March 2017 No.76

IDI
Infrastructure

Japanese Infrastructure Newsletter

Development

Institute—JAPANCONTENTS[Report] Example of Construction of Deep Underground Cable Tunnel in Singapore—EW1 construction section—

Infrastructure Development Institute – Japan (IDI)
Suido-cho Bldg, 3-1, Suido-cho, Shinjuku-ku, Tokyo, 162-0811, JAPAN
Tel: +81-3-5227-4107 Fax: +81-3-5227-4109 E-Mail: idi17@idi.or.jp Website: http://www.idi.or.jp/english/00index.htm

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Example of Construction of Deep Underground Cable Tunnel in Singapore－EW1 construction section －1. Outline of the Project
1.1 Cable Tunnel

end with two units of shield machines. The twoshield machines were started in May and June

In Singapore, to address future requirementsforincreaseinelectricity

demand,2014, respectively, and they were connectedawith each other in January 2016. Originally,transmission line cable tunnel, which extends

the units were scheduled to connect and befor 18.5 km in a north-south direction (NSdismantled at North Buona Vista Shaft locatedconstruction section) and 16.5 km in an

in the middle of the two shafts. However, due toeast-west direction (EW construction section),delay in progress of the slurry shield on thewas planned, and construction was started in

Holland side, the length of 1,330 m to be

October 2012. Completion is scheduled for 2018.

excavated by the mud pressure shield on the

In highly urbanized Singapore，utilities such as

Ayer Rajah side was extended by 920 m and thegas, water supply, telephone lines, etc., are laidtunnels were connected under Holland Village

underground, and MRT (subway) tunnels are

Station. As a result, excavation lengths werelocated deep underground due to the congested

2,250 m by the pressure shield method androute network. Since the cable tunnel in

1,640 m by the slurry shield method.

question, when viewed from above, intersects
with the existing Deep Tunnel Sewerage
System (DTSS)，it is designed to be located at a
depth of 55 m or more, which is deeper than the
DTSS. Figure 1 shows an overall location mapof the project. It is a design and build project,which comprises six construction sections,where two Japanese companies and three
Korean companies are responsible for theirrespective

construction

sections:

NS1:

Fig. 1 Overall location map of the projectSamsung; NS2: SKEC; NS3: Hyundai; EW1:
Obayashi; EW2: SKEC; and EW3: Nishimatsu
Construction-KTC JV.
1.2 Outline of EW1

2. Outline of the Tunnel
2.1 Segment
The tunnel lining comprises axial insertion type

Table 1 shows the outline of construction of
EW1, and Table 2 shows a tunnel route map.

m, a thickness of 300 mm, and a width of 1,400 mm

EW1 has the length of about 4 km extending(width of 1,000 mm at 140 mR curve section),from the western end of the East West Line,

which were manufactured in a segment factory inand a tunnel with a maximum depth of 68 m

Johor Bahru, Malaysia and transported by land.

was constructed from Ayer Rajah Shaft at theGaskets integrally formed with water swellable

western end and Holland Shaft at the easternseals were used as a water blocking material, skew

March 2017 No.76
2bolts were used for joints, and a cushioning
material made of thin plywood was attached
between rings for preventing cracks. These
specifications were standards in the construction of
MRT.
2.2 Excavation of Grounds to be Excavated
The ground of the west side comprises
sedimentary rocks called Jurong Formation

Fig. 2Tunnel route map(maximum unconfined compressive strength:78 MPa) and the ground of the east sidecomprises granite called Bukit Timah granite(maximum unconfined compressive strength:
227 MPa), and there was a section comprising
composite ground containing a weathered soil
layer. Before entering the composite ground, aprobe drill mounted inside the shield machine

Photo 1 Mud pressure shield

Photo 2 Slurry shield

searched the boundary with the weathered soil,
and the machine excavated the ground until
immediately before the boundary and thenchanged cutters in the rock ground.
Table 1 Outline of the construction of EW1Name ofconstruction
Application oftunnelOrdererForm of contract
Contractor
Construction period
Place of construction

SPPA Cable Tunnel EW1
Tunnel for ultrahigh voltage (400 kV) transmission lines
SP PowerAssets Ltd
Design Built
Obayashi Corporation (single)
October 2012-November 2016
Ayer Rajah - Holland
1. Shaft, buildingm)Intermediate shaft, facility building: one place (inner diameter of

