The DTSS will transform wastewater management in Singapore. A new conveyance, treatment and disposal system has replaced six sewage treatment work facilities, routing all wastewater through large tunnels under the island to two new, state-of-the-art wastewater treatment plants (WTPs).

The tunnel system works entirely by gravity, eliminating the need for more than 130 existing pumping stations. The project components include approximately 48 kilometres of tunnels, with internal diameter varying from 3.3 – 6 metres, and a total of 48 working and access shafts. A total of eight tunnel boring machines (TBM) were operated to excavate the tunnel.

The DTSS tunnels: experience and challenges

The DTSS project will provide many benefits for Singapore. These advantages include improved regional water quality; removal of pumping stations and plants from urban areas; prevention of sewer overflows; and the elimination of odour nuisance. The new system replaces approximately 140 existing pumping stations and six treatment plants, as well as releasing approximately 290 hectares of land for development for alternative purposes.

The design and construction of the tunnels was let out in six design and build contracts in 1999. The construction of phase one of the DTSS was completed in 2007. The completed tunnels connect to the influent pumping station built at the Changi WTP, allowing the treated effluent from the DTSS tunnel to be dispersed from the new outfall into the deep waters of the Singapore Straits. A link sewer of approximately 90 kilometres in length was built to convey wastewater from buildings, industry and households to the tunnel.

Deep tunnel design

CH2M/Parsons Brinckerhoff (PB) was engaged by the Public Utility Board of Singapore to carry out feasibility studies, concept design for deep tunnels, link sewers, treatment works and ocean outfall; site investigations, preliminary designs, contract documentation, and tender evaluation of deep tunnels; and program management of the six design and build contracts.

Three different types of sewer system — surcharge systems, gravity systems with intermediate pump station and complete gravity systems — were investigated with respect to short and long-term construction and operational advantages. Ultimately a full gravity system was adopted; a decision that has placed stringent tunnel construction and settlement requirements on the project.

The project alignment was established during the feasibility stage, based on specific criteria. The tunnel route predominantly followed expressways to avoid sterilising land. The horizontal alignment was selected to avoid known geological hazards. The tunnel depth was selected to minimise geological risk and the vertical alignment was selected to avoid existing underground structures, while maintaining the uniform gradient required for the gravity sewer flow.

A detailed geotechnical investigation program was implemented to assess the geological risk and to select the appropriate tunnel excavation methods and tunnel support system.

Major tunnelling challenges

The first phase of the DTSS tunnels was constructed under six design/build contracts. About 67 tenders were received and evaluated.

The major tunnelling challenges of the DTSS project included the high groundwater table; mixed face and very abrasive ground conditions; highly variable rock quality in the granite and sedimentary formations and the major expressways directly above the tunnels, making avoidance of settlement and/or face collapse critical.

Managing geotechnical risk

Bidders received a geotechnical data report (GDR) as part of the tender documents, which they were required to interpret and provide a subsequent preliminary geotechnical interprelative report (PGIR) as part of their bid. The PGIR was vital to the tender evaluation as it demonstrated how well the bidders understood the ground conditions in the corridor, as well as equipment selected and means and methods, which were required with the bid.

The TBMs were one of the most important considerations, as there would be up to eight operating simultaneously on the project, as well as the length of the run – up to 12.6 kilometres. The bid required advance rates, which represented substantial risks to timely completion. The PGIR was a key document in assessing whether actual encountered conditions differed from those anticipated by the successful contractor.

The successful contractors were required to perform additional site investigation and provide a final geotechnical interpretive report, which benefitted the contractors with valuable additional information and analysis in managing the excavation. The contractor was allocated all of the risks within their control.

The GDR provided was reasonably adequate, except at one location where a hard rock intrusion was encountered in a soft ground tunnelling area. This involved significant ground improvement works and modification of the TBM to excavate through the mixed face hard ground conditions.

Machinery and lining

The tunnels were excavated using TBMs. A total of eight TBMs were used for driving the 48 kilometres of tunnels. The eastern two-thirds of the North Tunnel were constructed through the Old Alluvium layer, a generally competent tunnelling material. The tunnel however, was deep below the groundwater table, in some areas more than 45 metres, requiring constant control of the tunnel face to prevent face instability. Earth-pressure balance shield machines were used to control face instability.

The initial ground support consisted of pre-cast concrete segments, and the annular tail void contributing to the excess ground loss was grouted to prevent the void from collapsing.

The western third of the North Tunnel and most of the Spur Tunnel passed through granite rock. The TBMs used in this section were selected to excavate through mixed face ground conditions.

Pre-cast concrete segmental lining was used as the primary support system. In order to protect the concrete segmental lining from internal corrosion, a corrosion protection lining (CPL) system was designed and constructed.

The CPL system consisted of a two component lining comprising a primary 2.5 mm thick HDPE membrane, and CPL2 — a secondary sacrificial concrete lining.

Conclusions

The construction of DTSS tunnels was completed without any major problems. The geotechnical and the tunnelling risk assessment methods were implemented effectively and contributed to the success of the project. All TBMs were operating as predicted, except for a couple of tunnel drives in mixed face ground conditions. The lesson learnt from this is that TBMs and excavation methods require careful planning for proper tunnel excavation.