Ice walls are to be used for the first time in Singapore, to stabilise the ground during construction of the Thomson Line Marina Bay station.
Japanese contractor Taisei Corporation will freeze a section of earth to prevent water from seeping in when workers excavate a 40m stretch of tunnel for the station. Engineers will insert vertical freezing pipes into the ground and pump in brine, a refrigerant, to freeze the groundwater into watertight ice walls.
Mr Shaik Sha Marican, Land Transport Authority (LTA) director of the Thomson Line Civil Team 4, said the ice walls are a precautionary measure recommended by consultants who carried out an engineering study.
The 40m stretch of Thomson Line tunnel will run below the live Circle Line and North-South Line tunnels.
Mr Sha Marican explained that exposure to water over a period of time can weaken the soil and affect mining operations. This could have an impact on the two MRT lines running above.
"We are working below two live tunnels, so we do not want things to go wrong," he said.
Ice walls have been used in Japan, and Taisei has employed the method previously. It was awarded a $425 million contract to build the Thomson Line Marina Bay station and its tunnels.
Several steps must be taken before inserting vertical freezing pipes to create the ice walls.
Engineers must construct mining shafts on both sides of the 40m tunnel, then create openings to build a pedestrian linkway about 20m below the surface.
Shield tunnelling machines will be launched to cut existing steel piles that support the North-South Line tunnel. The machine will work in 1m sections, excavating and cutting piles before steel segments are installed to support the excavated surface.
After the piles are cleared, engineers will strengthen a section of the ground made up mainly of soft marine clay, which Mr Sha Marican described as "toothpaste-like material". The marine clay section extends to a depth of between 20m and 30m.
Cement and water will be injected into the marine clay to solidify it - a process called jet grouting. Vertical freezing pipes will also be installed to freeze the groundwater in the soil bed below the marine clay to a depth of 41m.
Brine will be pumped into the pipes to freeze the groundwater around and form ice walls. The refrigerant can range from -10 deg C to -40 deg C, depending on how thick the walls have to be.
Mr Sha Marican said details are being finalised. It could take a month for the ice walls to form. The LTA will monitor ground temperatures closely, and pump more brine into the ground if needed.
The ice walls will be in place for about six months while the lower Thomson Line tunnel is being excavated. Those working in the tunnel may have to wear winter clothing if it gets too cold, Mr Sha Marican said.
The freezing pipes will be deactivated after the lower tunnel is completed, so that the surrounding groundwater melts. Structural work for the Marina Bay station is expected to conclude in 2018.