DTSS Experiences - TunnelTalk

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The good, bad and mixed on the DTSS Apr 2004

Fig 1. Alignment of the DTSS (Deep Tunnel Sewerage System)

By early 2004, the six design-build

contracts and eight EPBMs engaged on

the first phase of the DTSS in

Singapore had completed more than

87% (42km) of the 48km alignment.

Tunnelling had started in early 2001

and when TunnelTalk visited the

project in mid-January 2004, five of the

eight TBMs had finished excavation

and casting of the final in-situ concrete

lining, with its integrated corrosion

protective internal membrane, was

underway. The sixth and seventh

machines were just weeks from holing

through as the last of the set

continued to struggle through difficult

mixed ground conditions in the north.

For the most part the 48km alignment

divides into two geological zones.

The four contracts in the south-east

run mostly through old alluvium

deposits at up to 50m below the surface and beneath a high ground water table, while the two on the north and

west (Contracts T-05 and T-06) run through more challenging hard rock and mixed hard rock/soft ground

conditions. On the southern contracts in less permeable old alluvium, the four contractors used soft ground EPBMs

with open spoke cutting wheels and operated at pressures up to 3 bar. These four machines of 5.5m to 7.26m in

diameter (one from Germany and three from Japan), recorded consistent progress at averages anticipated by the

clients project engineers, the CH2M/ Parsons Brinckerhoff JV, and the design-buiId contractors and their

consultants (Table 1).

Penta-Ocean believes its 7.7km tunnel on Contract T-02 is the longest drive for a single EPBM of its size. The

7.16m o.d. NKK EPBM achieved a progress best of 602m/month and an overall average of 55m/week, working two

12h shifts/day, six days/week, to complete the 7.7km long drive in just under 35 months.

The machine performed well in 65% of the alignment, in quite good sandy conditions, in which only water was

required as a conditioning agent to create an effective pressure balancing plug in the 8m long screw. For the other

30%, polymers were needed to create a plug in more permeable, lower standard penetration test (SPT) soils. For

SITE REPORT - SINGAPORE

TunnelTalk Archive

At 48km long, the Phase 1 network of the Deep Tunnel Sewerage System (DTSS) in Singapore passes through

much of the island's highly variable geology. Conditions vary from soft compressible clays to loose running

sands under high ground water pressures, to zones of very hard, highly abrasive, fresh granite and stretches

of completely decomposed rock soils. Conditions change in some cases from metre to metre with menacing

mixed faces in-between. All are taxing the skill and abilities of men and machines alike. Shani Wallis,

TunnelTalk Editor, reported from Singapore.

Contract Contractor Design-build manager Length EPB TBMsFinished

dia

Award

price

T-01 Woh Hup/Shanghai Tunnel Babtie BMTHarris & Southerland 5.8km 1 Herrenknecht 6m i.d. S$80.6m

T-02 Penta-Ocean Construction Co T Y Lin South-East Asia 7.7km 1 NKK 6m i.d. S$95.6m

T-03 Kumagai/SembCorp JV Hyder-Geo-Consultant 5.3km 1 Kawasaki Hl 6m i.d. S$68.8m

T-04 Samsung Corporation Meinhardt/Halcrow 7.3km 1 Mitsubishi Hl 4.3m i.d. S$74.2m

T-05 Philip Holzmann/SembCorp JVST Architects &

Engineers/Babtie12.5km 2 Herrenknecht 3.6m i.d. S$139.5m

T-06 Ed Zblin Meinhardt/Geoconsult 9.7km 2 Herrenknecht 3.6m i.d. S$91.8m



The T-05 south drive Herrenknecht

a 200m stretch through an outcrop of hard granite on the drive, the TBM required modification. The outcrop was

discovered during additional site investigation work required by each design-build contractor after contract award

and before start of construction.

As a result, modifications were made during

TBM manufacture as well as on site prior to

entering into the granite zone. These

modifications included installation of additional

arms on the open spoke cutterhead, reduction

of the cutterheads open ratio, installation of

six thrust grippers in the forward can of the

TBM shield, and an increased number of disc

cutters.

Design-build contracting

While a great deal of the design criteria for the

fully gravity-fed trunk sewer tunnels is fixed,

the DTSS client team, part of the Singapore

Governments Public Utilities Board (PUB),

elected to award the construction contracts on

a design-buiId basis, "principally to optimise

the time saving advantages," explained

Chiang Kok Meng, Director of the PUB DTSS

Department.

The scope and size of these large diameter tunnelling contracts is a new undertaking for the PUB. "Under the

design-build concept, design, manufacture and delivery of the TBMs could progress concurrent with final design,

saving several months on the overall project schedule. On the down side, as the client, we have less direct

involvement than is possible under conventional design-bid-build contracts. Nevertheless we are pleased with the

design-build experience and would follow the same approach for future projects."

For the design-build contracting teams the room to manoeuvre is limited. The grade and alignment of the tunnels

is fixed and the use of certain methods such as dewatering is restricted.

