending joint set noted in all shaft exca-

vations, as seen in Fig. 4, is the key structural element within

this rock mass and any rock support system for the shafts must

sufficiently address this aspect.

The first assessment to be made in a rock engineering design is

to assess which domain controls the problem, so that the

appropriate tools may be used to quantify it. Most important is

the scale of the engineering works with respect to the rock zone

of influence. For a joint set with a frequency of only one to

three joints per metre, there is a high probability that any

randomly selected 3 m wide rock zone might have zero, one or

several joints running through it. In other words, the most

appropriate method of analysis would change for each situation

with one analysis yielding a result that no support is necessary

while another may indicate a high support pressure, where the

required rock anchors extending through to the adjacent shaft

could cause failure on the neighbouring shaft wall.

A review of several support design methods is instructive at this

stage to help focus ideas for an acceptable support solution. The

various approaches that may be reviewed are summarised in

Table 1 and applicable methods are discussed further. Most

parts of the shafts and tunnels can be assessed normally but a

thin rock zone at the narrow neck cannot satisfy all of the

assumptions in these methods. It is therefore unlikely that any

routine design solution will be entirely satisfactory. On the

other hand, proposals for a massive rock support scheme with

heavily loaded rock bolts, in what may appear to be a con-

servative design, could result in serious rock mass disturbances

if the support–interaction process is not understood and current

rock mass conditions not properly assessed.

In any circular excavation dominated by a single set of planar

features, such as the IPS basalt, two zones can be susceptible to

slip in each of the main excavations as described by Goodman6

(after Bray7). The method to define the zone of limiting-slip and

the required support pressures is presented below in equation

(1) and shown in Fig. 5.

pb ¼ ðN1p1 þ N2p2Þ1 � cot a tanfj

1 þ tan a tanfj

!1

where pb is the radial support pressure, a is the angle between

rock layers and excavation surface, fj is the angle of joint

friction, p1;2 are the larger and smaller initial stresses, and N1;2

are the initial tangential stresses at a point.

Substantiation of this behaviour is also provided by the rock

mass conditions in the original excavations. Concurrent with

initial excavation or at some time thereafter, large slabs of

rock are seen to have detached from the wall in both the north–

west and south–east quadrants of all three shafts. While this

can also be described as over-break back to any continuous

surface, the evidence of water seepages in the large joints of

these quadrants, while the same joint set in other areas are

dry, also suggests that these joints have dilated and possibly

slipped.

Fig. 4. The narrow neck in pump shaft 1, before excavation.(The poor quality of the retaining walls and the near vertical,persistent joint traces can be clearly seen in the narrow neck)


For the inlet shaft and pump

shaft zones away from the

narrow neck portions, the

slip-model is adequate to

develop a support scheme

that can be verified during

the detailed rock inspections

made before trimming. For

the narrow neck zone, how-

ever, the axis of the two

adjacent shafts is not coinci-

dent with the joint normal

and two further subdivisions

are created. Based on an

assumed joint friction angle

of 308, the slip zones for

each pump shaft overlap in

the bridge zone, while either

side, in the abutment zones,

only one excavation face has

the potential for slip. This

one characteristic of the

basalt therefore gives rise to

two general support zones

and three in the narrow

neck as shown diagrammati-

cally in Fig. 6 and in

Table 2.

Design basis Required parameters Design assumptions Considerations for rock support works inthe narrow neck zone

Deterministic and simpleclosed-form solutions

Rock material and jointestimates of strength anddeformation at anappropriate scale

All loads and physical featuresare known in position andmagnitude

Loadings are likely to be complex andhighly variable, positions of key featuresare uncertainIf one key feature dominates, the methodmay provide a lower bound to theproblem

Rock mass classificationschemes

Indices of material andjoint strength andorientation

Anticipated success is stronglydependent upon experience insimilar circumstances

Should be checked to provide firstapproximation to appropriate scale ofsupport needs at the excavation surface

Rock mass strengthcriteria

Rock material strengthand indices for intensityof rock mass fracture andcondition

Rock mass is assumed to becontinuous, homogeneous andlinearly elastic

Local variability will dominate any resultAs the rock section becomes thicker, themethod may provide an upper bound tothe problem at the narrow deck

