Construction of Pipe Roof and Support System in Proposed

Outlet Drain Undercrossing North Buona Vista Road, Singapore

T Sun, W& M Consultants, Singapore. wnmconsultants@gmail.com Mr. Chelliah_MURUGAMOORTHY, Land Transport Authority, Singapore,

Dr.Lingting Zhang, Or Kim Peow Contractors (Pte) Ltd, Singapore

Abstract: The project in the paper included design and construction of a permanent 120m long RC box culvert and

sewer diversion at North Buona Vista Road (NBV) Singapore. The pipe roof method was selected by the design

and build contractor in fulfilment of the contract requirement for the 53 m box section undercrossing NBV Road. The

adopted methodology comprised the installation by pipe jacking of numbers of interlocking steel pipes followed by

traverse series of steel frames for safe mining of the tunnel. This served to maintain uninterrupted service conditions for the

road, subways tunnel and other services. There are many challenges in the construction of pipe roof and mined tunnel. This

paper deals with the consideration in the design and construction of such works. It also highlighted the

problems encountered during the construction, subsequent solution adopted and protection of existing utilities as well as

monitoring of settlement of the surrounding areas of the project.

1. INTRUDUCTION:

The Land Transport Authority of Singapore (LTA)

Contract C 8288 for Tanglin Halt Outlet Drain at North

Buona Vista Road comprises the design and

construction of a 120m long permanent RC

(reinforcement concrete) box culvert, sewer diversion

and other related works located along Commonwealth

Avenue, Commonwealth Avenue West and North

Buona Vista Road.

Part of the works under this contract is the construction

of approximately 53m of drainage box culvert under-

crossing North Buona Vista Road using approved

tunneling method.

The Contractor Or Kim Peow Contractors (Pte) Ltd

(OKP) commenced the project on 15th December 2008

and the whole works were scheduled for completion on

14th December 2010.

The contract’s requirement mandated that uninterrupted,

smooth road and rail traffic be maintained, and that the

sewer and other utilities be protected against damages.

After careful consideration it was concluded that with the

adoption of pipe roof, mined tunnel method, the

requirements can be met. The outlet drain box culvert

under design and build contract was completed with

minimal deviations, good safety and quality standards

and within the time schedule and cost control.

The project was divided into various zones and areas as

shown in Fig 1. Fig 2 shows the longitudinal section view

along with the mined tunnel. The outer dimension of

drain is 7m wide by 5m high (inner 6m x 4 m). The

gradient of the proposed outlet drain is 0.1% ( 1: 1000).

Fig 1. Layout of proposed drainage box culvert

Fig 2. Longitudinal section of drainage box culvert

2. GEOLOGICAL / SITE CONDITION

The ground condition along the alignment of the mined

tunnel is given in LTA’s soil interpretative report (LTA

2009a). The profile is derived from the soil logs at

boreholes ABH 2, CC4739C, ABH 3 and ABH 4 and is

summarised in Fig 3.

Fig 3. General soil profile of the mined tunnel zone

The geotechnical design parameters adopted in the

design is shown in Table 1 (Parson B., 2009).

Table 1. Design soil parameters

3. DESIGN AND BUILD CONTRACT

The part of the contract involving the drain undercrossing

North Buona Vista Road was to be on a design and build

basis. The Contractor’s appointed design consultant

studied the soil condition and other information on the

project and designed the pipe roof over other method of

construction for the drain. This was achieved over

several months of presentations and detail proposal to

and close discussions with LTA and PUB (Public

Utilities Board), the later being the Authority which will

take over the drain upon its completion.

The key design considerations as detailed in various

communications with LTA(LTA 2009b) and other tender

and contract documents consisted of factors such as:

i. Geotechnical aspects, including soil-structure

interaction between pipe-roof and surrounding

ground, stability of the excavated face and

base stability during excavation;

ii. Structural aspects such as the design of the pipe-

roof and the structural steel frames supporting

the piped-roof;

iii. Soil improvements and types and extent of

protection to existing services;

iv. Other pertinent points related to method and

operation included: the adoption of 3 sided pipe

roof pipe, type and capabilities of jacking

machine used, their maintenance of face pressure,

excavation speed especially when encountered

with obstructions such as sheet piles, timber

pieces, skill of operators, details of pipe clutching

and damages expected to be caused by excessive

settlement , mitigation measures and others.