Details of
construction

2. Adit
Between each shaft and the shield tunnel
3. Shield tunnel
TBM1 (mud pressure): construction length L=2.25 km
TBM2 (slurry): construction length L=1.64 km
Girder height: 300 mm
Width: 1.4/1.0 m
Minimum curve radius: 140 m
4. Open-cut tunnel
235 m (including a section of 32 m where auxiliary construction
method was used in combination with NATM)
5. Entire utility work (ultrahigh voltage cable was constructed
separately)

Fig. 3 Longitudinal sectional view around theconnection

In the composite ground, earth pressure
control was set relatively high, and theslurry shield machine excavated theground by increasing the viscosity of slurry.
Further, even when the cutting face was
self-supported, it was required to maintainthe earth pressure control high due toconcerns about subsidence of the surface ofthe earth caused by lowering ofgroundwater level: on average, it was 3.5
bar for the mud pressure shield and 4.0 bar
for the slurry shield. Figure 3 shows alongitudinal sectional view around theconnection. The connection point wasdetermined to be about 80 m on the

March 2017 No.76
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sedimentary rock side from the boundary

shield. Due to the frequent cutter replacementwith the granite.

work, particularly the progress of excavation

2.3 Shield Machineby the slurry shield was delayed. A pneumatic

A total of two shield machines namely, one

facility and equipment for conducting injection

unit of mud pressure shield (Photo 1) and one

from inside of the machine were provided asunit of slurry shield (Photo 2), were used, each

auxiliary facilities for cutter replacement.

haWhen it was not possible to decrease theperipheral dome-type face plate with 6 spokes.

amount of influent groundwater to a specified

TBM1, whose ground consisted mainly ofvalue only with compressed air, it was

sedimentary rocks of relatively low intensity,

controlled by means of a reverse grouting

employed a mud pressure shield, and TBM2,

method to be described later. First, water stop

whose ground consisted mainly of hard granite,

injection from inside of the shield machine was

employed a slurry shield. Also, the maximum

planned, but it was not carried out in the

diameter of gravel capable of being taken in

construction.

was set at 300 mm, and aperture ratios of

2.4 Shaft and Adit

cutters were set to 24% and 19%, respectively.

A circular start shaft had the diameter of 14

Roller cutters were mainly employed andm, and a circular intermediate shaft had thescraper bits were provided on the side of eachdiameter of 12 m. A diaphragm wall was built

slit

major

in the upper part of the weathered ground andequipment and capacities were characterized

used as the main body and, a base rock layer

by a spherical articulation (respond to a curveat the lower part of the shaft was subjected toof 149 mR), a maximum thrust of 48,000 kN, ablasting excavation, shotcrete process, andmaximum cutting torque of 7,500 kN-m, and arock bolting, and then lining reinforced

cutterSoil

concrete was constructed. Lining of the adit

excavated by the mud pressure shield waswas conducted in a similar manner. Also, aconveyed to an excavated earth wagon byrecharge well was installed as an auxiliary

primary and secondary screw conveyors and amethod for constructing the shafts and thebelt conveyor, and then conveyed to outside of

adit,

the

injected into rocks.

opening.

Inrotationtunnel.

addition,

speed

The

of

slurrytheir

5.0shieldrpm.

machineandacement-basedmaterial

wasdischarged excavated soil by means of slurry
sending-discharging pump. Since most of the

3. Excavation Performances of Shield Tunnels
Major excavation performances of shield tunnels

ground to be excavated was hard granite, theareslurry shield machine employed 19-inch roller

workability during cutter bit replacement andcutters. The mud pressure shield employedmeasures for recovering delay in process will be

17-inch roller cutters, as the ground consisted

explained in detail in Chapters 4 and 5.

of sedimentary rocks sandstone and mudstone.

3.1 Mud Pressure Shield

Cutters were replaced 40 times for the mud

3.1.1 Excavation Datatransportation

using

a

slurrygiven

below.

