Farrer Road collapse in November a result of tunnelling machine maintenance: BCA

Roadworks along Farrer Road on Nov 16, 2022, a day after a sinkhole formed on a slip road leading from Holland Road to Farrer Road. ST PHOTO: LIM YAOHUI

SINGAPORE - In an update more than two months after tunnelling works caused part of Farrer Road to collapse, the Building and Construction Authority (BCA) said a tunnel boring machine (TBM) was undergoing maintenance when the ground in front of it destabilised.

The machine was excavating a tunnel for national water agency PUB’s Deep Tunnel Sewerage System (DTSS) Phase 2 in the vicinity when the incident occurred.

The project is undertaken by Japanese construction firm Nishimatsu.

A TBM works by cutting the earth in front of it and conveying it to the back of the machine, where it is transported to the surface. The machine can also install concrete rings at each excavated section. These rings form the tunnel walls.

In response to queries filed by The Straits Times a week ago, a BCA spokesman said: “Our preliminary findings revealed that the tunnel boring machine was undergoing maintenance when the ground in front of the tunnel face destabilised.”

This in turn caused a sinkhole to form on a slip road leading from Holland Road to Farrer Road on Nov 15. No one was injured, but the road was closed to traffic for about a week. While the slip road has since been reopened, the leftmost lane of the Farrer Road underpass and a short section of Farrer Road from the slip road remain closed.

The BCA said this is to facilitate grouting and strengthening works. In grouting, a mixture of materials such as sand, water and cement is pumped into the ground or structures to stabilise them.

It added on Wednesday that its stop-work order for DTSS Phase 2 is still in force. “Tunnelling works will be allowed to resume after strengthening works are completed, and measures have been put in place to ensure stability of the remaining tunnelling works.”

It is investigating the cause of the incident to see if there were “any contraventions of the Building Control Act and Regulations”.

“BCA will consider the appropriate enforcement action after the investigations have been completed,” the spokesman added.

In its response, PUB said it is aiming to complete the rectification works and fully reopen the closed sections by March.

Meanwhile, those in civil engineering circles are puzzled by the nature of the incident.

A veteran engineer who has been involved in major public and private projects said it is common to change a TBM’s cutter teeth, which become worn as tunnelling progresses. Other forms of maintenance work may also be called for.

“But this sort of intervention is done in good, stable soil,” he said. “If the soil is weak, it is pre-treated by grouting to harden it.”

Another civil engineer with tunnelling expertise said it is unusual for such an incident to occur while the TBM is stationary and undergoing maintenance.

“The ground would be more unstable if the TBM was pushing forward,” he said.

“I’m also baffled at how long the recovery works are taking. It has been two months.”

A third engineer posited that such incidents happen when the distance travelled by the TBM “is less than the amount of earth excavated, thereby creating a void in front of the machine”. “It’s carelessness,” he added. 

Industry watchers expect the incident to delay the completion of DTSS Phase 2.

This is not the first time tunnelling work has caused such an incident in the area.

In 2008, tunnelling for the MRT Circle Line caused a cave-in in the same vicinity. An 8m-by-7m stretch of Cornwall Gardens in Holland Village – barely 200m from the damaged Farrer Road – sank into a 3m-deep crater.

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