Tunnelling under major roads and MRT lines: North-South Corridor construction enters 'most challenging phase'

Singapore From rock blasting in Thomson to tunnelling a metre beneath the North-South Line, engineers describe the obstacles slowing down one of Singapore’s largest transport projects. Construction of the North-South Corridor has been ongoing since 2018 and will be completed in phases. (Photo: CNA/Raj Nadarajan) New: You can now listen to articles. This audio is generated by an AI tool. SINGAPORE: Construction of the North-South Corridor (NSC) is entering its "most technically demanding" phase, as work progresses amid challenging ground conditions and the unprecedented scale of its viaduct, the Land Transport Authority (LTA) said on Saturday (Sep 12).At several locations, the corridor has to be built above and below operational MRT tunnels, including those on the North-South and Circle lines. Ground conditions have varied considerably along the corridor, with harder rock formations and differing rock profiles complicating foundation, excavation and tunnelling works. This has required additional rock removal, ground stabilisation or changes to construction methods.Mr Ang Pui Boon, LTA’s second deputy group director for the NSC project, said more challenging ground conditions had been encountered as work progressed. “We therefore have to make adjustments to our design and construction methods to ensure that the work is carried out safely (without) compromising our surrounding infrastructure.”CONSTRUCTION ONGOING SINCE 2018Spanning more than 20km, the corridor is a multi-modal transport link designed to improve connectivity between the northern region and the city, serving towns such as Sembawang, Yishun, Ang Mo Kio, Toa Payoh, Novena and Rochor, while relieving traffic along Singapore’s north-south transport axis.The project comprises two major structures: an 8.8km elevated viaduct and a 12.3km underground tunnel section. Construction has been ongoing since 2018 and will be completed in phases. LTA previously said the viaduct section is expected to open from 2027. SINGAPORE'S LONGEST VIADUCTAt 8.8km long, about 40m wide and roughly 19m tall – more than six storeys – the NSC viaduct will be Singapore's longest. More than 6,000 precast elements have to be fabricated, transported and installed to form the structure, which LTA said presents significant logistical challenges. Major segment installation is carried out mainly during four- to five-hour off-peak windows at night to limit disruption to motorists and bus services. At some locations, the viaduct crosses live roads, major junctions and existing transport infrastructure, further limiting the time and space available for construction. Disruptions to these narrow work windows have also affected subsequent activities such as segment installation. Construction of the North-South Corridor next to residential properties at Yishun Avenue 5 on Jun 8, 2026. (Photo: CNA/Raj Nadarajan) Foundation works have also taken longer than anticipated at some locations, including Gambas Avenue and Woodlands Avenue 8, which LTA attributed to utility diversions and adverse ground conditions during piling and pilecap construction.“As the viaduct works are highly sequential, delays to the foundation work have affected subsequent activities, as above-ground works such as columns, crossheads and segment installation can only proceed after the foundation is completed,” LTA said.With more than 90 per cent of foundation works completed, construction is moving into another complex stage: launching large viaduct segments across existing roads while road traffic continues and safety is maintained."The current focus is on above-ground structural works, including the launching operation over the MRT track and major junctions. These activities require meticulous planning and close coordination to ensure they are carried out safely, particularly as the works are undertaken above an operational MRT line and live roads," LTA said. TUNNELLING BENEATH MRT LINEOne of the project’s most technically demanding sections is at Novena, where the corridor’s tunnel passes both above and below the North-South Line, with clearance as close as 1m.The southbound tunnel crossing above the North-South Line has been completed using the tunnel box pushing method. But as excavation progressed on the northbound tunnel below the line, fractured rock and groundwater inflows were encountered.“These ground conditions could not be fully identified in advance, as they are located beneath the existing operational NSL tunnels and only became apparent as excavation reached the lower crossing,” LTA said.LTA is now using a more controlled mining method, excavating the tunnel in smaller, separately supported sections alongside additional rock removal, grouting and monitoring, with each section stabilised before excavation moves to the next. Although more time-intensive, LTA said this is necessary to build the tunnel beneath the operational line while protecting the rail infrastructure and maintaining MRT operations. A man walks past a poster for the North-South Corridor at Woodlands Drive on Jun 8, 2026. (Photo: CNA/Raj Nadarajan) At Nicoll Highway, the NSC tunnel has to pass over existing Circle Line tunnels, with clearances of only about 1.5m to 2.5m at some locations. Given this proximity and the presence of soft reclaimed ground, LTA is using specialised pipe roofing and ground improvement measures to stabilise the ground and protect the Circle Line tunnels.At Kampong Java, stacked tunnels are being built beneath the Kampong Java Flyover on the Central Expressway. The flyover has to be underpinned while construction proceeds beneath it, with workers operating in a low-headroom space under complex and varying ground conditions.In the Thomson area, hard rock formations and large boulders encountered during excavation have required extensive controlled rock blasting and changes to the design of both temporary and permanent earth-retaining systems.“Additional rock removal and ground stabilisation works are required before excavation can proceed safely,” LTA said, adding that the measures help control ground movement during deep excavation in densely built-up areas. Construction of the North-South Corridor along Thomson Road on Jun 8, 2026. (Photo: CNA/Raj Nadarajan) IMPACT ON RESIDENTS, BUSINESSESThe specialised construction methods are necessary to manage the complexities of working near existing infrastructure, protect workers and commuters, and minimise disruption to the public, LTA said, adding that it will update on the project's completion timeline in due course.The impact on residents is expected to ease as construction progresses. With heavy foundation works completed, noise levels are expected to continue tapering off, LTA said. Most upcoming works will also be carried out underground beneath live traffic, cutting noise and the need for multiple traffic diversions.“LTA recognises that hoardings and noise barriers may affect business visibility and footfall, including in areas such as Novena," it said."Where feasible, LTA works with affected stakeholders on tailored solutions, including directional signage and advertising banners, and will continue engaging business owners as works progress,” the transport authority said.