ST Explains: What we know about the Tanjong Katong sinkhole so far
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Repair works on the sinkhole under way at about 9.45pm on July 28.
ST PHOTO: CHONG JUN LIANG
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SINGAPORE – As work continues to restore a section of Tanjong Katong Road South that caved in
PUB, Singapore’s national water agency, gave a brief sequence of events on July 27
Now that the ground situation has been stabilised, the Building and Construction Authority (BCA) is starting an independent investigation that could take several months.
The Straits Times speaks to engineers to piece together what is known about the incident so far.
What is a caisson ring?
The work being carried out next to the sinkhole involved the construction of a 16m-deep shaft to connect three existing sewer lines. Experts said the shaft was being built so that the contractor, Ohin Construction, could access those sewer lines underground.
Instead of digging up large trenches, equipment and workers could be lowered into the shaft to construct new sewer pipes using a method known as pipe-jacking.
This involves pushing the new pipes through the soil using hydraulic machines. It is similar to how underground MRT tunnels are built today, but on a smaller scale.
Mr David Ng, a professional engineer, said caisson rings are essentially the segments that make up the walls of the vertical shaft. The construction technique involves building each ring layer by layer while excavating the soil inside.
Mr Chong Kee Sen, an honorary fellow and former president of the Institution of Engineers, Singapore, said caisson rings are made of concrete reinforced with steel bars.
They should have sufficient strength to hold the surrounding earth in place and keep soil and water out of the shaft, he added.
A worker aligning a water pipe that will be installed underground using the pipe-jacking method, which obviates the need to dig large trenches at the possible expense of infrastructure such as the urban environment or underground cabling, sewerage and rail networks.
ST PHOTO: MARCUS TAN
What are some reasons for the failure?
The experts gave several possible reasons why a caisson ring may crack or break, but they were not keen on speculating too much without more information.
Mr Chong said it is not common for caisson rings to fail.
One possibility is that an unanticipated load such as heavy machinery was placed on the caisson ring. Soil movements, such as those from excavation work, can also cause significant stress on the shaft walls.
He said investigators will likely have to look at the design of the shaft, the quality of the concrete and the strength of the steel reinforcement, among other factors.
Mr Ng, on the other hand, believes that there might already have been a void in the earth underneath Tanjong Katong Road South, which could have caused the road to cave in.
Such a void could have also exerted uneven pressure on the shaft walls, causing the caisson ring to fail.
He said the construction of vertical shafts similar to the one in Tanjong Katong is done all around Singapore, and contractors usually put in place a host of preventive measures.
For example, a detailed investigation of the soil condition and computer modelling of the construction sequence are usually done beforehand.
Contractors also improve the ground conditions so that they are suitable for excavation, for example, by driving piles into the soil around the shaft to ensure stability, he added.
The experts rejected suggestions that Tanjong Katong Road South being on reclaimed land contributed to the sinkhole.
Mr Ng said: “No doubt, the ground there is challenging because it is made of soft clay and sand. But this is not an issue because a lot of Singapore is like that, and we have been able to construct many structures under such conditions, like the MRT station and tunnels there.”
How are sinkholes fixed?
Preliminary findings by PUB suggest that nearby water mains may have been damaged as a result of ground movement linked to the sinkhole, causing it to fill up with water.
Workers began pumping the water out from 9.45am on July 27, and this was completed by 1.20pm. Since then, the sinkhole and the adjacent shaft have been filled up. Tests are now being done to ensure that there are no air pockets in the soil before the road is repaved.
PUB said it used a mixture of soil, water and cement to fill up the sinkhole that will harden over time. This material is stronger than clay and sand, and it is hard to wash away, Mr Chong said.
He told ST that it should take a couple more days at most for road repairs to be completed. “They just have to be doubly sure that the sinkhole has been totally stabilised,” he added.
Past cases of road cave-ins, and what caused them
There have been several sinkhole incidents in Singapore, some related to construction activity, while others were not.
Farrer Road, 2022
The most recent reported case was on Nov 15, 2022, when a sinkhole formed on a slip road
Preliminary investigations found that a boring machine excavating a tunnel for Phase 2 of PUB’s Deep Tunnel Sewerage System project was undergoing maintenance when the ground in front of it destabilised
Changi, 2014
In April 2014, a tipper truck sank into the ground after a section of Upper Changi Road East caved in. The driver was uninjured. The incident happened about 50m away from a construction site for the third phase of the Downtown MRT Line (DTL).
Multiple incidents, 2013
In 2013, a number of sinkholes appeared.
One in Keppel Road was caused by an old water pipe bursting, and it led to a massive traffic jam as a car became stranded.
A sinkhole in Clementi, which reappeared days later, was caused by the erosion of soil into an abandoned manhole, and a motorcyclist was injured trying to avoid it.
A portion of Woodlands Road also caved in due to excavation works for the DTL2, which ruptured an underground water pipe, causing a sinkhole to form.
Cornwall Gardens, 2008
Tunnelling work for the Circle Line created a sinkhole, measuring 8m by 7m, with a depth of 3m, on a stretch along Cornwall Gardens near Holland Road.
No one was injured, but water supply to four homes was disrupted. Road repairs, which included stabilisation work, took more than two weeks.
