Singapore DTSS Phase 2 detailed design 07 Jun 2018

Detailed design of the deep sewer tunnels that comprise the Singapore DTSS Phase 2 is advancing ahead of work starting on excavation of the project’s main design-build tunnel contracts. Construction of the 50km of Phase 2 deep sewer tunnelling, running across the southern and western areas of Singapore, is awarded in five design-build contracts starting under Keppel Road at 35m below ground to the east and following the Ayer Rajah Expressway, under Tuas Bay to the Tuas water reclamation plant to the west (Fig 1 and Table 1).

Five construction packages of the Phase 2 deep tunnels

All 50km of the new gravity-fed DTSS Phase 2 deep tunnels will be TBM excavated, using up to four TBMs per contract, and are segmentally lined with a secondary inner corrosive resistant lining. Each contract includes several temporary working shafts as well as permanent drop shafts and odour control structures.

Designers working with the contractors on the design-build contracts are considering designs for steel fibre and synthetic fibre-only reinforcement of the primary segmental lining and designs to meet the required inner corrosive resistant lining. Preliminary designs, prepared by the Black & Veatch and AECOM joint venture for the client, PUB, Singapore’s national water agency, require an inner corrosive resistant lining of microbiological-influenced corrosion- resistant concrete and an inner high density polyethylene (HDPE) protective lining to protect against corrosion, particularly from bacteria and other micro-organisms present and from gaseous fumes in the sewer environment (Fig 2 left).

For tunnel sections that cross under the sea on contracts T-08 and T-11, an additional waterproof membrane layer is to be installed between the segmental lining and the inner secondary lining to minimise any water ingress into the tunnel (Fig 2 right). These linings are required to extend the design life of the tunnels.

Corrosive resisting linings for the deep tunnels (left) and of the under sea reaches (right)

Fibre optic sensors will be embedded into the linings to monitor long-term design life during operation

To monitor the design life of the new sewer system, the tunnels will also be fitted with a network of fibre optic sensors cast into the primary lining to enable remote monitoring of the long term structural integrity of the tunnels. This ability to monitor the structure will be critical also for when other construction works are underway in their vicinity, or during events of seismic activity.

Another innovation for the new DTSS Phase 2 system are gates within the permanent shafts that can be lowered into the deep tunnels to temporarily isolate the tunnels and provide dry environment for safe man access in the event that any repairs are required once in operation. During such repairs, the flow will bypass the isolated part of the tunnel via the link sewer network and rejoin the main tunnel downstream.

Among the detailed designers working with the contractors on the project, COWI in partnership with CPG Corporation are working for Leighton Asia, part of the CPB group, on contract T-09. The contract scope includes the design of an 8km-long section of sewer tunnel, with a 6m internal diameter, several shafts and the hydraulic structures and other facilities associated with the conveyance contract. It is the first win for the newly established COWI Singapore office.

Shaft gates will isolate flow should repairs be required during operation

COWI and CPG will work closely with Leighton Asia to devise solutions to various challenges on the contract, including investigating options to combine temporary work shafts, to minimise land take, and design ground improvement measures to reduce any impact on adjacent structures.

The deep tunnels in Phase 2 will connect to the existing deep tunnels in Phase 1, which serve the northern and eastern part of Singapore to create one seamless integrated system. DTSS Phase 1 was completed in 2008 and comprised construction of a 48km-long gravity-fed deep sewer tunnel to a new water reclamation plant at Changi (Fig 2).

In addition to the 50km of deep segmentally lined main tunnels, Phase 2 of the DTSS system includes another 50km of connection tunnels that are to be awarded as design-bid-build pipejacking contracts. Tenders for the first packages of these connection sewers have been invited and are due for return to PUB later in June 2018.

When Phase 2 is completed and in operation in 2025, the whole of Singapore will be served by the deep tunnel sewer system and its connection sewers directing all flow to three major water reclamation treatment plants that will recycle as much of the used water as possible and minimise the release of treated water to the sea outfalls.