T-06 Herrenknecht (2)

"To avoid the potential for surface settlement

we specified the use of positive closed-face

slurry or EPB tunnelling, and disallowed

dewatering without explicit approval," said

Mrs Ang, Deputy Director of the PUB DTSS

Department. "We also required the design-

build contractors to undertake further site

investigation borings, to supplement the

geological investigation data provided by us

during contract tender, and for them to

prepare their own geotechnical interpretive

report for the purpose of refining the design

of the selected TBMs, confirming their

methods for excavation, and for anticipating

conditions to be encountered. In addition,

and in all cases, contractors had to have

their designs reviewed by an independent

checker before they came to us for final

checking and approval."

For the most part the strategy has worked. The incidents of surface settlement above such a long distance of

tunnelling, through ground conditions of known treacherous nature, and beneath vital surface traffic infrastructure

has been low and of slight impact - by both Singapore and international standards.

An incident at a launch shaft as one of the TBMs was mining through the tunnel eye into the drive is the most

serious event of ground loss to date. It caused a face loss depression on a street some 40m above, which

resulted in road closure during the night hours to fill the depression and have the road fully available once again

for the morning rush hour. "We have had no other major face loss incidents and have not caused any major

settlement of either the roads or the MRT transit network, under which the DTSS tunnels pass several times, said

Robert Marshall, Project Director for the CM2M/Parsons Brinckerhoff (PB) JV. "Now, with most of the tunnel

excavation finished, the highest risks of surface settlement are behind us and I believe the record is very good

given the tunnelling conditions and DTSS tunnel routes"

Greatest challenge

The greatest challenge for contractors on the project is the most northerly of the eight tunnel drives. For

Singapore contractor SembCorp (now fully responsible for the contract following collapse of its joint venture



T-02 NKK

partner Philipp Holzmann in 2002) progress

on Contract T-05 using two 4.93m

Holzmann-procured Herrenknecht-supplied

rock-face EPBMs has been particularly slow

due to exceptional geological conditions.

The North Drive in particular was running

almost two years behind schedule when

TunnelTalk visited in January 2004.

One of the most difficult aspects is that the

fixed tunnel horizon runs along the highly

undulating interface between peaks of very

hard, massive fresh granite, of recorded

compressive strengths of 200MPa-30OMPa,

and valleys of weathered to completely

decomposed granite soils under a full head

of up to 52m hydrostatic ground water

pressure. "With a fixed gravity flow

alignment we cannot move either deeper

into more consistent hard rock conditions,

or higher into longer reaches of soft ground conditions," said David Helliwell, Construction Manager for SembCorp.

"As it is, the machine can drive out of an almost vertical interface of hard rock into a full face of water-charged

decomposed soils. To avoid over-excavating the running soil, the TBM operator must change immediately from

non-pressurised operations in full-face rock, to full EPB operation at the first indications of soft ground in the face.

The same then applies in reverse."

Also, the granite is highly abrasive. Discs break on impact with hard massive rock ledges in mixed faces, and

others are worn into several flat spots by being jammed and released in succession by chips of hard, massive rock

lodging in the housings. Interventions into the excavation chamber to replace worn discs can be as frequent as

every stroke, with an inspection at least after every fourth ring or 4.8m.

T-03 Kawasaki

On Contract T-05, a large percentage of

these interventions must be carried out in

compressed air to the maximum 3.4 bar

pressures allowable by Singapore law.

Hydrostatic pressures have been higher

however and on one occasion, ground

freezing was required to provide stable

enough conditions for an intervention.

Maintenance has also required repair of

excessive wear on the screw conveyor,

cutterhead and body structure.

Work has also been needed to strengthen

the cutterhead support struts against fatigue

caused by the high thrust pressures used to

penetrate the rock and high torque

encountered in the soft and mixed face

conditions. "At the start of the project we

did not expect to need such frequent

interventions," said Tay Ching Khiang,

Project Manager for SembCorp. "Also, most of the interventions we expected to undertake in free air in zones of

full faces of rock. The reality has been at the other extreme."

TBM design

With no room on the forward bulkhead of the 4.93m diameter direct variable frequency electric drive TBM for

modular air locks, a secondary bulkhead is located at the back of the 12m long leading shield can, just ahead of

the segment erector and ring build area. The space between this and the leading bulkhead is the single large air

lock. Through this, a maximum three workers can pass through into the forward chamber, as well as be

decompressed back into free air. This is a major consumer of time, since at the maximum 3.4 bar working pressure

and according to the German compressed air working tables that are being used, a maximum of 2hr working

requires a minimum 3hr 49min in decompression, which provides for a maximum possible 8hr of working in each

24hr period of man-entry intervention compressed air operation.

To avoid the time and cost of having to apply ground freezing to execute interventions, the contractor applied for

controlled dewatering along the alignment to lower the ground water and reduce hydrostatic pressures to the

within the maximum 3.4 bar man-entry pressure.