Risk assessment and hazard analysis including alert and

suspension levels for instrumentations such as

inclinometers and settlement markers were thoroughly

considered in the Report for Overall Constriction Impact

Assessment (CIAR) (Parson B. 2010) and adopted during

the construction stage.

4. CONSTRUCTION

4.1 Construction methodology

The temporary earth retaining structure to support the

ground during the excavation of the mined tunnel

consists of a series of 780mm diameter, 16mm thick

circular steel pipes interconnected by clutch to form an

inverted U pipe-roof structure around the footprint of

the proposed drain. This method known as pipe roof/

arch, have been deployed in USA and Singapore in the

last twenty years, as reported in Martha Railway

Underpass ,USA (Atalah, A.L. and D.T. Iseley, 1991)

Orchard-C.K .Tang Underpass Singapore (Darling, P.,

1993).

The pipe roof formed by 22 rows of steel pipes ,

comprised of 10 at the top and 6 on either side .They

were installed in predetermined sequence, starting from

the lead central row on top, and consecutively on left

and right before the vertical rows were done.

The interlocking steel pipes forming the pipe roof

together with the support system is shown in Fig 4.1 to

Fig 4.5

Fig 4.1 Transverse view of interlocking pipe roof

Fig 4.2 Details of Pipe Clutch

Fig 4.3 View of interlocking pipe roof with steel entrance frame

Fig 4.4 partially completed pipe roof

Fig 4.5 Complete pipe roof with interlocking pipes

The pipe roof (Parsons B 2009) is in-turn supported by

structural steel frames at every 1.5m’s interval. Each

excavation step is also 1.5m with the width of the

excavated slope also limited to 1.5m. Three sections of

soil improvements were provided and these are located at

the interfaces with the jacking shaft, the receiving shaft,

and a 10m zone at mid-point of the mined tunnel.

Drainage holes (2 nos.) were also provided to release any

built-up of excess pore-water pressure during the

excavation. Soil reinforcement in the form of

reinforcement bar arranged in a square grid pattern at

0.65 m spacing both ways, would be provided to

stabilize the face of the excavation in the F2 layer. The

face of the excavation would also be protected

temporarily during each stage excavation with a layer of

short-crete of 15mm thick.

After the pipe roof and the last portal frame( at the

tunnel opening into the receiving shaft) were installed,

the soldier piles and the sheet piles around the shaft were

cut. The permanent drain section inside the mined tunnel

between the jacking shaft and the receiving shaft were

then cast. Some of the piped-roof pipes were filled with

approved cement-based backfill material before the

casting of the final drain section, whereas others were

done after that to ensure that the steel pipes do not pose

any corrosion problems in the long run..

4. 2 Construction problems encountered

The major construction problems encountered include

those realted to utilities protecton, pipe jacking and

monitoring of settlements.

4.2.1 Protections of utilities

Many utilities were encountered in the project areas and

need to be diverted or protected. These were identified in

the CIAR Report ( Parsons, B 2010).

The West End line viaduct and the MRT (CCL 4& 5 )

Tunnel, which was laid some 4.4 m below the propsed

outlet drain were checked for the effect of the excaation

of the shafts, mined tunnel and found to have maximum

settlement of 6 mm, much less than the allwoable 15mm

according to LTA Civil Design Criteria ( LTA 2009 c)

and LTA Code of practice for RailWay Protecvtion.

(LTA.2009 d).

The plan and cross section view showing the utilities is given in Fig 5 and 6

Fig 5 Plan view showing major utilities

Fig 6 Cross section showing major utilities and the

relative distance from the proposed out let drain

Fig 7a shows the service MRT Line tunnels under the

mined tunnel, and the influence zone of the services

Fig 7a MRT tunnel underneath of mined tunnel

Fig Fig 7b,and 7 c shows the cross sections of the major services( MRT /Sewer lines and MRT viaduct ) .

Fig 7b- Section taken at Ch.. 0+055

Fig 7c-Section taken at Ch 0+065.

Fig 8 shows the general summary of utilities

encountered and the protection measures adopted.

Fig 8 . Utilities and protection measures adopted

The services and utilities and their allowable settlements

at the influence zones of the mined tunnel are indicted in

Table 2.

