Tanjong Katong Road sinkhole did not happen overnight: Experts

SINGAPORE - The large sinkhole in Tanjong Katong Road South seemed to appear suddenly on the evening of July 26,

swallowing a car and its unsuspecting driver

as two lanes of the road collapsed into a large pit.

But it was unlikely that the erosion of the soil beneath the road happened overnight. Instead, the collapse could have been the result of changes under the ground that had happened much earlier, experts told The Straits Times.

“Sinkholes cannot form suddenly. They need time for the surrounding soil to wash away, causing the cavity to get larger and larger,” said Associate Professor Wu Wei, a geotechnical engineering expert at Nanyang Technological University.

Prof Wu said the rate of ground settlement accelerates before it collapses. Before that happens, cracks might appear on the road surface before it caves in, but these might not always be identifiable to the naked eye, he added.

Mr David Ng, a professional engineer who is also chairman of the civil and structural technical committee at the Institution of Engineers Singapore, said that in some cases, it may be possible to spot the road being uneven, see crack lines on the road surface or water bubbling from the road surface.

Referring to the images of the road before it collapsed, he said the freshly laid tarmac may be a sign that repairs might have been carried out recently to fix surface cracks or uneven ground because of soil settlement, although it may also be because of other unrelated works.

While Mr Ng said it was too early to know for sure what caused the collapse, he said the erosion could have been caused by a leaking old pipe underground or soil movement triggered by excavation works happening next to the road.

In the case of leaking pipes, if it happens to be prolonged, it would create a void “large enough for the surface above to cave in”, he said.

Excavation works can also wash away materials supporting water pipes underground, he added.

Left suspended without support underground, the water pipe may burst, leading to water gushing out and flushing away the materials under the surface, causing the road to cave in.

“This is happening underground, so we may not be aware of it,” said Mr Ng, who is also executive director of One Smart, an engineering company that works on geotechnical and infrastructure works, including sewerage projects with PUB.

The sinkhole was located beside a worksite of national water agency PUB, where work was under way on a sewerage project.

Residents had earlier told reporters that there was a burst water pipe in the area the night before the sinkhole appeared.

But Mr Ng pointed out that in sewerage projects, precautions against cave-ins are taken from the design phase of the project in order to minimise disturbance to the surroundings, ground movements and changes in ground pressure.

Instruments like settlement markers, which are nails driven into the ground or concrete blocks in the vicinity of the works, are also used to monitor ground movements, he said.

Prof Wu said more frequent use of ground penetration radar could possibly have detected the cavity below the road before it collapsed.

Such equipment is typically used by engineers to examine the ground condition below the surface, including location of cabling and other infrastructure, he said.

But Prof Wu also said there are limitations to such technology, including that it can only “look” 3m to 5m under the surface, and is less accurate when water is present.

He pointed out that key to preventing incidents like this is taking more frequent ground measurements and closer monitoring efforts.

“In this case, someone observed a burst water pipe one day before. If monitoring is strengthened, at least early warning can be given,” he said.

This is not the first time a road has collapsed here.

In November 2022,

a PUB project caused a slip road

leading from Holland Road and Commonwealth Road into Farrer Road to be closed for a week because tunnelling works caused a cave-in. There were no injuries reported.

In the latest incident, the car driver, a woman, was rescued by workers and taken to hospital by the Singapore Civil Defence Force.