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

THE DESIGNAND

PE O NCEOF THE STATI D P NICHOLSON •'SENIOR

'EEF•

GEOTECNNiCA£ ENGIN ...........

SUMMARY long by 21m wide by 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 Newton Station is about 180m

15m

with the, field measurements.

INTRODUCTION

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

GROUND

CONDITIONS

At

depth,

the site is underlain

junction

Nine'

the Bukit Timah

with the

were

were

mainly

/ X/.X/

••heetpiles

FIGURE 1. SITE PLAN 147

inffiled with Fluvial Sand

the western and part of the eastern valley infilled with Lower Marine Clay (M•) to a level of

Federal

Hotel

by

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

Circus

rainfall.

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).

(F1), whilst to the Newton

heavy

Granite. The surface of this formation has been

THE SITE adjacent

of very

Geological Setting

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.

Road

OVE ARUP AND PARTNERS

was obtained by piston undrained strengths Laboratory 'and thin wall sampling. than the in situ lower samples were (C.) from these Upper The Marine Clay Figure 4. strengths, see ':vane

The soil for laborator• tests

shown in the seCtion in Figure 3. 12000 years ago, the sea level about Subsequently, the Lower Marine Clay. During dropped aga n, mixtur• of Fluvial Sands (F1), and Clays this period a deposited to about +91m elevation. About about

+90m,

as

exp£sing

(F2),

were

10000 years ago the

sea

rose again deposited to

level

Clay (Mu)

•,•

t•

and the about 101m.

was Upper Marine schemes .,..Over the past 50 years reclamation level. ground sitei'to its•present •-•;the

Marine

The

h

ave

filled

Upper and Lower Marine

silty clays

are

roots were

(electric)

(q=)

very soft to soft light greenish-grey soil with some intact shelts containing water. Very few and layers fabric features are present. Occasional peat in shown properties are encountered.

Clays