The most challenging part of the new MRT Downtown Line 3 project involves the diversion of the Singapore River.

While this is not the first time Singapore is tunnelling below the riverbed, it is the first time the river is made to change course for the construction of an MRT line.

The unusual construction method was necessary because the proximity of the Central Expressway (which also goes under the river) ruled out other methods.

Land Transport Authority (LTA) project director Chang Kin Boon told The Straits Times none of the 200 monitors installed in the nearby CTE to detect structural movements had triggered an alarm.

The LTA said tunnelling under the river between the Chinatown and Bencoolen stations will be completed by year end. Work to reinstate the original flow of the waterway should finish by next year.

Downtown Line 3 will link Chinatown to Changi by 2017.

Two main methods are used to build Singapore's MRT tunnels - the cut-and-cover method, and the tunnel-boring method. The latter is used to build Downtown Line 3 tunnels under the Singapore River.