Four construction workers have been killed and three more injured following the collapse of a retaining wall during open cut tunnelling works on Singapore’s 33km long, US$3.9bn Mass Rapid Transit (MRT) Circle Line.
The incident occurred on Tuesday 20 April at 3.30pm at the Nicoll Highway MRT station worksite Ð part of the Circle Line’s 5.2km long stage one, being constructed by the Nishimatsu Construction Co. Ltd/Lum Chang Building Contractors Pte Ltd. joint venture. Early reports indicated that the contractor was only 5m from completion having already successfully excavated the station box to a depth of some 31m.
The Singapore Civil Defence Force (SCDF) conducted a search and retrieved the bodies of two workers amid the debris, a Singaporean and a Chinese national. A Malaysian crane operator was killed when his crane collapsed into the cave-in. The twisted remains of support structures and equipment within the hole further hampered the search and by 24 April efforts were called off before recovering the body of a fourth missing worker, a Singaporean foreman.
The collapse has led to a 30m deep hole opening across six lanes of the Nicoll Highway over an area estimated to be 150m long. Luckily no motorists were travelling along the stretch of highway at the time of the collapse. The gas, water and electricity supplies in the area were also interrupted for 15,000 people and 700 businesses. Reports say the death toll could have been far higher had approximately 20 tunnel workers not been on a tea break at the time of the collapse. Eyewitnesses said they saw a fireball and heard a noise like an explosion, but the Land Transport Authority (LTA) told the press it had no evidence of an explosion and a spokesman added that the noise of the collapsing wall could have sounded like an explosion.
The surrounding buildings and structures have been assessed as safe to use by the LTA and Building and Construction Authority (BCA), while the LTA is monitoring ground movements in the area regularly. Road closures and diversions are in place and although other Circle Line works are ongoing, the LTA said it could be nine months before Nicoll Highway could re-open. The first stage of the recovery works needed was to stabilise the collapsed ground and to prevent water seepage. The LTA stated these works could proceed when they would not be disrupting the SCDF rescue operation.
With the rescue operations abandoned, completed parts of the tunnel and shaft have been sealed off and work has begun to fill voids with foam concrete and bind the mangled wreckage together. Structural concrete will be poured as a top layer to support the heavy equipment being brought in to remove the protruding steel beams.
An investigative committee has been set up to determine the cause of the tragedy. Reports have speculated that base slab heave which destabilised the toe of the retaining walls could have been responsible. Local press reported that the Nishimatsu-Lum Chang joint venture has offered an unconditional payment of nearly US$20,000 to the bereaved families.
On 29 April, an unrelated incident occurred on the FusionPolis construction site in Singapore, resulting in the death of two workers and injury to 29. Whilst working on a 30m deep basement, the chairs supporting a layer of rebar failed and the reinforcing steel fell onto workers below. FusionPolis incorporates work on a station on the Circle Line, but authorities emphasised it was a local event unconnected to Nicoll Highway.
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