Singapore's DTSS has thrown many a challenge at the contractors involved in its construction. Shani Wallis, TunnelTalk Editor, reported from Singapore.
- Celebrations echoed across the oceans on 24 January when the last TBM on the Deep Tunnel Sewerage System (DTSS) in Singapore finally holed through. The event, for local contractor SembCorp after a long, painful slog of more than three years to complete the 7.6km x 4.9m o.d. north drive for Contract T-05, also marked the completion of all 48km of tunnel excavation for the DTSS project.
Map of the complete DTSS scheme in singapore
- It also celebrated extremes. In its final stages the Herrenknecht EPBM set the record for all production rates achieved on the DTSS with a best shift of 22.04m in 12hr, the best week at 210m, and the best month at 631m. Earlier in the drive, the TBM struggled through difficult mixed face conditions under high hydrostatic groundwater pressures of up to 5bar.
- Ground freezing was required on one occasion to enter the plenum for inspection and cutter changes. Implementation of controlled dewatering in conjunction with compressed air entry facilitated tool changes and improved overall progress rates.
- This is a remarkable turn around for a machine that had at one stage been lifted completely out of the ground for major modifications.
- The previous project record of 625m in one month, was set on the 4.6m Herrenknecht EPBM on Contract T-06 by Züblin.
- Since modification to fit a new cutterhead, a new screw conveyor, and a new main bearing (a task of about three months from a purpose built shaft) the relaunched T-05 machine "completed just under 5km in 15 months," said David Helliwell, Construction Manager for SembCorp.
Reasons to be cheerful! Breakthrough of the T-05 north drive TBM in Singapore after more than three years of tough going
- Breakthrough of this last machine was also more than six months earlier than anticipated, based on the previous difficult and slow progress of the drive. A ceremony to celebrate the finish of all excavation on the project was being planned at the T-05 site. Project progress is now working towards commissioning the lower reaches of the scheme by August 2005. Treated affluent from the Seletar Water Reclamation Plant (WRC) will be diverted into Shaft G of the T-04 contract and flow south to a new influent pumping station shaft at Changi WRP and out via a new 5km long sea outfall pipeline into the Straits of Singapore.
- This meets the beginning of the agreement with Malaysia to discontinue discharge of all effluent by Singapore north into the narrower Straits of Johar. The lower reaches of the system are now complete with their corrosion protective membrane and in-situ concrete inner lining and the last section on T-04 is scheduled to finish in time for the August commissioning date.
- The design and supervising engineer for the DTSS tunnel network for Singapore's Public Utilities Board (PUB) is a CH2M/Parsons Brinckerhoff JV.
- On T-05, casting of the inner corrosion-protective lining for the 4.8km south drive will be complete by mid-March, using three 10m long shutters. "We will use four shutters and cast one pour/day, six days/week for a weekly production of 240m/week to complete the north drive tunnel by the end of 2005," said Helliwell.
- For the T-05 SembCorp team, the next tunnel job is C856 on Singapore's new MRT Circle Line. The contract was awarded in October and SembCorp will use three Herrenknecht EPBMs to complete the 7.5km of bored running tunnel.
View inside the 2.4m id remotely controlled EPB/slurry dual-mode pipejacking machine
- Elsewhere in the world others also celebrated the T-05 breakthrough. Senior managers who started off the project, before collapse of initial JV partner Phillip Holzman, are now working on the Kárahnjúkar Hydro Scheme in Iceland and in Malaysia on Kuala Lumpur's SMART Tunnel Project.
- The next phase of tunnelling for the DTSS in Singapore is not scheduled until 2015 or later. That second phase encompasses another 40km plus of deep level excavation on the west side of the island state.
Breakthrough for EPB/slurry link sewer drives
Just days later on 9 February, the last drive of Züblin's Upper Thomson Link Sewers contract, also for PUB and at a higher elevation, broke through into the U2 drop shaft on SembCorp's DTSS T-05 contract.
- Züblin's contract comprised about 1,000m of smaller diameter microtunnelling (400mm and 600mm i.d.) another 1,000m of 1.2m i.d. pipejacking. At the deepest level is a further 3,013m of 2.4m i.d. remotely controlled pipejacking at up to 40m deep.
- The final 65m long drive was completed by the Züblin's 2.4m i.d. dual mode slurry/EPBM. Believed to be the first of its kind, the machine is designed and supplied by Herrenknecht to cope with geological conditions that varied frequently and abruptly from hard fresh granite to soft marine deposits including estuarine clay. It was only designed to operate in a highly permeable interface zone between the upper residual soils and the rock below in which groundwater pressures were up to 3 bar.
- The screw conveyor of the dual EBP/slurry TBM stays in service in both modes and the change is achieved by interchanging a rock crusher unit and the transfer belt conveyor. In slurry mode, material transfers from the screw conveyor into the rock crusher box and from there, down to the slurry out-bound pipeline for treatment at a simple de-sanding unit on the surface. When transferred back to EPB mode, the crusher is moved back, out of the work area, and the belt conveyor is extended back beneath the discharge gate at the top end of the EPB screw conveyor.
Breakthrough on the last drive on Züblin's Link Sewer tunnel contract in Singapore of the Herrenknecht dual mode slurry/EPB pipejacking machine
- The operating mode was changed twice in mid-drive. In both cases the machine started in EPB mode, jacking through soft soils with negligible amounts of ground water, until, within a few metres, entering the soft soil face with sand and weathered to fresh granite under high ground water pressures. In order to avoid over excavation and high groundwater inflows through the screw conveyor, and to be able to maintain the face pressure, the mode was quickly changed, within a few hours, from EPB to slurry.
- In total some 1,423m of pipejacking was through rock. "In the early days, pipejacking through very hard rock caused a problem with the main bearing," said Josef Kofler, Chief Engineer on the project for Züblin. "With temporary measures the machine completed the last 70m of the drive.
- Then during the overhaul to fit a replacement bearing, we also made modifications to the cutterhead and to the suction system of the prototype machine's slurry mode. From there the machine was running," continued Kofler. "Our best progress rates recorded on the job were 25.2m in a single 12h shift; 25m in a day; and 201m in a week."
- "Due to the good performance of our men and our equipment the Upper Thomson Link Sewers Project will be completed eight months ahead the schedule," said Kofler. "For all our pipejacking activities a high level ground-monitoring system was implemented. Instrumentation included surface and deep level settlement points, vibration sensors, crack metres, water standpipes, piezometers and inclinometers. The effect of our pipejacking works on the surrounding ground and structures has been negligible."
The good, bad and mixed on the Singaproe DTSS - TunnelTalk, April 2004
60km of tunnelling completed for Iceland's Kárahnjúkar power station - TunnelTalk, April 2008
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