Accident investigators in Singapore are looking at whether a major cut and cover tunnel collapse was triggered by heave induced base slab failure.
They were also expected to examine the contractor’s decision to form an unreinforced jet grouted base slab 30m underground in soft marine clays.
The slab is thought to have failed, triggering the collapse of propped diaphragm walls progressively from the base.
Four workers died when the diaphragm walls of an open cut tunnel being excavated at Nicoll Highway for Singapore Mass Rapid Transit’s new Circle Line caved in at 3.30pm local time, on 21 April.
The excavation at the collapse site was some 15m wide and 33m deep. Nishimatsu-Lum Chang had reached a depth of 30m when the collapse occurred.
Failure over a length of around 80m, triggered major movement in the surrounding ground.
The scale of the failure is unprecedented. Settlement of up to 15m occurred over an area some 100m in diameter around the cave-in, destroying part of the Nicoll Highway, Singapore’s major east-west harbour-front road artery.
Three large office and retail towers bordering the site are also feared at risk from further ground movement.