"Given the circumstances and the logistics of the alignment in this not so heavily urbanised area of the island, we



T-04 Mitsubishi

agreed to strictly controlled dewatering at

certain zones along the route of the most

troubled North Drive," said Marshall for

CH2M/PB and the client. "In addition, the

tunnel route was moved horizontally within

the easement under the 6-lane Seletar

Expressway closer to the verge to allow for

the installation of a limited number of

dewatering wells and away from potentially

more serious impacts of any dewatering-

induced settlement. Tunnel alignment was

also adjusted to allow otherwise offset

access shafts to be directly overhead and

create further opportunities for easier TBM

wear repairs in free air."

While the South Drive machine of the two

identical Contract T-05 TBMs has struggled

on in its original configuration to finish its

shorter 4.8km long South Drive, conditions on the much longer 7.6km North Drive proved too onerous. In

September 2003, the contractor decided to retrieve the machine from an intermediate shaft and undertake a

major refurbishment that took two months. "Among other things, we fitted a longer screw conveyor and fitted a

new, more abrasive resistant cutterhead with larger openings to help clear the spoil from the front," said Helliwell.

T-01 Herrenknecht

The new cutterhead, it was said, has shown

a much-improved performance in the rapidly

changing conditions. Reports from Singapore

confirmed that the South Drive - scheduled

for a hole-through in March 2004 when

TunnelTalk visited in January 2004 - did

actually hole-through on 8 March. Meanwhile,

by late March, the North Drive had added

about 600m since the start of the year. It

had passed through another three sections

of very abrasive, mixed face material, which

required another complete replacement of

screw conveyor due to wear. By the end of

March, and with the ground conditions

remaining highly variable, the TBM was some

3.6km into the 7.6km long drive with 4km to

go.

On Contract T-06, through conditions that

also required hard rock EPB TBM excavation,

contractor Zblin made better progress. By January 2004, with one drive finished and the other about to hole

through, the contracts two 4.48m Herrenknecht rock EPBMs had recorded a best monthly advance of 625m on its

North Drive and an average weekly rate for both machines of 59m and 55m respectively. Conditions included full

faces of both hard granite rock and soft decomposed granite soils but with a significantly lower percentage of

mixed face conditions.

The Contract T-05 North Drive is the tunnel causing undoubtedly the most delay to the project but fortunately it is

the most northerly drive. The client will be able to commission the lower reaches of the network once they are

complete and the network of new higher-level interceptor feeder sewers are commissioned.

Protective inner lining

As the T-05 North Drive is left to battle on, all other contractors are well into the final stages of casting the inner

membrane-protected in-situ final lining. As part of the owner's 100-year maintenance-free design life criteria, the

corrosion protection PVC or HDPE membrane must be applied integral with the cast concrete. For each contract,

different methods of casting this lining are being applied. Some are casting the 330 membrane-protected arch

first, and coming back to cast the invert, while others are casting the invert first and the arch as the second pass.

The contractors have selected HDPE membrane from either AGRU of Austria, Steuler of Germany, or Engineered

Linings of South Africa. "The key to effective membrane installation is to place enough tension on the tough

inflexible minimum 2.5mm thick material as it lies draped over the arch shutter before starting the pour" said Owen

Griffiths, CH2M/PB's Resident Engineer on Penta-Ocean's Contract T-02. "Otherwise ripples will form and sections

of the membrane will have to be cut out and reinstalled."

Only on Contracts T-02 and T-04 have contractors Penta-Ocean and Samsung elected to cast the final lining using

a full round telescopic shutter. When on site, Penta-Ocean project manager, Nakayama, showed TunnelTalk an



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A completed section of the membrane-protected in-situ

concrete final lining of the 6m i.d. tunnel on Contract T-01,

with the 30 invert section yet to be cast as a second pass

illustration of the two 37.5m long

Japanese-manufactured shutters that

were at the time being assembled at the

mid-point of the 7.7km long tunnel. He

described how the invert element will hold

the membrane in tension while the full

round lining will be cast to the tunnel's

final 6m finished diameter. Hopes were

high that the system, and the two

shutters, would allow the lining to

progress at up to 1,000m/month.

One down, Phase 2 to go

Back in 1999, keen international

competition for the DTSS design-build

contracts attracted between 10 and 12

tenders for each, resulting in very

competitive prices. Few of the bids

received exceeded the projects budget

estimates. Under the design-build

procurement strategy and once

completed, the owner of the DTSS will

have acquired a 48km system of large diameter, 100-year design life infrastructure at a total contract bid price

cost of US$329 million. This is within a project total of US$2 billion that includes a new high capacity treatment

plant and a 5km long outfall comprising two 3m i.d. precast concrete pipes installed in sea bed trenches off the

south east coast of the island.

Phase 2 of the DTSS on the south-west side of the island comprises another planned network of deep level

tunnels with another large treatment plant and outfall structure. It is scheduled for construction after 2015 - long

after the current Phase 1 experiences have passed into the archive of Singapore's quite particular tunnelling

history.

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DTSS Experiences - TunnelTalk