North-South Corridor: Foundations for road tunnels about 50% complete

SINGAPORE – A $7.47 billion project to build an integrated transport corridor from northern Singapore to the city is approaching the halfway point of its nine-year construction timeline, after Covid-19 delayed it by a year.

Work on the 21.5km North-South Corridor (NSC), which starts in Admiralty Road West and ends in East Coast Parkway near Republic Boulevard, started in 2018, and is on track to be completed from 2027 onwards.

Comprising an 8.8km viaduct in the north and 12.3km of underground road tunnels in the south, as well as continuous bus lanes and cycling trunk routes, the NSC promises to cut travel time from Woodlands, Sembawang, Yishun and Ang Mo Kio to the city by up to 30 minutes when ready.

Providing an update on the progress of the project, the Land Transport Authority (LTA) told The Straits Times that the current work involves the installation of diaphragm walls and diversions of utilities and traffic that come with it – a key step before tunnel construction can begin.

Diaphragm walls are reinforced concrete walls that outline underground tunnels, acting both as a supporting structure during tunnel excavation and as a permanent foundation and wall after the tunnels are completed.

LTA has to make room for this construction work beforehand by diverting important utilities that cut across the NSC, such as power and telecommunication lines.

It said it is working with various agencies to sequentially divert affected utility lines. This is being done in phases to minimise disruption to these essential services.

The installation of the diaphragm walls for the NSC is about 50 per cent done, said LTA.

Once the foundations are set up, excavation of the road tunnels can begin, and the roof and base of the tunnels can then be built.

According to previous reports, the NSC road tunnels are expected to be completed in 2026.

The reinstatement of roads and other paths above the tunnels is estimated to be finished from 2027. LTA intends to work with residents in various towns to redesign these surface streets.

It said the NSC is Singapore’s largest cut-and-cover tunnel project, and the fact that it runs through areas that are densely built-up – both above and below ground – adds to its complexity.

Cut-and-cover refers to a traditional tunnelling method that involves excavating a deep trench, constructing the tunnel, and then returning the surface to its original state. The 9km Kallang-Paya Lebar Expressway tunnel was also built using this method.

LTA also revealed that a section of the NSC tunnels along Thomson Road will be stacked, with the south-bound tunnel on top and the north-bound tunnel below, as there is little room there for the tunnels to be built side by side.