PUB moves to test ground stability at Tanjong Katong sinkhole after filling cavity

CNA

a day ago

SINGAPORE: PUB has moved on to testing the stability of the ground after the sinkhole on Tanjong Katong Road South was filled on Monday (Jul 28).
The sinkhole was filled with liquefied stabilised soil, which is a mixture of soil, water and cement commonly used to fill underground voids, the national water agency said in an update.
When poured into the sinkhole, liquefied stabilised soil flows into and fills the cavity, forming a "competent soil-like material" in hours, said Mr Herman Ching, a PUB water reclamation department director.
This material does not require any mechanical compaction, reducing the time required for works, PUB added.
The agency said it was conducting "rigorous tests and scans to detect underground cavities and verify ground stability" before continuing with repair works.
It provided details on the three methods it is using to carry out these checks.
First, it is scanning the area with ground-penetrating radar, which uses radio waves to assess subsurface layers for voids and other anomalies.
It is also carrying out ground probing, which involves drilling a metal rod into the ground to a depth of 20m, to test the condition of the ground and detect if there are any voids.
On Monday night, members of the media were able to observe the ground probing in a visit to the worksite facilited by PUB.
Finally, it is conducting seismic scanning to record sound waves underground over a period of time to identify any anomalies that might indicate voids.
Seismic scanning provides a profile of the ground to a greater depth than the ground-penetrating radar, said Mr Ching.
"Collectively, these three methods will give us a good (sense) of the competency of the ground in this area, and to ensure that the ground is good for road construction," he said.
PUB also said it was conducting thorough safety assessments in consultation with the Building and Construction Authority (BCA) and Land Transport Authority before the road would be reopened to the public.
It added that it was "working round-the-clock to expedite the recovery of the road" and apologised for the inconvenience to residents and motorists caused by the road's closure.
The sinkhole appeared on Saturday next to a PUB worksite along Tanjong Katong Road South where sewer-laying works were being done.
A 16m-deep shaft connecting three sewer lines was being constructed at the site.
A concrete component in the shaft, called a caisson ring, failed at around 5.50pm on Saturday, PUB earlier said.
Around the same time, two lanes of the road collapsed, and a car fell into the sinkhole. The driver was rescued by migrant workers and taken conscious to the hospital.
Earlier on Monday, the BCA said that backfilling of the sinkhole and shaft had been completed, which stabilised the site.
Buildings adjacent to the sinkhole remained safe, it said.
BCA also said it would start an independent investigation to identify the root causes of the incident, which could take months.
PUB previously said it would call a time-out to review and ensure the safety of similar sewer construction works.
The night before the sinkhole appeared, some residents reported seeing water gushing from what appeared to be a burst water pipe at nearby Mountbatten Road and Jalan Seaview.
On Monday, a resident told CNA that the floor tiles on the front porch of her house along Mountbatten Road had cracked the day before the sinkhole opened.
The resident, who did not want to be named, said that the family discovered the damaged tiles upon returning from work at about 8pm on Friday.
She said they alerted the authorities, and BCA had come to the house to conduct checks.
The tiles were installed when they moved in 13 years ago, and this was the first time they had sustained such damage, added the resident.
While the damaged tiles look "ugly", she said she was not concerned about the structural stability of her home.