Numerical models, andkey block analysis

Rock material and rockjoint estimates ofstrength anddeformation, in situ stress

All loads and physical featuresare known in position andmagnitude

Characteristic models are informative,nevertheless, satisfactory results are verysensitive to input parameters and arelikely to be poor in this case, sensitivitystudies neededKey block analysis will provide likelyshape and positions of blocks but severalmodels will likely give rise to unwieldynumber of possible outcomes

Observational andpragmatic studies:NATM, mining,experimental work

Precedence, observationand experience, or,experimental conditions

Only particular aspects ofdesign assumptionsinvestigated

Generally, lower factors of safety aretolerated in miningThe use of pre-reinforcement ofexcavated rock masses and a boltedgravel plate experiment is highlyinformative for support scaling

Table 1. Comments on possible design approaches

Normal to discontinuity set

Tangent to line of support

Pb

Pb

P2

P1

Zone

of p

oten

tial s

lip

Zone

of p

oten

tial s

lip

2φ

θ

α

Fig. 5. Zones of potential slip along planar discontinuities around a circular opening


The support and excavation

scheme for each of these

type-zones can now be deter-

mined. Since the support

scheme must also address

the effects of subsequent

blasting, it is prudent to also

consider the support needs

for areas that are not to be

excavated, but are to be

secured to protect the works

and workers below. The

support scheme for the link

tunnels, the potential slip

zones of the inlet shaft, the

west face of pump shaft 1

and the east face of pump

shaft 2, represent about 22%

of the total shaft surfaces.

Failures will most likely

arise from wide, tabular

blocks and an area-grid

pattern of fully grouted

bolts, requiring only a

nominal loading, will likely provide sufficient restraint to

hold these blocks. For other areas, representing about

65% of the shaft surfaces, the basalt is considered to be

largely stable and spot bolting of suspected blocks can be

addressed at time of face inspections, in particular,

looking for evidence of tension in north and south faces that

may need support.

In the pump shafts, the excavation is undercut between 66?0

and 61?5 m PD. If the areas above the undercut zone are

0 5 10 15 20 25 30 35 40 45 50

Scale: m

Zones are defined by the joint angle of friction(taken as 30˚) about the joint normal

Cofferdam

Inletshaft

Screenchamber

Surgeshaft

Zoneof slip

Zoneof slip

Zoneof slip

60˚

60˚

Trend of principal joint set

Pump shaft 2

Pump shaft 1

Cofferdams

Bridgezone

RC bridge

Narrow necksupport zones

Abut

men

t zon

e

Abutment zone

Limit ofwidening

Fig. 6. Plan of support schemes for all shaft excavations and the narrow neck zone

Structure Location Area of application

PS1 and PS2(not excavated)

Bridge zone Defined as the zone of overlap of the potentialslip zones


Abutment zone Defined as the projected zones of a single slipzone


Upper shaft zone Other non-excavated zones (except supportcategories 1A and 1B)

PS1 and PS2 Undercut zone Inclined rock face, undercut, at the start ofexcavated faces in the pump shafts

PS1 and PS2 Narrow neck zone Defined as the excavated rock zone beneath thebridge and abutment zones

Inlet shaft, PS1 and PS2 Potential slip zones Defined as a 608 arc about the joint normal in allshafts (excluding narrow neck zone of PS)

Inlet shaft, PS1 and PS2 Other zones Other excavation zones in all shafts

Link tunnels Roofs and walls All tunnel excavations

Table 2. Proposed type-support zone areas


secured, the support may be considered as a tunnel roof and an

appropriate support scheme is devised accordingly from rock

mass schemes. The recommended support measures include

shotcrete and wire mesh to tie the surface and the convex

corner to the upper shaft areas. Also, the upper pump shaft

zones which are not to be excavated may require additional

support to minimise blasting disturbance and to secure the

foundations of the retaining walls above. Support for these

areas include bolting, grouting and shotcrete to provide a

secure surface around the entire periphery of all the shafts. Full

details of support scheme are given in Table 3.

