Hard rocks, water seepage: LTA flags unexpected construction challenges along North-South Corridor

SINGAPORE – Construction of the North-South Corridor (NSC) has hit unexpected ground conditions at Novena and Thomson, complicating works as the project enters its most complex engineering phase, the Land Transport Authority said.

In Novena, tunnelling crews encountered a mix of materials, including hard rocks, rather than the more uniform and softer clay material previously anticipated. This caused a change in construction method.

In the Thomson area, the hard rock layer was closer to the surface than expected and large boulders were also found. This slowed progress because more work was needed.

At a media briefing on Sept 8, LTA said that such conditions were fully detected only during excavation. Earlier investigations had been constrained by live traffic, utility networks and existing MRT infrastructure.

Ground conditions are among the challenges adding to the complexity of construction, which is being carried out close to operational rail lines, major roads, utility lines and densely built-up areas.

Asked if these engineering challenges would delay the completion of the corridor, a spokesperson said LTA is monitoring the progress of the works and will give an update on the completion timeline in due course.

LTA had previously announced that the NSC would be ready in phases from 2027, with the northern viaduct from Admiralty West to Lentor Avenue and the road tunnel from Lentor Avenue to East Coast Parkway to be ready in 2029.

During the media briefing, LTA detailed the challenges encountered in Novena, where the road tunnel is being built directly above and below the operational North-South MRT line.

While the southbound tunnel above the MRT tracks has been completed, progress on the northbound tunnel beneath the line has been slowed by mixed rock formations that are harder to dig and underground water seepage that can weaken the structure.

With active train infrastructure running just 1m overhead, LTA pivoted to a different excavation strategy to prevent ground movement and safeguard the tracks.

Instead of excavating the full tunnel width at once, workers are digging it section by section across the width. Work in each section is monitored closely and stabilised before excavation begins in the adjacent section.

Each section of excavation advances the tunnel by 2m in length, less than the 3.3m achieved when engineers built the southbound tunnel using the tunnel box-pushing method.

Also known as box jacking, the construction technique involves a system of powerful hydraulic jacks pushing a rectangular box into the ground material to form the southbound tunnel.

In the Thomson area, unexpectedly harder rock formations were found around 10m closer to the surface than anticipated.

Workers also had to remove large boulders before they could construct reinforced concrete wall panels that outline the tunnel.

Installing these wall panels can take more than four weeks, longer than the typical timeline of one week.

Besides the challenges in Novena and Thomson, complex diversions of gas, water and telecommunication lines have slowed construction of the viaduct in the north, in Gambas Avenue and Woodlands Avenue 8.

In some parts, the rock-level underground was found to be higher and harder than expected. This caused delays in laying the foundations.

This impacted subsequent works, like installing columns and segments, which can only be done after foundation works are completed, LTA said.

Despite the setbacks, over 90 per cent of foundation works for the viaduct in the north are now completed, said Ang Pui Boon, LTA’s deputy director of the NSC.

The focus is now on works such as the installation of sections over MRT tracks and major road junctions.

The major segment installation works can only be done during four- to five-hour windows overnight to minimise disruption to road users and bus services, said LTA.

As the heavy foundation works are largely complete, it expects construction noise to decrease further as most of the upcoming works are underground. The need for multiple traffic diversions will also be reduced, it said.

Originally planned as an expressway in 2008, the NSC was redesigned as a “transit priority corridor” in 2016.

Spanning 21.5km, it is made up of an 8.8km viaduct, 400m of ground-level roads and a 12.3km underground road tunnel.

It connects Admiralty West to the East Coast Parkway, cutting travel times to the city from towns like Sembawang, Yishun, Ang Mo Kio, Toa Payoh, Novena and Rochor.

Once completed, pedestrian and cycling paths, dedicated bus lanes, and community spaces will be added to the repurposed roads above ground.

Work on the NSC started in 2018 and was originally slated for completion by 2027.

However, Covid-19-pandemic-related disruptions delayed the project by more than a year, pushing the completion to 2029.