Measuresfor

improving

pressure shield, and 75 times for the slurry
March 2017 No.76
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Average excavation speed was 20.4 mm/min.

with many cracks, excavation speed increased

Backfill grouting was conducted by means of

up to 15 mm/min., decreased to 5-6 mm/min

co-injection from the tail of the shield machine,when the ground was hard and further

which was standard in Singapore, and average

decreased to 2-3 mm/min. when excavation

injection ratio was 106%. An add-in material

efficiency lowered due to wear of the cutters.

consisted mainly of foam, and as otherAverage backfill grouting rate was 107%.

ingredients,

water

was

contained,

andbentonite was used as needed.
3.1.2

Issuesand

3.2.2

MeasuresduringIssuesandMeasuresduringConstruction•Due to hitting and vibration of cutter fixing

Construction•The tunnel in question passed under thebrackets, boards of the housing gradually sanktunnel of MRT Circle Line, Buona Vistaand deformed, fixing bolts loosened, and fixing

undergroundVista

metal fittings dropped off. They were taken

elevated station of East West MRT Line.

care of by conducting clad welding andChanges in the states of the tunnel andabrasive operation with respect to the housing

underground

during replacement of cutters.

station,stationmeasured,andmonitored

by

and

were

they

Buona

automaticallyweremonitoring

staff

constantly
for

the

excavation management unit.

• In the section where ground had many
cracks, rocks of large mass were taken in,causing frequent clogging of the sludge•The section located 200 m from the starting

drainage pipe between the shield machine andpoint had ground consisting of highly abrasive

the crusher on following truck No. 1. A window

igneous rock, which caused not only partial

for taking out mass of rocks was installed on

wear of the cutters but also damage of thethe sludge drainage pipe in order to reduce

center cutter housing. In order to smoothly

time for recovery from clogging, and a baffle

take in excavated earth at the center of theplate was added on the slit section for

faceplate, an opening was made and a watersecondary crushing on the faceplate.

injection line was modified so that a discharge
port would not be clogged.
• In the middle of the construction, achip-insert type was used as a centre cutter,and its range of use was broadened to theouter periphery side, which was successful in
reducing the frequency of cutter replacement.
3.2 Slurry Shield
3.2.1 Excavation Data
Average excavation speed was 8.6 mm/min.
During the excavation, the pressing force ofthe cutters was mainly monitored. In ground

4. Measures for Preventing Inflow of
Groundwater by Reverse Grouting Method
4.1 Large Amount of Groundwater In-flow
during Cutter Replacement
In January 2015, nine months after the mudpressure

shieldreplacementofstarted,cutterswhen
wasscheduledcarriedout,

groundwater of 1,300 L/min. with a chamberpressure of 2 bar came in, which we had notexperienced within the construction section. The
upper limit groundwater inflow amount according
to analysis of lowering of groundwater level andMarch 2017 No.76
5subsidence of the surface of the earth was 200

When the injection pressure begins to rise

L/min, and lowering of groundwater inflow

with the low flow volume, the above process isamount was needed, since the operation was

repeated in another injection hole. When theimpossible even under the upper limit pressure ofprocedure for areas, which are scheduled to be

3.5 bar specified by a public agency..

taken care of, is finished, necessary time isFirst of all, polyurethane was injected into thetaken for curing, groundwater inflow amount

ground around the shield machine, which had

is checked, and reinjection is conducted if

been reported to be effective in othernecessary.

construction sections, but groundwater inflow

At the site, natural water pressure wasamount remained unchanged and thus water

basically 3.5-4.0 bar and grouting injection

was considered to come from the front. The

pressure was set to 5.0-5.5 bar, and injection

construction work was expected to beholes provided on the outer periphery of thesuspended for a few months if groundrear body of the shield machine were used for

improvement were to be conducted from theinjecting grout until water injection pressure

ground. Amid continued discussion, it wasincreased in each of the injection holes. Water

decided to experimentally introduce a reverse

injection and grout injection took at least 2-3

grouting method using back-filling material,

hours and took one shift at the longest, andwhich was conceived of in a shield work incuring time was set to be 8 hours after

Ohio, USA, in concurrence with preparation

finishing grouting work.

work on the ground.
4.3 Effect of Reverse Grouting Method
4.2 Principle of Reverse Grouting Method

Injection amount (amount of leakage of mud

Figure 4 shows a conceptual diagram of thewater) for making pressure of water injected

reverse grouting method. In this method, first,

greater than natural water pressure of 4.0 bar

water is injected into a chamber at a pressureby +0.5 bar at a point where groundwater

higher than a natural water pressure (at the

inflow amount was 1,300 L/min. was about 500

site in question, initially, +0.5 bar, and thenL/min. immediately before starting grouting.

