THE SINGAPORE cut and cover tunnel collapse in April, the biggest retaining wall failure in history, was probably 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 Circle Line caved in at 3.30pm, on 21 April.
Investigators are thought to be examining the contractor's decision to form an unreinforced jet grouted base slab 30m underground in the soft marine clays. 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 was about 15m wide and 33m deep and had reached a depth of 30m at the time of collapse. Failure over a length of 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.
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.
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.
Furthermore its design featured relatively short diaphragm walls based in the marine clay, with little or no embedment below the bottom of the jet-grouted base slab.
Concerns centre on the integrity of the jetgrouted base slab and the structural connection between the slab, walls and heave resisting tension piles.
In the weeks following the collapse, severe cracking developed in a number of low-rise buildings along Upper Paya Lebar road close to the Nicoll Highway collapse zone.
Affected structures have been shored up and two vacant buildings have been declared unsafe 'but pose no danager to passers-by' Andrew Mylius in Singapore, Paul Wheeler Highway history Problems with retaining wall excavations close to Nicoll Highway are well known.
Nicoll Highway was built in the 1950s on land reclaimed from beyond Beach Road, the original shoreline. Fill material would have been dumped over the soft marine clays, which are unusually deep in the area, extending to between 50m and 60m.
Contractors experienced serious problems with a 9m deep basement during construction of the Gateway complex in the late 1980s. At one point sheet piles started yielding.
A few years earlier, excavation for the 7m deep basement for the adjacent Concourse building ran into similar problems.
William Powrie, professor of geotechnical engineering at Southampton University, said with retaining walls in soft clays, base stability can be more critical than lateral, which is often the main focus of the design.
Safety reward Singapore's government is to launch an incentive scheme to boost safety in its construction industry, delegates at the World Tunnel Congress in Singapore heard in May.
Transport Minister Yeo Cheow Tong said contractors with good safety performance will be rewarded an additional 0.5% of the contract sum, capped at a maximum of S$1M (£340,000).
But those with poor safety performance may lose an equivalent amount off its contract value.
Yeo said: 'This new scheme will provide an added incentive for contractors to promote safe-work practices at their work sites.'
Bid questions Procurement of public works in Singapore has come in for severe scrutiny following the Nicholl Highway collapse.
In May it was revealed that the Nishimatsu-Lam Chang tender was 25% lower than its nearest competitor bid. But Singapore transport minister Yeo Cheow Tong said that as the tender was close to the LTA's pre-tender estimate it was 'only logical' to give it the contract.
The government says contracts are awarded on the basis of best value for money in terms of both quality and performance.
For the Nicholl Highway Circle Line contract, LTA received six tenders but filtered them down to three after evaluation. Two committees determined that Nishimatsu-Lum Chang's proposal was 'either technically equal to or better in various aspects than the other two'.
A spokesman said: 'It is not necessary that the contract will go to the lowest bidder; between April and December last year, 28% of all public sector contracts were not awarded to the lowest bidder.'
Investigation agenda Two investigations are being held into the cause of the collapse.
One, for the Land Transport Authority, is expected to report later this summer, but its conclusions are not expected to be made public.
The second, for the government started in June, and its results will be publicly available.
Both investigations will seek to establish whether negligence was to blame for the collapse, and whether any party will be criminally liable.
Issues on the agenda include:
ladequacy of temporary works design;
lquality of materials and workmanship;
ladequacy of supervision of the five or six tiers of subcontractors by the main contractor;
lwhether the LTA's supervisory regime was adequate;
laccuracy and adequacy of instrumentation;
ladequacy of the design against base heave;
lwhether struts had been removed to allow machine access and then replaced incorrectly;
lwhether jacks used to pre-compress the struts had been installed correctly, and whether they were working properly;
ladequacy of the connection between the base slab, diaphragm walls and heave piles.
Warning noises One gang of construction workers said their foreman had picked up on unusual cracking and groaning noises in struts towards the bottom of the 30m deep excavation at about 9am on the day of the collapse.
That afternoon, workers were alerted to the impending failure by loud cracking noises in those same deep struts 15 to 20 minutes before the diaphragm walls started moving.
The clayey earth reportedly flowed into the excavation 'like water'.
Workers described racing to outclimb the flood of material welling up from below and pouring in through the huge tears appearing between panels of the diaphragm walls in the shortlived but intensely violent collapse.