A machine operator's lapse in judgement during train tunnelling works and ground conditions contributed to a tomb caving in earlier this month at the Mount Pleasant Chinese Cemetery within Greater Bukit Brown.
The Land Transport Authority (LTA) told The Straits Times yesterday that the operator of a tunnel boring machine had continued running it in mixed ground conditions instead of stopping. "This led to over-excavation and a localised depression on the surface."
Tunnelling works in the area for the Thomson-East Coast Line have stopped since the incident, LTA said. But no delay is expected for the line, which will have 31 stations linking Woodlands to Bedok by the time it is completed in 2021.
LTA said the cave-in, which was reported on March 4, was about 3m long, 2m wide and 1m deep.
Meanwhile, the authority has engaged an engineer to investigate the incident and propose rectification works. Contractor Daewoo Engineering and Construction has "removed" the operator.
The company will also tighten supervision of future tunnelling operations, LTA said, without going into details. The authority added that it will work with the South Korean company to select a more experienced machine operator as replacement.
When contacted, a Daewoo spokesman directed all media queries to LTA.
Geologist Grahame Oliver, from the Asian School of the Environment at Nanyang Technological University, said: "It is possible that underground digging and tunnelling in the vicinity of the cemetery, plus heavy rain, could have caused this subsidence."
Chinese paper Shin Min Daily News had quoted workers in the vicinity as saying it was raining before the incident.
The affected tomb belongs to a Mr Chen Yi Kuan, and work to reinstate it has commenced, LTA said, adding that it will be completed before Qing Ming next month. Efforts to locate the family have not been successful and no other tombs were affected, LTA said.
Singapore Heritage Society president Chua Ai Lin said workers should exercise greater care "as much construction takes place in historic and built-up areas".
Tomb hunter Raymond Goh agreed, saying the cemetery is home to tens of thousands of tombs from the 1890s to 1970s.
Mr Chan Ewe Jin, vice-president of The Institution of Engineers, Singapore, said cave-ins can normally be avoided if prior knowledge of ground conditions has been established. Good instrumentation and monitoring of tunnelling work can also minimise such occurrences.
Green groups said they are concerned if tunnelling works for the Cross Island Line have to be done in the Central Catchment Nature Reserve. The authorities have found both areas have similar soil types.
Site investigation works are ongoing in the reserve to assess safety of tunnelling works, LTA said. Because such works could damage sensitive habitats as they involve drilling and off-trail surveys, LTA has put in place mitigation measures to reduce the impact, such as reducing the number of boreholes from 72 to 16.
However, this may reduce the amount of information gleaned on the geological profile of the reserve, said Mr David Tan, a biologist from the Love Our MacRitchie Forest volunteer group.
"Sinkholes in forest areas are catastrophic because of the domino effect of large trees collapsing into each other, not to mention the potential danger posed to workers underground," he added.
Green groups are hoping the line will go around, instead of through, the reserve.