Tunnelling works were stopped late last month on part of the C855 section of the Circle Line extension in Singapore to investigate subsidence at the ground surface over a slurry TBM and perform extra grouting works.
Following the rapid backfilling of the hole that opened up in the Cornwall Gardens road, works were continuing to restore the local area following some utility disruption to houses. The location of the hole is between the stations being built at Farrer and Holland Village.
A hole that was 3m deep and measured 7m by 8m had formed above the location of the TBM in the early hours of Saturday, 24 May. Tunnelling work at the location was stopped to enable inspection and further grouting works.
The 6.6m diameter TBM was on the final, northbound leg of its drives, on the section from Holland Village station to Farrer station where it will terminate, when the incident happened. The contractor on C855 is the joint venture of Woh Hup, Shanghai Tunnel Engineering and Alpine Mayreder (WSA).
The client, Land Transport Authority (LTA), anticipated that the damaged road would be re-opened by about the end of May, but progress details were not available as T&TI went to press. It added that the incident was not expected to affect the overall progress of construction work on stage 4 of the Circle Line, which is to be completed from 2009 onwards.
Four TBMs from Herrenknecht are being used by the JV on the contract – two EPBMs and two Mixshields. Geology on the C855 contract section comprises loam, alluvium, sand and weathered granite.
On the adjacent, unrelated contract C854, contractor Taisei has completed the refurbishment of two 6.6m diameter slurry Kawasaki TBMs and re-launched them from Botanic Gardens station on southbound drives to Farrer station, where they will also terminate. The other pair of machines on the contract have been stripped out and track and M&E fit-out is underway (T&TI, March 2008, p6).
