The popular tourist destination of Sentosa Island sits just south of the Singapore mainland. The 1.3 km Sentosa Gateway Tunnel is a two-lane, one-way road tunnel connecting outbound traffic from Sentosa Island directly to major roads to alleviate congestion by bypassing busy traffic junctions at Sentosa Gateway, Telok Blangah Road and Kampong Bahru Road.
WSP was engaged by the project’s main contractor to carry out geotechnical analysis and design the temporary earth retaining system to allow the tunnel to be constructed. This was a critical aspect of risk mitigation for the project, as a poorly designed earth retaining system can have catastrophic impacts on the project timeline and overall cost.
The main road tunnel was constructed using the conventional cut-and-cover method with a permanent diaphragm wall as its retaining system, supported by multiple layers of steel strutting. Temporary soldier pile walls with steel strutting were used for some of the shallower excavations for ancillary structures.
Comprehensive design analyses encompassing multiple design sections were carried out to address the varying soil profiles, required excavation depths and the potential impacts on adjacent structures. The design also had to cater for traffic diversion requirements by carrying out excavation in different stretches concurrently.
For two stretches of the tunnel, the cut-and-cover method could not be adopted due to the presence of existing rail tunnels below the excavation and viaducts supported on piles above the excavation. This reduced the available space to construct the tunnel support elements, as well as undertake road diversions. For these areas, a piped roof temporary support system along with sequential excavation methods were used to construct the tunnel. While these methods addressed the surface constraints, the design solution still presented considerable challenges to the project team due to the shallow cover and presence of existing infrastructure above and below the proposed tunnel horizon.
As the work progressed, our team optimised the design to suit the actual soil condition encountered on site. We adopted many practical strategies to meet site requirements and expedite construction, including revising a bottom-up sequence to a semi top-down sequence for a tunnel launching shaft, and incorporating temporary steel frame supports into the permanent structures for the two-lane piped roof tunnel.
The opening of Sentosa Gateway Tunnel to public use in 2017 has eased the longstanding congestion challenges at the busy traffic junctions and improved traffic flow off the island for both tourists and locals.
* This work was performed by Golder professionals who joined WSP in an acquisition completed in 2021.