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Local engineers say design solution was unorthodox

NISHIMATSU-LUM Chang tendered for contract C824 of the Circle Line with a 'floating diaphragm walls' design, with relatively short walls based in the marine clay and with little or no embedment below the bottom of the base slab.

This solution is claimed by many Singapore engineers to be 'unorthodox' given the ground conditions.

Singapore subcontractor L&M Geotechnic formed the 3m to 4m thick jet-grouted base slab.

The South East Asian arm of Bachy Soletanche installed 600mm-800mm thick diaphragm walls to a depth of about 40m.

I-section steel struts spaced at centres of 4m to 5m horizontally and 3m vertically provided further support as excavation advanced.

This design approach has been used before in Singapore at Marina Bay Station, but the construction method was completely different. In this case contractors sank a 30m long composite H-pile/sheet pile cofferdam, which was then flooded.

Soil was then excavated, effectively dredged, with the water pressure inside the cofferdam resisting base heave. Contractors then tremmied in a 1.5m thick unreinforced concrete base slab, which was tied down with tension piles before the excavation was dewatered.

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