Fresh approach to Singapore MRT

Tunnelling work on the latest section of Singapore's Mass Rapid Transit Circle Line is to resume on a new alignment after the temporary works collapse last April.

Tunnelling work on the latest section of Singapore's Mass Rapid Transit Circle Line is to resume on a new alignment after the temporary works collapse last April in which three construction workers and a technical officer of the island's Land Transport Authority were killed.

Failure of the retaining wall system resulted in a massive cave-in on both sides of the braced excavation, including collapse of a stretch of road along the Nicoll Highway.

The new approach marks a complete break with the construction methods being employed at the time of the accident.

The Land Transport Authority and the design-build contractor Nishimatsu Construction/Lum Chang Building JV have come to an agreement whereby the twin tunnels between the Millenia and Boulevard stations will bypass the area that collapsed.

The new Nicoll Highway MRT station will now be built on the revised alignment approximately 100m south of the original location.

LTA has explained that as the civil works contractor for the Nicoll Highway and the MRT stations and adjacent tunnels, NLC will also be the contractor for the new tunnels and the new station at Nicoll Highway.

Bored tunnels will be driven in place of the cut-and-cover operation in progress at the time of the falsework failure.

Nishimatsu is bringing two TBMs to drive the 1.8 km tunnels at a depth of approximately 25 m.

The investigating committee appointed by Singapore's Manpower Ministry following the fatal collapse is due to submit its final report in a few weeks' time.

Its interim recommendations insisted that there should be a comprehensive engineering review of design and safety measures on all on-going projects, particularly where deep excavations are being carried out.

The contractor's project team for the reconstruction works is a new one made up of professionals said to possess the necessary experience and skills to carry out the works.

LTA will also have a new project team and says it will implement improvements for the reconstruction of the MRT tunnels and the station at Nicoll Highway.

The transport authority has engaged 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 current practice requires.

The transport authority is appointing a specialist instrumentation contractor who will report directly to the client to ensure better control over the instrumentation staff as well as the responsibilities of the instrumentation contractor.

Risk management is also being improved following the Circle Line collapse.

The risk management process installed by LTA now requires that during the progress of the contract all site staff are under a duty to report any instrument readings which indicate a breach in the specifications.

This rule will be monitored by a committee comprising senior design and project staff who will keep the contract works under constant review.

Top-down construction for new station The method now adopted for building the new Nicoll Highway station under Republic Avenue is top-down construction.

This involves installing diaphragm walls of 1.5 m thickness up to 60 m depth, socketed into hard soil strata to minimise ground movement.

This will be followed by casting the roof slab in the form of struts or support beams.

The transport authority says that the amount of open excavation required by this method will not be great as the components of the roof slab which will sit about 6 m below ground level will be cast immediately that depth is reached.

Excavation will then continue through openings in the roof slab until the station's concourse level is reached and cast.

This process is repeated until the base slab at the deepest part of the station box is completed.

The Land Transport Authority says that construction of the new MRT tunnels and station will start in the middle of this year.

The decision to introduce TBMs and adopt the 'top-down' method of construction for the Nicoll Highway station means that the resumed contract dispenses with technologies which the Committee of Inquiry described in its interim report as unfamiliar or extended beyond the normal range of their application.

In calling for a comprehensive engineering review, the committee specifically mentioned the floating cofferdam system formerly proposed for the Nicoll Highway station.

The new arrangements have plainly taken into account the observations of the Committee of Inquiry's interim report about efforts to minimise the risks involved to construction workers in deep excavations.

The Singapore standard code of practice for safety management of construction worksites they said did not refer to such complex projects as were in hand at the time of the accident.

The outcome of this review should be made public at the time the committee's final report is published, understood to be around the anniversary of the accident which occurred on 2004-04-20.

An industry-wide review of the safety management system was also necessary, they said.

"One way to achieve this is to integrate the SMS into the design, construction trials, execution of works and maintenance phases.

On ownership and resolution of design and site problems, the committee said that the LTA as authority and developer of this project took upon itself the roles of both supervision and building control unit.

Nishimatsu-Lum Chang JV was the design and build contractor and therefore had control over worksite risks.

NLCJV had engaged Maunsell as the consultants for design of the permanent works as well as certain elements of the temporary works.

"This complicated relationship inevitably led to conflicts in the ownership and resolution of problems in [Contract No.] C824".

"This critical decision-making gap adversely affected judgement calls necessary to deal with the crisis and had caused uncertainty at the worksite.

The committee of inquiry was led by Senior District Judge Richard R.

Magnus, assisted by two assessors: Associate Professor Teh Cee Ing, Head of the Division of Geotechnical and Transportation Engineering at the School of Civil and Environmental Engineering, Nanyang Technological University, and Er.

Lau Joo Ming, Director of the Singapore Housing and Development Board's Building Technology Department.

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