Table 2. Existing utilities within the area of mined tunnel

Utilities Protection

measures

Allowable

different settlement

Allowable

absolute settlement

The West

End line

viaduct

NA(Within

allowable limit) 15 or 1:1000 *

NA

CCL 4&

5 Tunnel

NA (Within

allowable limit 15 or

1:1000*

150 dia

water pipe

(Abandoned)

300 dia

water pipe

Re-propping

1:240

SingTel/

Star Hub cables

Re-propping

1:100

1650 dia

sewer

Within allowable limit

(estimated less than 1: 300)

1:300

230kv

cables

Support by

TAM Grouting

1:200

22kv

cables

Support by

TAM Grouting

Joint bays(4 m

above

Culvert)

Support and jack up

1:275 20

mm

*Whichever lesser , NA Not Applicable

Instrumentations were installed to measure the effect of

the works on them.

The mandatory requirements for the maintenance of

uninterrupted service for the services offered many

challenges to the project. At rare occasions when

settlements occasionally exceeded the suspension levels,

the pipe jacking and tunnel mining works were stopped

to enable lifting of the services to adjust the base

condition to normal to avoid damages to the utilities.

This is done in compliance of the approved( by the

authorities) stringent method. The laborious monitoring

process, with daily /weekly instrumentation meeting in

the presence of LTA, PUB and other agencies helped to

ensure that all such measures were carefully evaluated

and solutions decided upon expeditiously and promptly

carried out to prevent any further excessive settlements.

The requirements of Power Grid were particularly

strenuous on the joint cable bay at the centre of box

culvert along North Buona Vista Road, and cumulated

in the complex web of instrumentations installed (refer to

Fig 9 ).

Fig 9 . Layout of Joint Bay & Instrumentations

4.2.2 Pipe Jacking

Pipe jacking force for each pipe was calculated to be 50

tonne for the 53 m of jacking. This was mostly achieved

with the jacking force ranging from 50 to 80 tonne for

all except three rows of pipe that encountered

obstruction, which were unforeseen. These obstructions

were in the form of sheet pile, galvanised steel pieces and

timber residues, which were left in place by earlier

contracts. These had not identified during the tender

stage. The jacking force encountered for these cases went

up to over 150 tonne and additional time was needed to

complete these rows. However with skilled operators

and proper design and site management, no significant

damages were encountered in the works or utilities

during the process.

The particular case of encountering of sheet pile during

jacking proved to be very challenging indeed. Jacking for

this particular row progressed smoothly until the cutter

head of the machine hit very hard object, later found to

be sheet pile. A thorough investigation was carried, and it

was decided that a series of steel caisson of diameter

1.8 m were to be driven and the soil excavated , so as to

enable the cutting and the extraction of the sheet pile

for a number of locations. The works were carried out

overnight with partial lane closures to ensure minimal

traffic interruption along the North Buona Vista Road.

Luck does seem to play a big part as the sheet piles

were encountered only in one row of jacking.

The pipe jacking works were planned on a 24 hours

basis, alternating between actual pipe jacking and butt

welding of the steel pieces and testing of the welds,

which took over 4 hours for each joint. This resulted in a

cycle of 9 days in between each row of jacking. Two

jacking machines were utilised and the teams’ work to

jack the pipes in set predetermined designed sequence to

enable proper pipe roof to be formed after all pipes

have been jacked.

The three rows which hit obstructions - took much longer

(over 30 days each). In all, the jacking 22 pipes was

completed from Aug 2009 to Jul 2010, a total of 11

months. This is within the overall construction period

planned for the pipe jacking phase.

4.2.3 Settlement Control and Monitoring

The CIAR Report (Parsons B., 2010) helped to design

and adopt a comprehensive monitoring system as shown

in Fig 10 . This provided the systematic monitoring of

the ground settlement of the surrounding areas and other

changes such as in ground water table and other as a

result of construction of the pipe jacking and the mined

tunnel.

Fig 10. Layout of instrumentation and monitoring system

The allowable limits for settlement as set in CIAR are as follows:

• Alert level =0.7 x deisgn values

• Suspension level =deisgn value

The maximum settlement at the ground level was

expected to be along the centre of the mined tunnel

axis, at some 20 m from the jacking shaft. Fig 11 shows

the predicted settlement (Wong K S, 2009 ) along the

traverse section of 51mm, or maximum gradient of 1/250

or 0.4%. This was well within the LTA allowable limit of

1/100 (LTA 2009e).

