ST Explains What We Know About the Tanjong Katong Sinkhole

As work continues to restore a section of Tanjong Katong Road South that caved in on July 26, swallowing a car and its driver, who escaped with the help of nearby workers, speculation has been rife about what caused the 3m-deep sinkhole.

- Kok Yufeng

PUB, Singapore's national water agency, gave a brief sequence of events on July 27, noting that the failure of a concrete structure known as a caisson ring was observed at an adjacent PUB worksite, around the same time the sinkhole formed at about 5.50pm on July 26.

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.

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.