The Design and Performance of the Retaining Walls at Newton Station - Nicholson.pdf

THE

DESIGNAND

PE O NCEOF

THE

STATI

P NICHOLSON

...

'EEF•

D

•'SENIOR

GEOTECNNiCA£ ENGIN

OVE ARUP AND PARTNERS

SUMMARY

Newton Station is about 180m

long

by

21m wide

by

15m

deep.

Ground conditions at the site

comprise

soft

f•arine

Clay

up to,20m

deep overlying

Fluvial

§ands.

• .id

Clays

which are underlain

by

Cor0!bletel•'

•:•

Decomposed

Granite.

Inv,estiga.,tions

i•entified.•threeZ:.-:•:il

'buried'chan•ls •nning t•ansve•'sely

•cross

the station

The paper discusses the

design

and

performance

of the

perimeter diaphragm

walls

at the eastern end of the

station,

where a

reasonably

complete

set of

instrumentation data are

available.

The

predicted

movements of the

top

down excavation are

compared

with the, field measurements.

INTRODUCTION

Newton Station forms

part

of the _new

Singapore

Metro which is

being

constructed for the Mass

Rapid

Transit

Corporation

(MRTC).

In addition the station has been

designed

_{as a} Civil Defence Shelter. The main

contractor for this

design

and construct contract was

Dragages Sembawang

Construction. eve

Arup

and

Partners _were their

designers.

THE

SITE

Road

adjacent

to the Newton Circus

junction

with the

Bukit Timah

Road,

see

Figure

1. At the western end of

the station is a crossover which also formed

part

of the

contract but will net be discussed _{in this paper.} Ground levels at the site vary from 103.0m in the

west to 102.5m in the east. This is close to the mean sea level of 100m.

Occasionally

the eastern end of the

site•is•fi0ode•

durin•

periods

of _very

heavy

rainfall.

GROUND

CONDITIONS

The

results

of five

phases

of site

investigations

comprising

56 boreholas and 25 Dutch _cone tests

revealed

highly

variable

ground

conditions at Newton

Station and the

CrosSover.

The soil classification in this •.paper is

in-accordance

with Andrews and Morton

(1).

Geological

Setting

At

depth,

the site is underlain

by

the Bukit Timah

Granite.

The surface of this formation has been weathered in situ to

Completely

Decomposed

Granite

(G4).

Contours of the

top

of the G4 are shown in

Figure

2. These _{show the presence of} three buried channels

running

in _a N-S direction across the station. These

channels _were

probably

eroded

during glacial

periods

when the _sea level was

Iowar,

as discussed

by

Pitts

(2).

The

investigations

showed that the eastern and

central,

valleys

were

mainly

inffiled with Fluvial

Sand

(F1),

whilst

_the

western

_and

_part

of the eastern

valley

were infilled with Lower Marine

Clay

(M•)

to a level of

Hotel

Federal

Nine'

/ X/.X/

••heetpiles

FIGURE 1. SITE PLAN 147

about +90m, as shown in the seCtion in

Figure

3.

Subsequently,

about 12000

years

ago, the sea level

dropped

aga n,

exp£sing

the Lower Marine

Clay. During

this

period

a

mixtur•

of Fluvial Sands

(F1),

and

Clays

(F2),

were

deposited

to about +91m

elevation.

About

10000 years ago the sea level rose

again

and the

Upper

Marine

Clay

(Mu)

was

deposited

to about 101m.

•,•

.,..Over

the

past

50 years

reclamation

schemes

h

ave

filled

t•

•-•;the

sitei'to

its•present ground

level.

Marine

Clays

(M)

The

Upper

and Lower Marine

Clays

are very soft to soft

light

greenish-grey

silty clays

with some intact shelts

containing

water.

Very

few

soil

<:'<

fabric features are

present. Occasional peat layers

and

roots were

encountered.

Index.

properties

are shown in

The soil for

laborator•

tests was obtained

by

piston

'and thin wall

sampling. Laboratory

undrained

strengths

(C.)

from these

samples

were lower than the in situ

':vane

strengths,

see

Figure

4. The

Upper

Marine

Clay

undrained

strength design

line is also shown in

Figure

4. It

represents

about 80% of the average field vane

strength.

For _a

plasticity

index of 70•/o

Bjerrum

(3)

recommended

a 70% reduction in the field vane

strength.

