For the first time, the Land Transport Authority (LTA) is using ice walls to stabilise the earth before conducting MRT tunnel excavation works.
The ground-freezing method is being employed about 40m underground for construction work on the Thomson-East Coast Line's (TEL) Marina Bay station, which is targeted to open in 2021.
It is required because the soil which workers are tunnelling through is old alluvium, which is highly permeable and susceptible to water seepage.
Giving an update during a site visit yesterday by Transport Minister Khaw Boon Wan and members of the Government Parliamentary Committee for Transport, the LTA said the ground-freezing process started in late March.
After about two months, two ice walls have been formed along a 40m stretch underground.
They are about 1.8m thick and will allow excavation for the northbound TEL tunnel - towards Woodlands - to be carried out safely.
Tunnelling using excavators has also started, and a 6m stretch has been excavated so far.
The freezing was done by installing 96 vertical pipes into the ground and circulating chilled brine - a mixture of salt and water - with a temperature of minus 30 deg C, to allow the surrounding soil to freeze and form ice columns.
These ice columns have expanded and joined together to form a continuous ice wall, which will help prevent water seepage during mining works.
The works are being carried out by Japanese contractor Taisei Corporation, which won a $425 million contract in 2014 to build the TEL's Marina Bay station and its associated tunnels.
The LTA said: "The ground freezing will also help ensure that works have no impact to the safe operations of the Circle Line and North-South Line tunnels located above the TEL tunnels."
The ice walls will be in place for about three months until the tunnel construction is completed.
Mr Ng Kee Nam, group director for the TEL and Cross Island Lines (Civil), said that throughout this period, the temperature of the ice walls will be monitored to ensure it is maintained.
Once the tunnel construction is finished, the freeze pipes will be deactivated, allowing the ground to thaw. Construction works for the upper eastbound TEL tunnel above will then commence.
Mr Ng said that multiple site investigations and soil checks were carried out before deciding on the ground-freezing method.
The 43km, 31-station TEL will open progressively from the end of next year, connecting commuters living in the eastern region to the city centre, as well as those living in the Woodlands and Thomson areas. It is expected to be fully operational in 2024.