THOROUGH soil investigations must be carried out before an underground construction project starts.

These usually take one to six months and make up to 10 per cent of the project's cost, said engineers.

Boreholes are drilled to extract soil samples, which are tested to determine their strength, permeability, compressibility and other engineering properties.

The Building and Construction Authority (BCA) has requirements for the number of boreholes to be drilled based on the project's size, such as its depth and length.

The engineers said their peers should get foundation records of structures near the projects from the owners or the BCA.

"Many buildings in Singapore undergo renovation and could have different types of foundation. This could cause cracks in the buildings during underground construction," said Mr Lim Peng Hong, a fellow at The Institute of Engineers, Singapore and former president of the Association of Consulting Engineers Singapore.

During construction, instruments such as settlement markers, water standpipes, piezometers and tilt and vibration meters should be installed to monitor ground conditions, and to provide early warning of potential damage to nearby buildings.

Engineers have to submit safety plans and monthly reports on the instruments' readings to the BCA.

With good site investigations, risks and surprises during construction can be minimised, said Associate Professor Leong Eng Choon from NTU's School of Civil and Environmental Engineering.

The engineers said it is impossible to have a "100 per cent accurate" picture of the underground rock and construction's effects.

The records of older buildings may be missing, for example, and it may be difficult or too costly to dig too many boreholes in Singapore's built-up environment. Singapore's shallow ground may also be crowded by buried utilities such as electricity cables, water and sewage pipes, and telephone lines, said NTU Assistant Professor Louis Wong, an expert in rock mechanics and underground engineering. These present an obstacle to ground investigations.

Still, risks of incidents such as sinkholes can be reduced by investing in better technology and learning from other countries, the engineers said.

NUS' Dr Grahame Oliver recommended doing more extensive seismic surveys using multiple survey methods. "In this way, areas suitable for deep underground construction can be identified. Future underground projects are likely to be deeper than 50m," he said.

A registry of boreholes and soil data will also help engineers, said Mr Lim. "It is not possible to drill boreholes inside an occupied building, but engineers can interpolate from boreholes in the vicinity. There will always be roads serving all structures, and therefore space to carry out soil investigations," he said.

Last year, the BCA, Land Transport Authority, Singapore Accreditation Council and the Geotechnical Society of Singapore jointly launched an accreditation scheme for site investigation firms to improve the industry's standards.

The Singapore Geology Office was also set up in 2010 under the BCA to create a database on the country's geology to facilitate underground developments. A spokesman said that surveys are being carried out using boreholes up to 200m deep.

It is also using sound waves and borehole televiewers to identify dangerous rock faults and fractures. It declined to say when the work would be done.

While underground construction may be up to four times pricier than for a surface project, Singapore may not have a choice in future, said the engineers.

"You can build up, but there is a limit, because we have airports. You can reclaim, but there is also a limit, as you need to keep fairways and anchorages for your port," BCA chief executive John Keung has said.

He added: "The only thing left is to go underground."

FENG ZENGKUN & GRACE CHUA

This story was first published in The Straits Times on April 16, 2013