+1.0-1.5 bar) to induce flow of water toward

After the first grouting, an effect that thecracks in the surrounding ground (i.e., leakage

groundwater inflow amount was decreased toof mud water) and, at the same time, cement

825 L/min. at 2 bar was observed, and it

grouting material is discharged to outside of

decreased to 200 L/min. at 1.5 bar after second

the shield to let it naturally flow toward theand third grouting. Cutter replacement work

cracks to fill them. Water injection pressure

was started under compressed air, and it was

rises as the cracks are clogged, and water

finished

injectionamountamountarewithoutanincrease

of

the

and

grouting

injection

groundwater inflow amount, which had been adecreasedwhile

carefully

concern.

monitoring the clogging condition. Injection ofwater and grouting is continued until the
injection speed is decreased to about 10 L/min.
March 2017 No.76
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through the intermediate shaft and during
maintenance of the machine.
Incidentally, adjacent EW2 was reported tohave conducted, in addition to a similar
reverse grouting method, reverse grouting bypouring only solution A into the chamber forstabilizing the cutting face. Also, it wasreported that, at a location where subsidenceof the ground occurred, microfine cement
Fig. 4 Conceptual diagram of reverse grouting

slurry was poured into the chamber and

The reverse grouting method was conducted

decreased groundwater inflow amount toalso in the subsequent cutter replacement

almost zero.

operations when groundwater inflow amountexceeded the established groundwater inflow
amount (40-200 L/min. depending on areas),and it became possible to conduct most of thework without compressed air. However, there
was a case where groundwater inflow amount
could not be decreased to or less than thespecified value in the Jurong Formation of the
mud pressure shield section. Excavation was
continued and the cutter was overused under
the situation, but in granite of the slurryshield section, groundwater inflow amount
could be lowered to the target amount eachtime even though compressed air was used
several times.
In the reverse grouting method, a back-fillingmaterial consisting of solution A made ofcement bentonite (CB) and solution B made of
soda

was

used.

Underground Connection PointAs described above, predetermined progress
was not achieved by the slurry shield on the
Hollandside.

Therefore,

in

order

to

compensate for the delay, excavation was
conducted also from the opposite side of theslurryshieldtunnelsection

after

the

excavation reached North Buona Vista Shaft,
so

that

the

two

shieldmachines

wereconnected underground to reduce the totalmining time. In consideration of the progressof the two shield machines, at first, the mud

4.4 Grout Composition

silicate

5. Reduction of Excavation Process by Meansof Underground Connection
5.1 Background to Determination of

Since

grout

is

discharged from exploration drilling holes
provided radially on the rear body of the shieldmachine, gel time was set to be relatively long,such as 15-20 seconds, so that the insides of
the holes would not be clogged. For the mudpressure shield, injection holes were added on
the front body when the machine wentpressure shield was decided to pass the shaft,and then the length of excavation from the
opposite side was gradually increased to 500 m
and then to 800 m. Finally, the excavation
length of the mud pressure shield was decided
to dock under Holland Village station of MRT
Circle Line where impact on the groundsettlement was considered to be small.
The boundary between the Jurong layer andBukit Timah granite was located near theeastern end of the station building. In thisMarch 2017 No.76
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construction

section,Timah30) while it had many slight cracks, andgranite had less groundwater inflow than the

groundwater inflow amount was less than 10

other, there was a plan to shift the junction

L/min. RQD was expected to be within thelocation further to the east. However, the mud

wide range of 0-60 based on the result of soilpressure shield, which was configured forinvestigation in the vicinity, and therefore it

Jurong Formation, was provided with 17-inch

was difficult to predict whether the groundcutters, which means that it could have broken

condition was good or bad when excavation

easily while excavating Bukit Timah granite.

was further proceeded. For this reason, theTherefore, it was eventually decided to connect

place whose cutting face was stable and hadtheJurongless in-flow underground water compared withFormation under the station building. Figure 5

cutting grounds which had been observed byshows a plan view of the undergroundthat

connection point.

docking/connection point.

excavatedsincetunnels

in

Bukit

the

5.2 Segment Layout
After the slurry shield entered the Jurong
Formation, the faceplate and the chamber
clogged frequently, preventing the shieldmachine from excavating in accordance withthe target schedule. Therefore, the connection
point was moved to the eastern side of thestation building (close to the slurry shield side).
thiscausedanexcesswasdetermined

as

the

Fig. 6 Schematic drawing of improvement range of theconnection point

Fig. 5 Plan view of the underground connection

Consequently,

time

or

deficiency in quantity of tapered segments, but
this was adjusted by exchanging segmentswith the adjacent construction section.
5.3 Process for Arrival of Shield MachineFirst, the slurry shield arrived at theintended connection point. The cutting face
was self-supported and stable (visual RQD of

The slurry shield maintained the cutting
face pressure by feeding slurry until the mudpressure

shieldarrivedatthedoacking/connection point. In order to prevent
excessive excavation, the mud pressure shieldarrived by decreasing cutter rotation speed
and excavation speed.
After the mud pressure shield arrived at the
specifieddocking/connection

point,

earth

inside the chamber was discharged andchecked. The upper part collapsed withexcavated earth accumulated by being pressed
towardtheslurryside.

