Farrer Road sinkhole: Tunnelling machine under maintenance when ground destabilised

1 week ago 32

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

The TBM was excavating a tunnel for national water agency PUB’s Deep Tunnel Sewerage System (DTSS) Phase 2 in the vicinity. The project was undertaken by Japanese construction company 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. It 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 left-most lane of the Farrer Road underpass as well as a short section of Farrer Road from the slip road have remained closed.

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

The BCA added Wednesday that its Stop Work Order for DTSS Phase 2 was 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.”

The BCA said 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,” a spokesman added.

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

Meanwhile, 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...

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