ACCIDENT INVESTIGATORS are looking at whether a major cut and cover tunnel collapse in Singapore was triggered by heave-induced base slab failure. The tunnel is being excavated at Nicoll Highway for Singapore Mass Rapid Transit's new Circle Line.
Investigators were also expected to examine the contractor's decision to form an unreinforced jet grouted temporary 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 the tunnel caved in at 3.30pm local time on Tuesday 20 April.
The collapse occurred over a length of around 80m, triggering major movement in surrounding ground.
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 were feared to be at risk from further ground movement.
The collapse occurred on a section of the Circle Line being built by Japanese contractor Nishimatsu in joint venture with Singapore firm Lum Chang Construction.
It is believed the base slab was designed to work as a strut and provide a watertight seal at the base of the excavation.
Tension piles would have been required to resist pressure building up under the base slab. Even if these were provided, the unreinforced jet grouted slab would still be put under considerable pressures.
If the base slab heaved and cracked, the clay beneath it would have welled up into the excavation.
As soon as the slab started to buckle, its strutting function would have been lost.
Load would have been progressively transferred onto the lower struts, which then failed, allowing the diaphragm walls to collapse from the base up.
Nishimatsu-Lum Chang's contract involves construction of two stations at Boulevard and Nicoll Highway, a section of bored tunnel under a stretch of water known as the Kallang Basin, and connecting sections of cut and cover tunnel.
The S$209M (US$115M) project is being carried out under contract C824 for the government's Land Transport Authority (LTA).
Tunnels on contract C824 are relatively deep with base slabs at 33m. They are being excavated through soft marine clay under a high water table.
This material has the consistency of toothpaste with highly flowable properties, Singaporebased engineers told NCEI.
Nishimatsu-Lum Chang was constructing the station boxes and cut and cover tunnels within diaphragm walls. It had put in place a jet grouted base slab before excavation started.
But 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 an engineer involved in another of the Circle Line contracts.
Excavation had reached the 30m mark - just 3m above formation level - when the collapse occurred.
Concerns centre on the integrity of the jet grouted base slab.
Jet grouting involves drilling down to the required depth and injecting cement grout at high pressure. This is carried out on an overlapping matrix, creating a blanket of hard unreinforced material underground.
Sources close to the project said that 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 NCE believed the ground slab had failed in an area of weakness under uplift pressure from the surrounding ground.
Two investigations into the cause of the collapse at Nicoll Highway are under way.
One, ordered by the Land Transport Authority, got under way in April and is expected to report back to the LTA in two to four months time. Its conclusions are not expected to be made public.
The second starts this month. It reports to the government and results will be publicly available.
Both investigations will be seeking to establish whether negligence was to blame for the collapse, and whether any party will be criminally liable.
Issues on the agenda include:
Adequacy of temporary works design
Quality of materials and workmanship
Adequacy of supervision of the five or six tiers of subcontractors by the main contractor.
Whether the LTA's supervisory regime was adequate
Accuracy and adequacy of instrumentation
Adequacy of the design against base heave
Whether struts had been removed to allow machine access and then replaced incorrectly
Whether jacks used to pre-compress the struts had been installed correctly, and whether they were working.
Adequacy of the connection between the base slab, diaphragm walls and heave piles