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Singapore opts for bored tunnel after cut and cover failure

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CUT AND cover construction has been abandoned in favour of bored tunnelling and top down methods on Singapore's Mass Rapid Transit's Circle Line following last April's catastrophic collapse.

Joint venture contractor Nishimatsu Construction-Lum Chang (NCLC) will rebuild the line in 1.8km long twin bored tunnels between Millenia and Boulevard stations on a new alignment beneath the Kallang Basin, bypassing the collapsed area.

The stretch was originally being built using both bored tunnelling and cut and cover and was about 1.9km long.

Four workers died when diaphragm walls along 80m of a cut and cover section east of Nicoll Highway Station caved in on 21 April 2004 (NCEI June).

The excavation was about 15m wide and was down 30m at the time of the collapse.

NCLC has developed the new design and construction methodology which has been accepted by the Singapore government's Land Transport Authority (LTA). The LTA does not expect costs to rise.

Ground conditions along the new alignments are similar to the original, with reclaimed fi ll from the 1950s covering 50m to 60m of soft marine clay.

The new route allows parallel bored tunnels to be built at 25m below ground level. Two tunnel boring machines, launched from a shaft near Boulevard Station on the eastern side of the basin, will work westwards to the receiving shaft at Millenia Station.

Nicoll Highway station, planned to be built by cut and cover, will now be built top down about 100m south of its original location.

Tunnelling is due to start in the summer.

LTA plans to appoint an independent consultant to check NCLC's temporary works and comprehensively review the design and construction risks. Permanent works will be endorsed by an accredited checker.

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