The Land Transport Authority (LTA) and Nishimatsu Construction Co. Ltd/ Lum Chang Building Contractors Pte Ltd Joint Venture (NLC)¹ will reconstruct the Circle Line (CCL) MRT tunnels between the Millenia and Boulevard MRT stations along a new alignment, bypassing the area that collapsed. The collapsed area has debris and obstructions, and NLC has proposed a new alignment which LTA has accepted. The Nicoll Highway MRT station will also be built approximately 100 metres south of its original location.

A new set of twin MRT tunnels will be constructed from the west end of Boulevard MRT station towards Millenia MRT station. The tunnels, which are approximately 1.8 kilometres long, will cross under the Kallang Basin and will be about 25 metres deep.

The Nicoll Highway MRT station will now be sited under Republic Avenue and the adjacent land. The station will have two entrances: the north entrance will be linked via an overhead bridge to the Concourse, and the south entrance will serve the park there. Please refer to Annex A for a map of the new location of the tunnels and station.

The realigned tunnels will be constructed using the bored tunnelling method ² while the new Nicoll Highway MRT station will be built using the top-down construction method. These methods differ from the cut-and-cover method used ³ to construct the original tunnels and station.

The top-down construction method⁴ , which will be used to construct the new Nicoll Highway MRT station, involves the construction of 1.5-metre thick, permanent diaphragm walls up to 60 metres deep. These diaphragm walls will be socketed into the hard soil strata to minimise ground movement.

Construction of the new MRT tunnels and station will start in the middle of 2005, and this stage of the Circle Line (CCL1) is scheduled to be opened by 2010.

The NLC project team for the reconstruction works will be a new team and will comprise professionals with the necessary expertise and skills to carry out the works. LTA will also have a new project team and will implement improvements for the reconstruction of the MRT tunnels and station at Nicoll Highway.

LTA will engage an independent consultant to check the design of temporary works prepared by the contractor. The permanent works will be endorsed by an Accredited Checker, as with the current procedure.

LTA will also appoint a specialist instrumentation contractor who will report to LTA directly. This will ensure that there is better control over the proficiency, skill set, experience and training of instrumentation staff, as well as the responsibilities and duties of the instrumentation contractor.

LTA has a risk register to comprehensively review the design and construction risks. This has been improved following the incident by requiring that during construction, all site staff immediately report instrumentation readings which have been breached to a committee comprising senior design and project staff for review and follow-up.

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¹ As the civil works contractor for the Nicoll Highway and Boulevard MRT stations and adjacent tunnels, NLC will also be the contractor for the reconstruction of the MRT tunnels and station at Nicoll Highway.

² The bored tunnelling method involves tunnelling underground without having to cut open the earth. Two Tunnel Boring Machines (TBM) will be sunk into the Launch Shaft up to the required depth. The TBMs will then bore through from this shaft until they reach the Receiving Shaft, where they will be dismantled and removed.

³ The cut-and-cover method involves excavation to the final required depth, while using retaining walls and struts to support the soil at both sides. When excavation is completed, the base slab of the underground structure is cast at the bottom-most level, followed by the side walls. Casting of the concrete progresses upwards, level by level, until the roof of the structure is completed.

⁴ The top-down method involves installing the diaphragm walls first, followed by the casting the roof slab of a station. Open excavation will not be too deep as the roof slab, which is about 6 metres below ground, will be immediately cast once this level is reached. The roof slabs are thick and will act like solid struts, or support beams, for the next level of excavation. Excavation continues through openings in the roof slabs until the station's concourse level is reached and cast. This process is repeated until the base slab, the deepest part of the station box, is completed.