5. ASSESSING ROCK SUPPORTNEEDS IN THENARROWNECKThe problem of a rock support scheme for the narrow neck zone

between the two pump shafts remains unresolved. Between the

two pump shafts, including the upper sections of the bridge and

bridge abutment areas as well as the wider opening beneath,

the rock mass must be secured, excavated and supported. The

problem may be considered by several methods, including

computer models, deterministic single-joint models, homoge-

neous rock mass models, precedent practice and theoretical

studies.

Faced with an unusual situation, particularly when complex

three-dimensional problems exist, computer models, if avail-

able, can help a visualisation of intuitive ideas. Unfortunately,

only the most basic models were available during the design

stage. Based upon assumed rock mass characteristics and the in

situ stresses discussed earlier, boundary element models of the

narrow neck show that a viable rock mass with a strength–

stress ratio of around 1?1, can form between the original

openings. However, when enlarging and deepening the shafts,

these models also show that widening may result in a strength–

stress ratio of around 0?8 at the narrowest section, even without

consideration of the major joint set. The two shafts, in effect,

become a large oval opening, aligned perpendicular to the

direction of maximum in situ stress. This encourages the

formation of tension in the north and south rock faces and

indicates a potential failure in the narrow neck and/or the

adjacent abutment zones. Conveniently, the models also

indicate that the extent of de-stressing roughly coincides with

the position of the abutment zones already defined.

Considering a possible failure on one of the persistent joints,

several deterministic methods can predict the effects of single

discontinuities. However, there is likely to be more than one

discontinuity that may be critical and the loading of each

critical feature is uncertain because of the three-dimensional

nature of the problem. In the extreme case, one feature may

dominate and a minimum external loading must be available to

resist the potential failure. Due to the problems of scaling

laboratory strength estimates, some form of joint strength

criterion is appropriate. The strength criterion as proposed by

Structure Supportcategory

Location Type and description

PS1 and PS2 D Reinforced concrete bridge Install bars into cofferdam walls and foundationsGrout underlying rock mass

PS1 and PS2 1A Bridge zone Tie-in through-bolts, length as required, 3 m centres, grout rock masscomplete with concrete works and before commencing narrow neckworks

PS1 and PS2 1B Abutment zone Fully grouted rock bolts, 3^5 m long, 1?5^3 m centres, avoidinginterference from bolting from other pump shaft, grout rock massComplete before narrow neck works

PS1 and PS2 1C Upper shaft zone Fully grouted rock bolts, 3 m long, 1?5 m centres, 50 mm shotcreteComplete before narrow neck works

PS1 and PS2 2A Undercut zone 100^150 mm shotcrete with mesh on undercut sections upon excavation

PS1 and PS2 2B Narrow neck zone Grouted/de-bonded through-bolts, length as required, 1?5^3 m centresRemove face-plate for blasting operationsApply 100 mm shotcrete after comparison of excavation programme

All shafts 3A Potential slip zones Fully grouted rock bolts, 3 m long, 3 m centres, upon final excavationMesh curtain to be erected during base excavation and concrete works

All shafts 3B Other zones Check for tension in north and south faces. As locally required, fullygrouted rock bolts, generally 3 m long, 3 m centres, upon final excavationMesh curtain to be erected during base excavation and concrete works

Link tunnels 4 Roof and walls As locally required, fully grouted rock bolts, 1^1?5 m long, 1?5^3 mcentres, upon final excavation apply 50^100 mm shotcrete

Support category 1 is for upper areas that have no widening.Support categories 2^4 are for areas that will experience excavation works.The term rock bolts is used on site but, only lightly loaded, should be termed rock dowels.

Table 3. Summary of support zones and description


Barton and Bandis,8 with the currently available rock mass data

and low stress levels, is ideally suited to this situation. In a

simple two-dimensional model, assume that before excavation

a through-going discontinuity with an angle of friction of 308and JRC value of 5 exists at a critical angle of 608, then the

shear strength may be estimated using the Barton joint strength

model, as

tt ¼ ssn tan fb þ JRC log10

ssj

ssn

� �2

where tt is the joint shear strength, JRC is the joint roughness

coefficient, ssn is the normal stress on the joint, fb is the joint

basic angle of friction, and ssj is the joint compressive strength.

