Prequalification is about to get under way for construction of the multi-billion dollar Phase II of Singapore’s Deep Tunnel Sewerage System (DTSS).
Prequalification for TBM excavation of 50km of sewer tunnels of between 3–6m i.d., plus associated shafts and manholes, will begin on 29 April (2016). The new 40km network of deep tunnels, plus a further 10km of larger diameter link sewers, will extend the existing DTSS Phase I to serve the western and southern parts of the island (Fig 1).
DTSS Phase I, comprising 48km of TBM-driven conveyance tunnels in the north, east and central parts of the island state, a 3km outfall tunnel at Changi Water Reclamation Plant, and a further 60km of 0.75m–3m diameter link sewers, was completed in 2008 at a cost of US$3.4 billion and following six years in construction. A total of eight EPBMs (5 x Herrenknecht, 1 X NKK, 1 x Kawasaki and 1 x Mitsubishi) were procured to deliver mechanized excavation of the main tunnel.
Phase II preliminary design for the owner, the Singapore National Water Agency (PUB), was completed by Black & Veatch/AECOM earlier this year (January 2016) following a 21-month design period and with Ramboll acting as sub-consultant.
“We are now looking for industry partners with established track records to take us another step closer to the completion of DTSS Phase II,” said Yong Wei Hin, PUB DTSS Phase II Project Director. “This is an opportunity for the underground construction industry to participate in the delivery of one of the world’s most anticipated water infrastructure projects that will ensure Singapore’s water sustainability for generations to come.”
At this stage it is envisaged that the 50km of TBM-bored and segmentally lined tunnels and link sewers will be let in five design-build packages (Fig 2). The secondary lining is to consist of Micro-organism Induced Corrosion (MIC) resistant concrete with an HPDE lining. For the 5km-long undersea outfall tunnel at the Tuas WRP (which will be let separately), a waterproof membrane between the primary and secondary linings is being considered to ensure water-tightness along the alignment.
The remaining 50km of smaller diameter link sewers will be constructed using the pipejacking method (Fig 3). The link sewers will be tendered on a design-bid-build basis, with three detailed design consultants, who are yet to be appointed, managing up to 18 construction contracts.
Geological conditions in the south of the island are characterised by the limestone, sandstone and mudstones of the Jurong Formation, but featuring many locations where mixed face soil conditions are likely to be encountered. Generally speaking, however, conditions in the south and west of the island are considered more favourable for mechanised excavation than those encountered in the north and east during Phase I construction (see TunnelTalk site report on Phase I excavations in the References below).
DTSS Phase II includes a number of new design concepts that differentiate it from Phase I. For Phase I there were a mixture of offline and online shafts, but for Phase II all shafts will be online to facilitate the launch of remotely operated vehicles (ROVs) for tunnel inspection purposes. As an added feature DTSS Phase II will also explore the use of fibre optic cables cast into the tunnel lining to provide real-time monitoring of structural integrity.
Another new feature of the Phase II tunnel will be the ability to isolate sections using large modular sliding roller gates that will be incorporated into the main shaft structures. Similar gates, measuring 9m x 7.6m, have been used on the TARP tunnels in Chicago USA.
Expected to complete by 2025, DTSS Phase II will extend the deep tunnel system to cover the western part of Singapore, including the downtown area and major upcoming developments such as Tengah Town. A NEWater factory to be integrated with the Tuas WRP will be built to facilitate water recycling, contributing to the goal of increasing the overall water recycling rate from 30% to up to 55% of total water demand in the long term.
Once Phase II is in place, the existing conventional WRPs at Ulu Pandan and Jurong, as well as intermediate pumping stations, will be progressively phased out and the land freed up for higher value development. The implementation of the entire DTSS will result in a 50% reduction in land taken up by used water infrastructure once it is fully completed.
Construction awards for the main tunnel lots are expected to begin in 2017, with contract rollout for the smaller diameter link sewers starting in 2018.