Further,sinceintermittent fall of earth was observed, slurry
was fed again from the slurry shield side inorder to maintain the cutting face pressure.

March 2017 No.76
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After a dry spray machine was carried intothetunnelandprepared,discharged,

and

spray

prepacked

materialslurrywhen ground improvement was necessary. In

wasorder to prepare an improvement zone in theusing

ground near the docking/connection point, it iscompression

easier to do so from inside of the machine,strength: 50 N/mm3, bending strength: 6

because boring distance can be shorter andN/mm3) was sprayed at a thickness of 50 mm

boring and injection can be conducted at an

and thereby suppressed the fall of earth.

optimum elevation angle, and it was also easy

Eventually, an articulation jack of the mud

to conduct additional boring and injection.

pressure shield was stretched and separation

Figure 6 shows a schematic drawing of

between the roller cutters and the cutter bits

improvement range of the connection point.

concrete(28-dayof the slurry shield (their roller cutters hadAperiodofgroundreleaseatthealready been removed) was decreased to 50

docking/connection point was determined to be

mm. The hollow on the upper part was filled

2 months, and the upper limit groundwaterwith cement-based grout after the shield

inflow amount was set to be 60 L/min. based

machines were dismantled, and water stopon the analysis result.

iron plates were installed.

Actually, groundwater inflow amount after

5.4 Survey Accuracy for Connecting Shield
Machines
Duringarrival of the two machines was as low as 10
L/min. and further, it decreased to a few

the

daily

L/min., which was difficult to measure due toinspection of control points and automaticthe aforementioned reverse grouting andtracking survey of the shield machines were

supplemental injection of an aqueous material.

conducted by a total station. In each tunnel, a

Therefore, boring & injection operation, which

control point was subjected to gyro survey

had been planned to be conducted as needed,

after completion of the initial boring andwas not conducted.

before arrival of the shield machines. Owing to

5.6 RC Lining

the

in

RC structure was employed for lining of theconnection of the shield machines was 5 mm inunderground connection point including inside

the horizontal direction and 20 mm in theof the shield machine. Thickness of RC lining

vertical direction.

was set to be 240 mm without taking into

resultsshieldofthese

excavation,

surveys,error

5.5 Grouting at Connection Point
There

was

also

a

plan

to

consideration the outer shell of the shielddecideamachine left behind. Spraying was conducted

docking/connection point in advance andon the ground from which the faceplate of the

conduct ground improvement from the ground,shield machine was removed at a thickness ofbut it turned out difficult to prepare an

50 mm, and water stop iron plates were

improvement zone with accuracy by obliquely

welded on the entire ground to connect it withboring the ground at a depth of 70 m.

the outer shell of the shield machine. The total

Therefore, it was decided to improve thelength of 18 m was subjected to four times ofground from inside of the machine after

concrete placements: the lower part: once, andprimary dismantling of the shield machine

the upper arch: about 6 m x 3 spans.

March 2017 No.76
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All reinforced steel & steel formwork

tunnel, then it was delivered by a 5-t forklift

materials were carried in by a storage battery

for about 900 m from there to a fixed concrete

locomotive after dismantling of the shieldpump installed before the docking/connection

machines was completed and before removal ofpoint, and then it was subjected to pressure

underground rails and crossties were started.

feeding and placement.

Concretewastransported

by

means

of

Concrete with design intensity of 60 MPa

pressure feeding from the ground of the

had slump of 230±30 mm and coarse aggregateintermediate shaft to a 2-m3 bucket in theof 10 mm, and coagulation time was 4 hours.

Technology developer: OBAYASHI CORPORATION
Contact：IMAZATO Toshiya
E-Mail: imazato.toshiya@obayashi.co.jp
TEL: +65-6471-2870 / FAX: +65-6471-5136

HAII Kohei

Construction chief, SP Cable EW1 Construction Office, Overseas Branch, Obayashi Corporation

IMAZATO Toshiya

General Manager, SP Cable EW1 Construction Office, Overseas Branch, Obayashi Corporation
21A One-North Crescent #01-01 Shingapore 138537

March 2017 No.76
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