Fig 11 The predicted settlement along the traverse

section during various stages of excavation

During construction, instrumentation monitoring

meetings with the contractor, specialists, sub contractors,

the instrumentation personnel, authorities (such as LTA,

Power Grid, SingTel, PUB) and consultants were held

daily / weekly accordingly to the needs. In the meetings,

the participants evaluated and decided on rectification

measures and adjustments to the procedures to ensure

working within the alert and suspensions levels and in a

safe and acceptable manner.

Plots in Fig 12 depict the readings taken with the prisms

installed to measure the combined effect of pipe roof and

mined tunnel. The prisms were installed along pipe L1

next to the lead pipe (The lead pipe is in the centre of the

top rows of pipes) which was expected to produce the

maximum settlements. The maximum settlement at this

pipe was 30 mm at about 20 m from the jacking shaft,

after the completion of the tunnel mining . This is below

the allowable of 125 mm , and predicated of 51 mm. At

two other prisms locations in the centre of the left and

right vertical pipes, maximum settlements were much

less than this figure.

Fig 12. Plots showing readings of Prisms installed along

the axis of pipe, after jacking and tunnel mining

5. SITE ORGANISATION AND MANAGEMENT

The contractor engaged a number of specialists and

subcontractors such as grouting, shaft construction , pipe

jacking, instrumentation and monitoring experts, as well

as experienced design consultants and supervision staff

to complete the design and build outlet drain . The well

coordinated efforts of these stakeholders resulted in

overall completion of the project with minimal time and

cost overrun.

During the course of the construction, the authorities

such as LTA and PUB are constantly on the scene to

provide the necessary monitoring and back up support,

and give prompt decision to enable problems to be

resolved.

Daily instrumentation meetings were troublesome but

useful to ensure all stakeholders are up to date and to

provide promptness of action.

In all, the project represents a successful implementation

of the good design and build construction mode for a

difficult and delicate project.

6. CONCLUSION

The pipe roof component of the contract C 8288 is the

most challenging of the project, and it was dictated at the

tender stage by the Client ( LTA) that it be done on a

design and build basis. Many stakeholders including the

client and other authorities such as PUB, Power Grid,

and SingTel etc were initially apprehensive on the risk

posed by such method. However with the direct and

active involvements of all parties, including designers,

and the accredited checkers, and with skilled

operations of the jacking and other specialists, the pipe

roof and the tunnel mining were done with uninterrupted

service of the bored tunnel, roads, sewer lines and

other utilities.

The strict procedures adopted on the work control

especially on monitoring and remedial measures taken to

contain settlement issues were strictly successfully

implemented. The tight control and reporting

requirements ensured that works were carried out within

safe and quality guidance, and within the project’s time

frame and contract cost approved. The lessons learnt

from this project, especially in mutual consultations and

control can be applied universally in similar projects.

ACKNOWLEDGMENT

The authors would like to thank the Contractor, Or Kim

Peow Contractors (Pte) Ltd (OKP) and Land Transport

Authority (LTA) for permission to publish this paper.

The views presented in the paper are solely that of the

authors, unless otherwise stated.

REFERENCES:

Atalah, A.L. and D.T. Iseley, 1991. "Pipe Arch

Horizontal Drift Method for MARTA's Transit Extension

Under I-285," Proceedings of Rapid Excavation and

Tunneling Conference, SME, Chapter 24, June, pp. 435-

453.

Darling, P., 1993. "Jacking Under Singapore's Busiest

Street,” Pacific Rim Supplement to Tunnels &

Tunnelling, summer, pp. 19-23. Jacking under

Singapore’s Busiest Street.

LTA 2009a, LTA interpretive soil report for C 8288.

LTA 2009b, Correspondence between OKP/LTA and

Authorities (LTA comments on contractor’s response to

LTA earlier comments on DAR 8288/CON/0019(DATED

7 May 09).

LTA 2009c ,LTA Civil Design Criteria

LTA 2009d ,Code of practice for RailWay Protecvtion.

LTA 2009e, Code of practice for works on Public Street.

Parsons Brinckerhoff, 2009, Design Report for Mined

Tunnel submitted to LTA.

Parsons Brinckerhoff, 2010, Report for overall

constriction Impact Assessment (CIAR).

Wong K S, O/ 2009, Independent report on Tanglin Halt

Outlet Drain-mined tunnel.