ACCIDENT INVESTIGATORS in Singapore are looking at whether last month's major cut and cover tunnel collapse was triggered by heaveinduced base slab failure.
Four workers died when the diaphragm walls of an open cut tunnel being excavated for Singapore Mass Rapid Transit's new Circle Line caved in at 3.30pm local time, on 21 April.
Investigators are also expected to examine the contractor's decision to form an unreinforced jet grouted base slab 30m underground in the soft marine clays of the site on Nicoll Highway.The slab is thought to have failed, triggering the collapse of propped diaphragm walls progressively from the base.
Joint venture contractor Nishimatsu-Lum Chang's excavation at the site was about 15m wide and 33m deep and had reached a depth of 30m when the collapse occurred.
Failure over a length of about 80m triggered major movement in the surrounding ground.
The scale of the failure is unprecedented.
Settlement of up to 15m occurred over an area 100m in diameter around the cave-in, destroying part of Nicoll Highway, Singapore's major east-west harbour-front road.
Three office and retail towers bordering the site are feared to be at risk from further ground movement.
The collapse took place on a section of the Circle Line being built by Japanese contractor Nishimatsu in joint venture with Singapore firm Lum Chang Construction.
The S$209M (£69.7M) project, for the Singapore government's Land Transport Authority (LTA), was being carried out on a design and build, lump sum basis. It is understood that Nishimatsu designed all the temporary works in-house.
The contractor opted for a cut and cover tunnelling solution that is widely used in Singapore - but not normally for excavations as deep as that at Nicoll Highway, or in such difficult ground conditions.
Engineers close to the Circle Line project said that, because of the depth of construction, the approach carried significant risks.
'The methods being used aren't uncommon for Singapore, but it is unusual to use them on excavations as deep as those at Nicoll Highway, ' said one.
Concerns centre on the integrity of the jetgrouted base slab. Sources close to the project said it can be difficult to achieve uniformity at such depths due to deflection of the drill string and local variations in ground conditions. Grout can end up being weakened by excessive inclusion of soil or water.
Many Singapore engineers contacted by GE believed the ground slab had failed in an area of weakness under uplift pressure from the surrounding ground.
It is believed the base slab was designed to work as a strut and provide a watertight plug at the base of the excavation.
But given the depth and poor condition of the marine clay, the clay below the slab probably became overstressed as the excavation deepened. If this resulted in base heave, the unreinforced jet grouted slab would have been put under considerable pressures. If the slab started to buckle, its strutting function would have been lost.
Load would have been progressively transferred on to the lower struts, whose failure would allow the diaphragm walls to collapse from the base up.