This

design

line

generally

corresponded

to _a

ratio of

c•/p=

0.23 where p• is the

preconsolidation

<tpressure. The direct correlation between Dutch

(electric)

cone

resistance

(q=)

and field vane

(%)

was 12

and

15

for

Upper

and Lower Marine

Clay

respectively.

Typical

consolidation

parameters

obtained

from

oedometer

and Delft

piezocone

tests are

summarised

in Table 1. An

undrained

soil modulus

(E•)

was derived

assuming

Eu/c•

450.

_.[

q

mopofG4

/

\

s'l

k_/,

i" )

_t

₎

J

1

°s,

Borehole ' Piezometer •'°'Set-t

em•Rt

point

i

In&linometer

FIGURE

2. PLAN OF

STATION

metres 100- 90- 80-- 70--

Ftii

Mu

FIGURE

3.

SECTION

A A -,/Mglm 40 80 120 1.4 Cu kNlm= 1.8 0 20 40

Laboratory

• Vane

i• Design

o'•:

line

FIGURE

4. MARINE

CLAY PROPERTIES

TABLE 1. CONSOLIDATION

PARAMETERS

Soil

Oed6rfiet•r'

Piezocone

Fluvial

Deposits (F)

The Fluvial Sands

(F1)

and

Clays (F2)

between the M arid

M•,!ayeps.•ar,•:•hig•!y

variable. The Sands

comprise

very oose to

;Ioose,-gFey

fine to _coarse

ang,ular

quaGz

sand with

•some•:si!t•,:,Th•,

Clays

comprse

firm

m6•led

•:e•LI•6WH

"•ii•

and

•lays•

desiccation and a

design

c. •kN/m• was

adopted.

A

drained

angle

of internal resi•ance

•

35.5 was

determined from trivial and shear •x tests.

Howeve•

a

value of •o was used in the calculations. Index

prope•ies

and SE results for the Fluvial soils •are

:•p•ed in-Figure'

5.:

•

:•:

•:

•'%"•:•:•'•"

•onsolida{ion

p•ram•tePs

for:the

:Clays

(F2)•Areset

out in Table 1. The Delft

•iezocones

recorde•,,

5ydrost&tiC

•Snditions:

•n

•e

S•H•

(F1):•sh•]•:}•t

they

were

h•hly

per•eabl#..A,correlation,be•een

Dutch

(ele•ric)

cone

resis{a•c'e

(q• •:•

0.SN MN/m 2 _was established for

•the

clays..(E2)

E

j%

450 was

adopted. •(,the•saBds

a drained modulus

•

_•.5q•

was

esed:"

•

Completely Decomposed

Granite

(G4).

•his

s0il

comprises

medium dense

becoming

ve• dense with,

"'8•#th;

sCeckled grey-gree6Lbrown-red

clayey

sil•

withT

some fine to coarse

angular

qua•

sand. Index

prope•ies

and S• results are shown in

Figure

5. At

the eastern end of the station numerous inta•

granite

boulders were encountered,

paGicularly

be•een the

central and eastern buried

channels,

see

Figure

2.

These were

exposed

during

excavation

and were oEen

found to be more than 2m across. Rock head was not established

[n

any of the boreholes.

The

silty

nature of the G4 soil means that it is

sensitive to

sample

disturbance and

rapidly

becomes

partially

saturated. Therefore in situ tests were relied

upon to derive

parameters.

The

following

correlations

were

used;

q= 0.165N MN/m

•,

q¢

30%

and c.

5.5N kN/mZ•

Consolidation parameters

for the G4 soil

"are

set out in Table 1. The soil modulus was assessed

;_,.u.sing,E•c•_

650.

A drained

angle

of internal

•i•hce

•:•

3•

-•

was

used.

Pumping

Tests

Pumping

tests were conducted to assess the

dewatering requirements

for the station and the

possibility

that the Fluvial Sand

layer

might

be

horizontally

continuous. Well W3 was installed at the

position:shown

in

Figure

2. Fluvial Sand was

-encountered between 16.7 and 20.0m

depth

in this well. To monitor

drawdown,

piezometers

were

installed

in

separate

boreholes in the F1

layer

and in the G4 at about the

depth

of the

proposed

toe of the

diaphragm

wall

(80m

level).

These

piezometers

were installed

along

the

length

of the station and were

subsequently

used to

monitor the excavation

dewatering.

Pumping

from the 275mm diameter well was started on the 2.4.84 and

continued for 7

days.

