The Ministry of Sustainability and the Environment (MSE) will convene an internal investigation panel to probe the sinkhole which opened up on Tanjong Katong Road South last Saturday (July 26).

Speaking to the media onTuesday (July 29),Minister Grace Fu said the internal investigation panel formed by the ministry, which manages PUB, will examine all available evidence.

"We want them (investigators) to tell us what caused the incident and how we can prevent it from happening," she said without going into further details.

Calling it a serious incident, Fu said the sinkhole should not have happened.

"On behalf of MSE and PUB, I would like to extend my sincere apology to the injured driver for the injury suffered, as well as (for) going through a harrowing experience."

She also apologised to members of public and residents in the vicinity for the anxiety and unease caused by the incident, and to motorists and commuters inconvenienced by the road closure and diversion.

Fu added that MSE will work with the Building and Construction Authority, which said on Monday that it is starting an independent investigation.

"I think in this case, having two teams looking at it is helping us to find out more thoroughly and systematically what are the causes of this incident," she added.

Investigations are expected to take several months to complete.

Fu told reporters that more tests will be done, and it will be "a couple of days" before the road can be reopened.

A stretch of Tanjong Katong Road South adjacent to a PUB worksite had caved in at about 5.50pm on Saturday, swallowing a car and its female driver, who managed to climb out with the help of workers.

The female driver is recovering in hospital, according to Fu.

Both sides of the road between Mountbatten Road and East Coast Parkway have been closed since Saturday night for repair works.

AsiaOne observed seismic scans being conducted along the road on Tuesday, a method used to identify any anomalies that may indicate voids in the soil. 

The most affected stretch has been backfilled with liquefied stabilised soil, but reinstation works have yet to be completed.

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The national water agency had said on Sunday that a caisson ring at the PUB worksite, where a 16-metre-deep shaft was being built to connect three existing sewer lines, was observed to have failed at about the same time the sinkhole formed on the adjacent road.

A caisson ring is a cylinder concrete structure that is sunk underground to construct a shaft.

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lim.kewei@asiaone.com