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3/12/2014 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
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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
3/12/2014 DTSS experiences - TunnelTalk
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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
3/12/2014 DTSS experiences - TunnelTalk
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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
3/12/2014 DTSS experiences - TunnelTalk
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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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