In this case, the narrow neck is found to be stable. But, after

excavation, the upper rock zone is not thinned and so the

vertical loading on the interior remnant of the same existing

joint may be increased by around 60% since the much smaller

shearing surface is only partially compensated by a reduced

weight of overlying rock. Excluding any other external forces,

this calculation provides factors of safety of 0?99 before, and

0?94 after excavation and although the actual values of these

factors of safety are not important at this stage, the calculation

shows that any critically orientated through-going joint would

already be close to a factor of safety of 1.

The other extreme to a single critical joint is to consider that

the rock volume in the narrow neck is large enough to be

considered a homogeneous rock mass. It should be noted that

designers must apply caution when using such schemes in

respect of an assumed continuous nature of rock masses (most

particularly, for transversely isotropic masses with through-

going discontinuities) and the situation at the narrow neck

should, ordinarily, be discounted on this basis. However, a

rapid assessment can be useful, particularly as the method does

become more applicable the further one moves into the abut-

ment zones where a check on the bearing capacity is required

for the support available for a natural rock arch. The generalised

strength criterion proposed by Hoek et al.9 is as follows

ss01 ¼ ss03 þ ssci mbss03ssci

þ s

� �a

3

where ss01;3 is the effective axial and confining stress; mb, s and

a are rock mass constants (from GSI values); and ssci is the

intact rock material strength.

From rock mass classification data, the complete stress–strain

curve and the uniaxial rock mass strength may be estimated.

Using a geological strength index (GSI) value of 55, mi of 17

and an intact material strength of 100 MPa, gives a rock mass

uniaxial strength, sscm, of around 13 MPa, using

sscm ¼ 2c cosf1 � sinf4

where sscm is the rock mass uniaxial strength, c is the rock mass

cohesion, and f is the rock mass angle of friction.

Clearly, an intact rock mass could safely support the new

uniaxial loading of around 1 MPa at the narrow neck, if the

effects of the major discontinuities are ignored. Equally, the

abutment zones can provide sufficient reaction to a natural

rock arch.

Lang10 has experimented with gravel plate models in develop-

ing rules for critical rock bolt spacings and the results are

particularly relevant here since a thin, discontinuous ‘gravel’

layer used in the experiments is analogous to the discontinuous

rock mass that exists between the pump shafts. The conclusions

of these experiments do give some reassurance that a thin rock

wall can be a viable structural element if properly designed.

The mining industry also has many examples of working close

to the limits of the ultimate strength of rock masses. Indeed,

some mining techniques demand the controlled failure of a

rock mass to operate efficiently, or, routinely rely upon thin

rock walls for hanging wall support. Despite these analogies, it

is difficult to apply mining techniques in this situation since

they are frequently operated with lower factors of safety than

normally required in civil engineering and the consequences

associated with failure are much greater. Similarly, NATM or

other observational methods of construction may not be

reliable because the interpretation of deformation trends of an

already disturbed rock mass may be an impossible task. NATM

requires a known performance model to compare actual

deformations. In a small rock volume, locating or detecting the

one point that may indicate impending rock failure may not be

possible at all. Nevertheless, if failure did occur, it could

possibly be both rapid and catastrophic from one or several

blocks in either shaft and giving little time to react with

appropriate countermeasures.

In summary then, the review may appear to be pessimistic,

since all of the methods examined only give a series of

limitations to their use. Very simplistic design methods have

shown that the rock mass would probably fail without restraint.

A picture emerges that shows the rock mass to be of a good

quality (ignoring the persistent joints), but more importantly,

there are indications that parts of the rock mass may already be

in a post-failure condition. The joints are relatively strong, but

their shear strength must be maximised to maintain the current

levels of stability with an anticipated increased load and still

allow other activities to continue within the shafts. Rock

support must, therefore, aim to limit joint dilation while

ensuring an adequate face stability. Since there are indications

that the rock mass could already be in a post-failure condition,

the support should be stiff, but largely passive and should not

load the surface, because the consequences of an active loading

on this system are unpredictable.