The water level in the well was

kept

at 86m elevation and the

pumping

rate settled down to about 1.2m3/hr.

The

piezometers

in the F1 reached their

steady

state

after 1

day

whilst those in the G4 stabilised after 2

days.

The initial

piezometric

levels

together

with those after 7

days

pumping

are

plotted against

the distance

from the well in

Figure

6. This shows that drawdowns of

about 2.5m and 2.2m occurred in the F1 and G4

respectively:to

• distance of 100m from the well.

Beyond

this distance little drawdown occurred. This showed that

the F1 _was

hydraulic•Jly

continuous over most of the

station

length.

It should also be noted that the drawdown in the F1 sand was

always

greater

than in the G4. This is

consistent with an

upward

flow of

water

into the F1

layer

-and then

horizontally

to the well. The

calculated

permeabilities

of the F1 and G4 were about 8 x 10 "s and

5 x 10 • m/sec

respectively.

The

rapid

response of the G4

confirmed

the use of

effective

stress

parameters

and

steady

state

drainage

conditions

rather than

adopting

undrained

strength

parameters

for the G4.

During

excavation the roof slab was also used _{as a}

support-from

which the

£oncourse

sl.ab

was

temporarily

hung.

Th,

e roof also

provided

a reaction for the

inclined

struts, see

Figure

7 temporary

stage.

Once the

permanent

base

slab,

central

columns

_and

internal walls had been

constructed

the

hangers

and

inclined struts were

removed

see

Figure

7

permanent

stage.

Tl?e

central

columns

enabled

the roof slab to

be

In

addition,

ground

surface

settlements

were

i:monitored ddring

the

.pumping

test. The

recorded

used

:tO

r•ist thelong

term

hydrostatic

pressure

acting

nts

between-the

2nd and

12th

April

1984

al;e

•

"•settteme

[

ed •'

•."

Tl•t•.t•porai•':•hangers

and inclined struts were

plotted

in

Figure&

These

settlements deveop

steadily

throughout

the

pumping

test and resulted from

the

consolidation

of the Marine

Clay.

Ti•

e

•T,.;i-I'

'•

efe

n

•

•

requiremeni-for,

a:

heaviiy

r•inforced'.

2.0m thick roof slab was

incorporated

into the

top

down

construction

by using

it as the

top

strut for the walls.

relatively

short and

therefore

easy to manoeuvre in the

very soft

ground

conditions

encountered

whilst

excavating

below the roof slab. Another

advantage

of the

'iiqclined strut#

was that

they

did not obstruct the

excavation

to the

final

formation

level.

Studies

had

indicated

that if the inclined strut had

be•en•.•it{•l•¼th•n-il•

•0utd have been necessary to increase the

diapt•ragm

wall thickness from 0.8 to 1.0m

and to

deepen

the toe

embedment

depth.

zFinal

9.4.84

10 20 50 100

DISTANCE FROM WELL W3 metres

'/•

KEY 7

Borehole

No •' Initial

(2.4.84)

:•-•:;•'•

FIOURE

0,

R•S•LTS

OF

P•PINO

TEST

metres

Roof slab Central column

)urse

slab

Internal

walls 0.8m

Diaphragm

wall

r

DIAPHRAGM

WALL

DESIGN

The

design

of the

diaphragm

wall involved the

following:

i)

Seepage

analysis.

ii)

Lateral

stability

analysis

to determine toe levels.

iii)

Wall deformation

analysis

for

bending

moments.

the

passive

earth pressure coefficients in accordance

with CP2

(4).

Full wall adhesion was assumed in the M

and F2

clays

whilst wall friction of 0.5

#•1

was

adopted

in

the F1 and G4.

iii)

Deformation

Analysis

The wall deflections and

bending

moments associated with excavation of the

station were calculated

using

the

computer

program

Bearing capacity

for vertical loads. BILL described

by Pappin

et

al,

(5).

A cracked wall

iv

v•)

Lon _g term h

_•

drostatic u-lift

_•

resistance section stiffness El 355 MN.m 2 was

used,

this

being

:L.:•..:•..

i

:._•: •

-:::-•

.i:•about_hatf•the•al.astic

section

stiffne.ss.

N,o,

factors

of.

These

analyses

are shown

diagrammatically

in

Figure

safety

were

,a,pplled

to

!he

"best

e.stlm,ate,

.de•sl•gn

,,s,o,•l,

8. To cater for the

varying

soil conditions the

perimeter

parameters.

