Measures in place to manage risk of sinkholes during construction works

We refer to Mr Desmond Cheng’s letter “Take steps to prevent sinkholes caused by human activity”, 6 Sep 2024.

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:

1. Underground tunnelling or excavation works, either directly or through soil erosion associated with these works.
2. 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. For areas where there are cavities, usually small ones, BCA implements more stringent measures for the project parties 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:

1. BCA requires a Qualified Person (QP) who is a Professional Engineer to conduct a thorough site investigation and recommend prevention measures prior to the commencement of such works.
2. Both the builder and QP are required to install instruments to monitor ground and building movements throughout the construction process, and to implement additional safety measures where necessary. This includes the deployment of a 24-hour ground surface watchman to monitor any signs of excessive ground settlement.
3. BCA also requires the builder and QP to undertake necessary measures to obviate any immediate danger and rectify the excessive ground settlement promptly to prevent sinkholes.
4. During tunnelling, continuous monitoring of the volume of soil removed is required to prevent over-excavation and key tunnelling operations are closely tracked to maintain a safe and stable tunnel face at all times to prevent collapse of soil. 
   

In the event of a sinkhole formation during construction, BCA will prioritise public safety by issuing a Stop Work Order. The builder and QP are then required to carry out immediate actions 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 soils. The builder and QP are also required to carry out thorough investigations in relation to the sinkhole, review the construction and supervision regime, and implement measures to mitigate the risk of another occurrence.

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