6. THENARROWNECK ROCK SUPPORT SCHEMEThe cast-in-place concrete beam between the cofferdams (as

shown during construction in Fig. 7) will encourage the

formation of a natural rock arch through the completed upper

shaft zone. Further reinforcement and grouting of the under-

lying rock would improve rock continuity and even if

subsequent works cause a rock failure below, the cofferdams

will be secured. Nevertheless, for safety reasons, the rock

beneath the arch still requires support as well, but it is difficult

to predict the behaviour of this zone during and after

excavation.


A grid of through-going cables was originally proposed,

subsequently modified to bars, to constrain the narrow neck

and limit any further joint dilation, as shown in Fig. 8. The

spacings of through-bolts can be determined as half the narrow

neck thickness at 1?5 m rising to 3 m away from the middle, but

not greater than four times the mean joint spacing, that is, less

than 4 m. The 1?5 m spacing, not surprisingly, also satisfies the

recommendations of both Rock Mass Quality Index (Q) and

Rock Mass Rating Index (RMR) systems for surface rock

support. A 30 mm deformed steel bar was proposed, placed in

oversized holes to ease installation and allow a free flow of

grout. Grout was a high water–cement ratio mix (0?35–0?40) to

flow freely into any voids and provide an expected compressive

strength of at least 20 MPa.

The pre-installation of support provides a passive system,

designed to limit rock mass dilation as much as possible and is

also sufficiently robust so as not to be damaged by blasting.

The bars were painted with a thick bituminous paint in portions

to be excavated to discourage any hard contact with grout. The

face-plates were installed to allow a nominal straightening load

and the hole was then grouted. During excavation, at any level,

the face-plates were temporarily removed to allow rock to freely

detach around the bars. Following the blast and a face inspection,

the face-plates were replaced to the new rock surface. This

scheme provides support by maintaining the pre-blasting loads

on the interior of the rock mass. As the excavation progressed,

the tendency of any area to fail was restrained in equal measure

by the presence of passive through-bolts acting in the central

portion of the narrow neck. Monitoring was completed at each

stage and no progressive movements were seen.

The reinforced concrete beam tied into bedrock and into the

cofferdams. A rock grouting programme ensured a solid

structure over the narrowest section of rock wall. In the upper

parts of the bridge zone, where the shafts do not require

widening, the through-bolts could be entirely grouted. In the

ever-widening arch of the abutment zone, long, fully grouted

rock bolts were fixed into the rock mass to resist sliding and

provide a stable foundation for a natural rock arch and

including the bridge structure and retaining walls.

In summary, a systematic, staged support and excavation

scheme is required, which is simple to construct and at least

maintains the current safety levels. The scheme must be

developed and scaled so it can provide for safe working before,

during and after excavation, until the excavations are deepened

and lined. The adopted support scheme was a system of

through-bolts of pre-reinforcement, which could maintain the

existing loading in the interior of the rock while blasting

operations continued around the support elements.

7. ASSESSING ANEXCAVATIONMETHODOLOGYThe excavation tasks include the removal of the existing

reinforced concrete rafts at the base of both pump shafts, the

deepening of the pump shafts and rock trimming in all shafts.

The trimming of the rock on both sides of the narrow neck gave

most concern.

Fig. 7. General view, looking north over the inlet shaft and pump shafts 1 and 2 (note the construction of the concrete bridge at thenarrow neck)


Generally, for the areas away from the narrow neck, the

perimeter blasting of the shafts was excavated in a series of

12 m wide panels of two rows of 20 radial blast holes at

600 mm centres. Horizontal holes were drilled to the required

clearance depth with charge weights of 0?22 kg/m used with

nine delays, firing four holes a time. The resultant peak particle

velocities were generally less than 5 mm/s. In the 16 m zone

either side of the narrowest part of the narrow neck, charge

weights were halved and only 6 m panels were used. The

spacings, simultaneous charge and charge weights of each blast

were adjusted locally to suit actual profile requirements. Each

panel comprised two rows of ten holes at 600 mm centres. Each

row was fired by stepping around the shaft perimeter, firing

lower rows first, then the upper row of the previous panel.