Mowever, the momen[s CalCu]a[ea oy •[.

wall was

eventually

divided into 9

design

cases, were

increased

by

1.2 and 1.6 for the reinforced

i)

Seepage

Analysis

Ove

Arup

and

Partners'

finite

element program SEEP was used to calculate the

steady

state

piezometric

pressures

around the toe of the wall. These

piezometric

pressures

were used in the

calculations for toe embedment

depth

and deformation

analysis.

SEEP also

predicted

flow rates so that the

pumping

requirements

inside the station could be

assessed. Where the buried channels contained F1

(s,•,r•d

it _was necessary to extend the toe of the wall 2m

the

underlying

G4 to

p•'ovide

.a

hydraulic

cut-off and

limit

drawdown outside the station

during

excavation. For

design

purposes

the extema

-drawd•wn

was

•l!m!ted

to

101m in the Fluvial

Sands.

ii)

Toe

Depth

The toe

embedment

stability

was assessed

using

a factor of

safety•f

2

on

concrete

design

in the

temporary

and

permanent

conditions

respectively.

Ground settlements were estimated from the BILL

predictions

of tateral wall movements

using

the method

proposed

by

Milligan

(6)

with a small modification based

on field observations

reported

by

Tan et

al,

(7)

for the

neighboudng

MOE

building

shown in

Figure

1. Details

of these movements are

given

in

Figure

9 where D is

the

depth

at which the maximum lateral wall

movements occurs. Additional allowances were also

made for

diaphragm

wall installation and consolidation

settlement,

iv)

Bearing

Capacity

During

excavation the

diaphragm

walls

provided

the vertical

support

for the roof and

C•ncourse

Slabs.

Excavation within the station led to a

•:ed•ctionlof

the

_overburden

_{pressure and effective}

stress at

the

toe of the wall and hence a reduction in

Seepage

Toe

stability

Bending

/

Deformation

Bearing

capacity

FIGURE 8.

DIAPHRAGM

WALL DESIGN

20 10 ' 0 ':•,l•.•i 20 10

6.8m

incIin0meter

bearing

capacity.

The horizontal effective stresses for

calculating

wall friction were taken from the BILL

deformation

analysis.

The maximum

allowable

piezometric

levels at the toe of the wall were calculated

at various

stages

of the

excavation

to maintain the factor

of

safety

of 2 on the ultimate

bearing

capacity.

During

the final excavation

stage

about 30% of the

working

load was

calculated

to be taken

by

end

bearing

on

the

base of the

diaphragm

wall.

v)

Hydrostatic

Uplift

On

completion

of the base slab and internal walls the water

pressure

was allowed to

return to its

original

level.

Calculations

showed

that the

dead load of the station and the

diaphragm

wall were

adequate

to resist the

hydrostatic

uplift

pressures.

FIELD

PERFORMANCE

Monitoring

The

performance

of the wall

during

excavation

was

monitored

using

inclinometers, settlement

points

and

piezometers.

The inclinometers were either

installed

within the wall

('"•,anels

or

just

outside, as with inclinometer

I3,

see

Figure

2. Ground surface settlement

points

were located

at different distances from the

excavation.

The

piezometers

were

installed

in

boreholes

outside the

excavation either in the G4 or in the

overlying

Fluvial

Sand. In addition

piezometers

were

installed

immediately

below the base of the

diaphragm

wall

and

inside the station

excavation,

e.g. TP6 and IPI.

The toe

piezometers

and wall inclinometers were

installed

by

rotary

coring

through

steel tubes

attached

to the

diaphragm wall

pane!,

reinforcement

cages. This

provided

the

opportunity

to core the soil

immediately

below the base of the wall and

confirmed

that all

slurry

and

excavation

debris had been

removed

and therefore

good

end

bearing

was

provided.

Diaphragm

Wall

Installation

Previous

experience

with

completely decomposed

granite

in

Hong Kong

has demonstrated the

importam•

of

maintaining

an

adequate

net

slurry

pressure to maintain trench

stability

and limit

ground

_movements,

see Davies and Henkel

(8).

At Newton Station the

table was often less than 0.5m below the ground

surface and therefore the

guide

walls _were

_extended

lm

above the

ground

level,

to

provide

the net

slurry

pressure. Ground settlements of 20ram were

allo•d

_let

in the

design.

The

ground

movements recorded

by

inclinometer

during

the

excavation

of

panel

58 followed

by

57 are shown in

Figure

10. These show inward movements of

up to 15ram

occurred

in the G4

during

excavation.

