As an island state, Singapore obtains a significant portion of its water supply from state-of-the-art rainwater and used water treatment. In the 1990s, Singapore’s National Water Agency, PUB launched a massive project to strengthen the nation’s water security: when completed, the Deep Tunnel Sewerage System, (DTSS), will include a total of approximately 200 km (124 miles) of newly built sewers running up to 60 m (196 ft) deep underground. It is designed to meet Singapore’s long-term needs for used water collection, treatment and reclamation. This underground system collects used water from homes and industries and conveys it entirely via gravity to state-of-the-art water reclamation plants without the need for additional pumping stations. The newest of these is currently being built in the Tuas industrial area and will have a capacity to treat of up to 800,000 cubic meters of sewage per day. Phasing out the intermediate pumping stations and other aged used water infrastructure will also free up around 150 hectares of land in the densely populated metropolis (around 8,750 inhabitants / square meter), for higher-value use.
Marking the completion of tunneling works, Goh Si Hou, Chief Executive of PUB, said in a statement: “As one of the most water-stressed countries in the world, the ability to effectively collect and recycle our used water in a closed water loop has been a game-changer in our quest for water security. The Deep Tunnel Sewerage System is not only an engineering feat, but a key pillar in strengthening Singapore’s water resilience to meet the long-term challenges of climate change and growing water needs.”
PUB is implementing the DTSS project in two phases: in Phase 1 (DTSS 1), between 1999 and 2008, 48 km (30 miles) of main tunnel and 60 km (37 miles) of link sewers were constructed in the eastern part of Singapore. Five Herrenknecht EPB Shields excavated 28 km (17 miles) of tunnel. The second phase (DTSS 2) is now extending the system in the south and west of the main island by a further 100 km (62 miles) of sewers, of which just under 49 km (30 miles) were excavated by Herrenknecht TBMs. Construction for DTSS 2 officially began in April 2019. The five contractors involved completed tunneling in the summer of 2023. DTSS 2 is expected to be operational two years from now.
With Herrenknecht TBMs having proven themselves in Phase 1 already, in the second phase of the project the decision makers for all five construction lots once again opted exclusively for mechanized tunneling technology from the manufacturer in Schwanau in the south of Germany. 18 mixshields and one EPB shield (Earth Pressure Balance Shield) with diameters between 4.5 and 7.56 m (14.7 and 24.8 ft) were used to bore the sewage collectors and line them with segments. Herrenknecht also supplied 12 separation plants for the Mixshield drives and provided comprehensive support services throughout the entire project. This included, for example, remote access to individual machines, enabling them to be monitored and controlled remotely.
Among the challenges for machines and miners was the city state’s complex geology. The “Jurong Formation,” for example, consists of sandstone, siltstone, mudstone, limestone, dolomite and some conglomerate, which are extremely folded due to tectonic plate movements. As a result, diverse rock types with different weathering grades often alternate along a tunnel alignment and even in the cross section of a tunnel. “In close cooperation with the customers, we therefore adapted the design of the TBM to the complex geological conditions,” said Dirk Schrader, Herrenknecht’s General Manager Asia Pacific.
Special solutions were also required for construction of five of the 24 shafts in Tunnel Contract T-11 with diameters between 10 and 12 m (33 and 39 ft). In this particular section, with depths of up to 60 m (197 ft) there was high water pressure the shafts constantly had to withstand. “For this reason, for the first time in the Asia-Pacific region, a Vertical Shaft Sinking Machine (VSM) from Herrenknecht was used for sinking the shafts,” said Schrader.
DTSS1 and 2 are further beacons in a success story of Herrenknecht tunnelling technology in Singapore spanning a quarter of a century: since 1998, Herrenknecht TBMs have been used at numerous jobsites for subway and electricity tunnels. Tunneling for the subway extension by a large diameter machine (Ø 12,680 mm) is due to start next year.