We refer to Mr Desmond Cheng’s letter “Take steps to prevent sinkholes caused by human activity” (Sept 6).

Sinkholes can occur when the earth beneath the surface is eroded, typically by water, resulting in underground voids or cavities. Sinkholes may be caused by underground tunnelling or excavation works, either directly or through soil erosion associated with these works.

Naturally occurring sinkholes are usually formed by the erosion of soluble carbonate rocks, including limestone and dolomite. Singapore’s geology is generally not susceptible to such sinkholes, as these rocks are not near the surface. In areas where there are cavities, usually small ones, the Building and Construction Authority (BCA) implements more stringent measures before allowing construction to commence. There are no known sinkhole incidents caused by natural processes in Singapore.

There are also stringent measures to manage the risk of sinkhole occurrence during large-scale underground tunnelling or excavation works.

BCA requires a qualified person (QP) who is a professional engineer to conduct a thorough site investigation and recommend prevention measures before such works start.

Both the builder and QP must install instruments to monitor ground and building movements throughout the construction process and implement additional safety measures where necessary. These include deploying a 24-hour ground surface watchman to monitor any signs of excessive ground settlement.

The builder and QP must also undertake necessary measures to prevent any immediate danger and rectify the excessive ground settlement promptly to prevent sinkholes.

During tunnelling, the volume of soil removed must be monitored continuously to prevent over-excavation, and key tunnelling operations are closely tracked to maintain a safe and stable tunnel face at all times to prevent soil collapse. 

If a sinkhole forms during construction, BCA will prioritise public safety by issuing a stop work order. The builder and QP must act immediately to obviate any potential danger, including cordoning off the affected area, filling up the surface void, conducting a geophysical survey to detect any remaining underground void, and carrying out grouting works on any detected underground void or loosened soil.

The builder and QP must thoroughly investigate matters concerning the sinkhole, review the construction and supervision regime, and implement measures to mitigate the risk of another occurrence.

Tan Chun Yong
Group Director, Building Engineering Group
Building and Construction Authority