The existing pump shaft bases each comprise a massive 2?2 m

thick of reinforced concrete, each containing some 2500 m3

of concrete and 400 t of steel. Drilling and blasting this

structure is a difficult task, inevitably producing high wear

for equipment designed for rock blasting. The slab was divided

into sectors, allowing effective blast designs to be exploited.

Inspections were completed after each blast and concurrent face

and shaft base works were permitted, although all shafts were

cleared for the routine late-afternoon blasts.

8. EXECUTIONAND PROGRESSSite mobilisation, de-watering, access, surveying and logging

took place between April and December 1996. All temporary

support (including shotcrete, grouting and bolting) and the

construction of the concrete beam followed up to March

1997.

Between January 1996 and February 1997, raft-breaking

(5000 cubic metres), enlargement (9200 m3) and deepening

(22 100 m3) took place. Blasting each day at two or three

locations, the concrete rafts required 400 blasts and yielded

some 12 m3 per blast, enlargement yielded 28 cubic metres per

blast and deepening some 62 m3 per blast. Typically four to six

blasts were taken each day and included inspection, barring

down and any additional temporary support installation. Fig. 9

gives a view from the base of pump shaft 1 and provides a

good impression of the scale of the works.

Work commenced on civil construction in February 1998 and is

expected to be complete by June 2001.

9. DISCUSSIONAND CONCLUSIONSIt is the case in many projects around the world that local

contractors are less experienced (or indeed confident) in the

latest techniques of rock engineering and it is important that a

team effort provides a consensus between designer and builder.

It is, therefore, a necessary requirement that the proposed

methods give confidence, by adopting the simplest schemes

that can be built. For the Bandra project, in the development of

type-zones for rock support, the on-site capabilities were

recognised and frequent discussions enabled a consensus on

After blasting 50–100 mm thickshotcrete layer

Blast holes

Face plates aretemporarily removed

during blastingand replacedon new face

Debondedportion

Cementgroutedportion

0 1 2 3 4 5 6 7 8 9 10

Scale: m

Existingexcavation33·0 m dia.

Enlarged excavation37·0 m dia.

Narrowneck

Undercut zone

(Grouted above undercut)

Bridge and abutmentzones of narrow neck

61·50 m

66·00 m

74·00 m

79·00 m

Through boltsgrouted/debondedat 1–5 m centres

Through boltsfully groutedat 1–5 m centres

100–150 mm thickshotcrete layer

Cofferdam walls

Reinforced concrete beam, 10·4 m long tied in tocofferdam walls with 24 No. 25 mm dia. steel dowels

Pumpshaft 2

Pumpshaft 1

Fig. 8. Section through narrow neck and support detail


methods and materials to be

agreed. For the excavation of

the pump shafts, some

through-bolts were replaced

with bars installed only to the

boundary of the excavation

zone in the other shaft, that

is, not completely through the

rock. The remaining empty

hole was blocked with timber.

This practical modification

was a little easier to use,

although it was not possible

to re-tension any bars after

blasting.

The situation that led to reas-

sessment of the design of

rock support and the design

of the pump shaft excava-

tions arose because of a series

of quite reasonable but

unfortunate assumptions

concerning the condition of a

previously occupied site. Due

regard was given to what

was readily available in

terms of materials and equip-

ment, for installing the tem-

porary support by the

contractor.

The support scheme adopted

was, necessarily, innovative

and had to be rapidly devel-

oped prior to excavation.

Basic theoretical rules were

followed and although many

parameters could not be

established with real confi-

dence, useful schemes and

guidelines were produced that

helped determine both the

type and scale of support. The

size of the rock mass in the

most critical zone of the

narrow neck, was a larger

volume than appropriate to

use material or single joint

analyses and, conversely, was

a smaller volume than could be reliably described as a

homogeneous rock mass. A wide spectrum of analyses had to

be reviewed and any guidelines taken from them to apply a

sensible support scheme that was not in conflict with any single

approach.

At Bandra IPS, a good-quality basalt rock mass could not be

used as an integral part of the station structure and was the

source of many problems and concerns when the basalt rock

mass was excavated during the narrow neck widening. On a

virgin site, other criteria could have led to a different design

and layout for the pump station.