However outward movements of 20mm occurred in the

Marine

Clay during concreting

to about 0.5m

above

ground

level. The

concreting

also resulted in the

adjacent

ground

heaving by

about 10mm.

Excavation

Before

commencing

excavation,

new datum

readings

were

established

for all settlement

points

and

inclinometers. The

excavation programme

is shown at the

top

of

Figure

11. This

figure

also shows the

settlement records at

$1,

$2 and

$18,

together

with the

changes

in

piezometric

levels inside the

perimeter

wall

(]P1),

below the toe of the wall

(TP6)

and outside the

wall

(B5a

and

B5b).

Their

positions

are shown in

Figures

2 and 3.

Pumping

from

the

wells within the

diaphragm

wall

box was started at

ihe beginning

of

January

1985. The

rapid

drawdown

in the F1

layer

inside the box is shown

on

Figure

11. The more

gradual

response

at the toe and

outside the

excavation

can also be seen. The

pumping

rate was controlled to suit the

calculated

piezometric

level

requirements

Set

for the toe

piezometers.

Once the base slab was cast. the pumps were

switched

off

(August

1985)

and the wells allowed to overflow onto the base slab.

To reduce drawdown outside the

excavation external

recharge

wells were installed in the F1 sand

layer.

These were fed with water

pumped

from the internal wells. T3 metres 0 Fill -20 -10 • 102.6m 2¢ 58 57 56 .:

L__.-_:•::•".

:•!:

± -:;: F

GURE::I0,

i3RO.UN[•MOVEMI•.-N•.,S.--:D•E[•TO;INSTA•

LATION OF -:" !

::PANEL:,58

•bLEO•E•"iB•?P'RNEIL

57:

,;;.

152

o

•o-

104 100- E •" 92- LU 84- 80-

Roof

Concourse•

Base _•I

• • • Excavation [• • Concrete

r-11984

1985 N O N D j

_F

M A M J 1J A S O

•B5a

BSb TP1

FIGURE

11. MEASURED

SETTLEMENTS

AND

PIEZOMETER

READINGS. 102.6 CaseA 1"3 Design DEFLECTION

((•)

mm

L40

80 0 0

40x•

80

' 7--I-FI

Roof

Concourse 40 80 120

/'

"28.685

:Base

FIGURE

12.

PREDICTED

AND

MEASURED

DEFLECTIONS.

The lateral movements

recorded

by

inclinometer I3 for .the

roof, concourse•and

base slab

excavation

stages.ar•

shown in

Figure

12. The

predicted

wall

deflections

for

the Case A soil

profile

at the same

stages

are also shown.

The roof slab

excavation

to 100m level in this

part

of

the station was

completed by

10.10.84. At this time about

20mm lateral

deflection

had

occurred

at the

ground

surface,

see

Figure

12. The roof slab was not cast until

the end of November and

by

22.11.84 creep effects had

-increased the movement to

60ram,

see

Figure

12. This

was similar to the

predicted

surface deflection.

However

below

12m,

depth

in the F2 and

G4,

the

predicted

deflections were about

twice

those

measured.

This may be associated with

underestimating

the small strain

•.stiffn-(•ss•of.theq•situ-soil•:-•h•_predicted

and

measured

deflections for the

subsequent

excavation

stages

were •n

.reasonable

agreement,

see

Figure

12. The maximum

predicted

deflection

was about 100mm

compared

with 110mm

measured.

The inclinometer I3

deflections

recorded

on the 28.7.85 are

plotted

in

Figure

13. Also shown are the

'"undrained"

surface settlements

predicted

from I3

using

the method

given

in

Figure

9. In

addition

Figure

13

shows the

settJements recorded

for

$1,

$2

and $18 on the 28.7.85. It can be seen that these settlement

points

have settled about 100mm more than those

predicted

from I3. This

.additional

settlement is due to the

consolidation

of

the Marine

Clay

resulting

from drawdown in the Fluvial

deposits.

The effect of the

consolidation

settlement can also be

seen from the record of $2 shown on

Figure

11.

Rapid

increases in

&ettlement

were

recorded

during

the

concourse an•I base

excavation

stages

in

February

and

May

1985.

However,

a more

gradual

settlement

of about

:10mm/month

occurred between these

excavation

stages.