As a foundation material, a uniform, compact basalt is one of

the best available. It is widely used as a construction material

with a massive bearing capacity and a largely predictable

performance. Overall, it could be argued that the rock

excavated to allow the construction of concrete walls may have

been as good, if not better, in strength and deformation

characteristics than the concrete that replaced it. The weakness

of this argument at Bandra IPS, was that the rock contained

obvious discontinuities that, for very high loading structures,

could lead to some degree of anisotropic elastic response and,

consequently, adverse bending moments in any structure built

within it. However, these poorer characteristics are relatively

Fig. 9. During excavation of pump shaft 1. (The scale is provided by the man seen at the base andthe contact is seen between basalt and volcanic breccia across the bottom of the shaft)


straightforward to resolve with either bolting or grouting being

used to even out this anisotropic character.

Watertightness is a necessity for a structure designed to contain

sewage and exclude groundwater. The resulting buoyancy of a

watertight structure is largely resisted by self-weight in the

current station design. However, in a structure with less rock

excavation, that is, with thinner internal lining walls, tension

anchors connected into the base slab and the lower wall

sections would probably be required to resolve these concerns.

None of these options could be used because of the constraints

imposed by the existing site layout and the advanced civil,

mechanical and electrical contracts. Although the present

design is a highly effective station, similar situations could

employ a simpler construction with a more effective use of the

site geology. As in many underground structures, using the

surrounding rock mass to form an integral structural element

may provide useful cost benefits. Working in a good-quality

rock, such as that at Bandra IPS, should be seen as a freely

available resource and an opportunity to be exploited.

10. ACKNOWLEDGEMENTSThe authors wish to acknowledge and congratulate the

engineer, Mr D. Kell, the project manager Mr A. Beattie and

staff of Binnie Black & Veatch and Tata Consulting Engineers,

the Municipal Corporation of Mumbai and the staff of Hindu-

stan Construction Company on the successful completion of the

work described in this paper.

Particular thanks are offered to the original engineer for the

works, Mr N. Dawes (previous partner of Binnie & Partners,

UK). His positive support and clear direction during the difficult

period at the start of the works, is gratefully acknowledged.

11. REFERENCES1. POWAR K. B. Evolution of the Deccan Province. Proceedings

of the 74th Indian Science Congress, Bangalore, Part II:

Presidental Address, 1987, pp. 1–30.

2. AVASIA R. K. and GANGOPADHAYAY M. Distribution of

secondary minerals in the Western Deccan Traps of

Bombay–Baroda coastal tract, India. Indian Mineralogist,

1984, 215–230.

3. BARTON N. R., LIEN R. and LUNDE J. Engineering classifica-

tion of rock masses for the design of tunnel support. Rock

Mechanics, 1974, 6, No. 4, 189–239.

4. BIENIAWSKI Z. T. Engineering Rock Mass Classifications.

Wiley, New York, 1989.

5. ANTONIO E. C. The identification, effects and control of weak

sub-horizontal discontinuities in a TBM excavated tunnel

(in preparation).

6. GOODMAN R. E. Introduction to Rock Mechanics. Wiley, New

York, 1980.

7. BRAY J. A study of jointed and fractured rock-part II.

Felsmechanik und Ingenieurgeologie, 1967, V, No. 4,

197–216.

8. BARTON N. R. and BANDIS S. C. Review of predictive

capabilities of JRC–JCS model in engineering practice.

Proceedings of an International Symposium on Rock Joints,

Loen, Norway (Barton N. and Stephansson O. (eds)).

Balkema, Rotterdam, 1990, pp. 603–610.

9. HOEK E., KAISER P. K. and BAWDEN W. F. Support of Under-

ground Excavations in Hard Rock. Balkema, Rotterdam, 1995.

10. LANG T. A. Theory and practice of rockbolting. Transactions

of the American Institute of Engineers, 1961, 220, 333–348.

Please email, fax or post your discussion contributions to the secretary: email: [email protected]; fax: +44 (0)20 7799 1325; orpost to Lesley Wilson, Journals Department, Institution of Civil Engineers, 1^7 Great George Street, London SW1P 3AA.


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