This settlement rate is similar to

$18,

which is

outside

the influence of

exctavatien

,movemen•,•n.•'•is4her•fo•

considered

to reIlect the

consolidation

of-the Marine

DISTANCE

FROM WALL

metres

DEFLECTION

30 20 10 0 100mm

•100

200

28.7.85

!

Inclinometer

T3

_---• /

profile

28•7.85

30-

FIGURE 13. GROUND SURFACE SETTLEMENTS.

CONCLUSIONS

Pumping

tests and

piezocone

tests

provided

valuable

information on the

permeability

and

hydraulic

continuity

of the Fluvial Sand

layers.

These tests also demonstrated

that

steady

state

(flownet)

drainage

conditions would be

rapidly

established in the

underlying

Decomposed

Granite and therefore effective stress soil

parameters

were

adpoted

in these soils.

The

top

down construction

technique

proved

to be

successful. This used the 2m thick roof slab to

support

the concourse slab and

provide

the reaction for the

temporary

inclined struts which

propped

the final excavation

stage.

The Ove

Arup

and Partners'

computer

program BILL

was used to calculate deflections and

bending

moments

in the

diaphragm

walls. The program

incorporates

the

effects of wall

stiffness,

propping

system

and

ground

conditions. Reasonable

agreement

with the measured deflections _was obtained.

As the excavation

progressed

the

predicted

wall

(•-'•

_{deflections and toe}

_piezometric

levels _were

compared

with the field instrumentation records. This enabled the

Contractor,

Designer

and MRTC to monitor the

performance

of the wall at all

stages

of the excavation. A

simple

method of

predicting ground

surface settlements associated

with

wall

deflection

_was

adopted.

This method differs from

the

approach

described

by

Peck

(9)

because the

predicted

surface settlements _are

directly

related to the wall deflections rather than the excavation

depth.

It has been used to

separate

the

undrained deformation settlements outside the wall from

the consolidation settlements associated with

dewatering.

REFERENCES

(1) ANDREWS,

D. and

MORTON,

K.,

Geotechnical Studies for

Singapore

Mass

Rapid

Transit. Proc. 2nd

Conf. _on Mass

Transportation

in

Asia,

Singapore,

1984,

pp 272 283.

(2) PITTS,

J.,

A Review of

Geology

and

Engineering

Geology

in

Singapore. Quarterly

Journal of

Eng.

Geol.,

London, 1984,

Vol.

17,

pp 93 101.

(3)

BJERRUM, L.,

Problems of Soil Mechanics and

Construction on Soft

Clays.

Proc. 8th Int. Conf. Soil

Mech. and Found.

Eng.,

Moscow, 1973,

Vol

3,

_pp. 109

159•"

(4)

CP2.

Earth

Retaining

Structures. The Institution of Structural

Engineers,

London,

1951.

(5)

PAPP1N, J.W.,

SIMPSON,

B., FELTON,

RJ. and

RAISON, C.R.,

Numerical

Analysis

of Flexible

Retaining

Walls. Proc. Int. Conf. on Numerical

Methods in

Engineering: Theory

and

Applications,

Edited

by

Middleton,

J. and

Pande, G.N.,

Swansea,

1985,

pp. 789 802.

(6)

MILLIGAN, G.W.E.,

Soil Deformations Near

Anchored

Sheetpile

Walls.

Geotechnique,

London,

1983,

No.

1,

pp. 41 55.

('7)

TAN,

S.B.,

TAN. S.L. and CHIN

Y.K.,

A

Braced

Sheetpile

Excavation in Soft

Singapore

Marine

Clay.

Proc.

11th Int. Conf. _on Soil Mech. and

Found.

Eng.,

San

Francisco, 1985,

Vol.

3,

_pp. 1671 1674.

(8) DAVIES,

R.V. and

HENKEL, D.J.,

Geotechnical

Problems associated with the

construction

of Chater Station. Conf. on Mass

Transportation

in

Asia, Hong

Kong,

1980.

(9)

PECK, R.B.,

Deep

Excavations and

Tunnelling

in

Soft

Ground.

Proc. 7th lnt. Conf. Soil Mech. and

Found.

Eng.,

Mexico

City,

1969,

State of the art volume,

pP.

225 290.

ACKNOWLEDGEMENTS

The Author would like to thank his

colleagues

in Ove

Arup

and Partners and

Dragages

Sembawang Construction

_who

jointly

developed

the

design

and monitored the excavation

performance.

The assistance

provided

by

MRTC

in