A new dual-layered transport corridor for Singapore

World leading innovation on a transit project

Challenging ground conditions and proximity to existing structures have made our team’s decades of expertise invaluable in finding innovative solutions on the North-South Corridor (NSC) project. A dual-layered transport corridor, it is planned to run from residential areas in the north of Singapore into the city centre.

The 21.5km corridor will relieve congestion through a mix of tunnel and elevated viaduct, while providing dedicated bus lanes, cycling and pedestrian paths at the street-level. Its first 12.5km route comprises a stacked box tunnel that dives beneath the majestic modern skyscrapers and traditional street markets in the central business district (CBD). Emerging north of the city centre, vehicles will continue their journey at surface level before the route transitions into 9km of elevated viaduct.

1m proximity to metro tunnels in places

Engineering design for this project is made particularly complex by its tight proximity to the underground transport infrastructure and the complex network of utilities that support the city. It will run next to, above and beneath the tunnels of the city’s metro network, with clearances of as little as 1m.

To protect the existing built environment and to ensure the best possible design for the new route, the engineering teams must analyse and understand the impact of the wide array of ground conditions on both the new route and the existing infrastructure. This presents an enormous range of challenges for Mott MacDonald’s engineering teams, who are providing a full range of consultancy services under design and build contracts across five of the 11 major civils contracts. These are N101, N102, N105, N106 and N107 (see figure 1 in contracts panel).

“This is definitely one of the hardest engineering projects I’ve ever worked on,” says Nick Osborne, project principal for Mott MacDonald. “There is the geology, which is very difficult. In the southern area, we’ve got some very soft marine clays that need to be treated before construction can begin. And then we go all the way through to granites further north,” he says. These very hard rocks can be difficult to excavate. “That is further compounded by the fact that the projects are right in the heart of Singapore,” says Nick, noting the interfaces with the existing mass rapid transit system and underground utilities. Each of the contracts has its own challenges, but as an example Nick points to Marymount Road where the MRT line runs close beneath the excavation for a considerable portion of the alignment.

Diagonal struts on the N107 tunnel section between Toa Payoh Rise and Marymount Lane

Fortunately, Nick and his team are bringing many years of experience working in Singapore’s geologically changeable strata. Over the past two decades this includes a range of cut and cover road projects from the Kallang-Paya Lebar Expressway to the Marina’s coastal road and 32 of the 130 stations on the 200km mass rapid transit network. In fact, Singapore’s first ever power plant, the St James power station, was designed by our founding partners Mott, Hay & Anderson in 1926.

“We’ve got lots of experience of engineering in these ground conditions, and we learn as much as we can from our previous projects,” says Dr Nick Mace, technical excellence director leading NSC geotechnical design. Data from 2500 boreholes has been used to populate a modelling tool called Leapfrog, which Mott MacDonald helped software provider Seequent to develop back in 2018. “This is where Leapfrog’s very useful. Because boreholes are quite far apart, you know exactly what the geology is at those two locations, but you don’t have sight of what is happening in the middle. Leapfrog uses some quite clever processes to do predictions of what’s going on in between them.”

The next step is the analytic modelling to show how the soil interacts with proposed and existing structures. This finite element analysis is carried out using a software called Plaxis. But it doesn’t stop there. After this comes the back analysis, to ensure that what contractors measure on site is in line with the predictions. “The best way of seeing what the properties of these materials is by looking at how much they actually move, and then you can recalibrate your model,” says Nick. Taking this process further, Mott MacDonald with Spanish firm SAALG Geomechanics has developed a software called Daarwin that can use real-time construction data to back analyse performance and then run this through a genetic algorithm to determine the optimum design. “The development of these digital tools is super important for us,” says Nick.

Five complex contracts

Each of the contracts on the North-South Corridor project presents its own complex underground challenges for designers, from difficult ground conditions and proximity to existing buildings, to multiple interfaces with existing transport and utility systems.

North-South Corridor route and contract packages

Contract N101: Tunnel between East Coast Parkway and Victoria Street

Client: GS Engineering & Construction Corp

Scope: Consultancy services for the design and construction of 990m route length of northbound and southbound NSC tunnels, ramps, at-grade roads, facility building and vehicular viaduct structures and demolition of an existing flyover.

Contract N102: Tunnel between Victoria Street and Kampong Java Road

Client: Ssangyong-Wai Fong JV

Scope: Consultancy services for the design of both temporary and permanent structures for the construction of 1420m of a twin-cell box vehicular tunnel structure with four slip roads; this includes a challenging tunnel crossing above existing mass rapid transit tunnels with tight clearance.

Contract N105: Tunnel between Suffolk Walk and Novena Rise

Client: Penta-Ocean Construction Co and Bachy Soletanche JV

Scope: Consultancy services for the design and construction of a 1040m stretch of four lane stacked carriageway tunnel and commuter facilities. The contract includes a segment of mined stacked tunnels across the existing North-South line.

Contract N106: Tunnel between Novena Rise and Toa Payoh Rise

Client: Samsung C&T Corp

Scope: Consultancy services for the design and construction of 1250m of main tunnel as well as all slip roads, a facility building, eight escape staircases, six pedestrian overhead bridges, reinstated at-grade roads and associated infrastructure.

Contract N107: Tunnel between Toa Payoh Rise and Marymount Lane

Client: Samsung C&T Corp

Scope: Consultancy services for the design and construction of 1.37km of twin-cell box vehicular tunnel structures, a ramp structure for transition to at-grade roads, commuter facilities and the proposed Marymount Facility Building.

Visualising geotechnical challenges

The route encompasses a range of geological conditions, with distinctive demands on each section.

Leapfrog model of the varying geological strata along the route

Singapore has three main geologies. Nick Mace describes the first of these as a granitic core, which is very hard – but also weathered, making it potentially problematic. Secondly the island has sedimentary rock formations, but the NSC route won’t run through these. Finally, along the southern end of the route there is the third type of geology, which is the most difficult to engineer: soft marine clay sitting atop good quality alluvial deposits. An added difficulty is that Singapore’s extensive land reclamation work means some of these areas are more geologically dynamic than others. “Because it’s still consolidating. It compresses significantly, which is a key challenge,” says Nick.

“We started work on contract N101 first, and it is just outside our office,” explains Heidi Seah, a geotechnical engineer on the contract. The marine clay needs a lot of ground improvement work, she says, noting that the team are seeking to find the right balance between ensuring there is enough ground improvement work for structural integrity of the marine clay, without being overly conservative in the use of the cement used to stabilise and strengthen.

N101 excavation beside the Kampong Glam conservation area, where the ancient shophouses need to be kept stable

To the north of this contract package are the historic shophouses of Kampong Glam. These must not be affected by the new tunnel, but the terraced buildings are problematic because of historic improvement works. Previously if a shophouse was renovated the contractors put in micro-piles to support them in the soft ground. If one structure sits on deep foundations but the one beside it doesn’t, the movement at the interface is concentrated and this can lead to significant damage.

To limit movement of the shophouses, the team is recommending extensive ground improvement and thick diaphragm walls.

Clever engineering

Faced by demanding and variable conditions, our team adopted a sensitive, adaptive approach supported by careful monitoring.

A BIM model of part of the N101 section shows the location’s complexity

The southern end of contract N101 highlights a challenge facing the team on all the contracts, and that is the combination of difficult ground conditions and proximity to the existing infrastructure. Nick Mace describes this section as “supercomplex” as the road tunnel splits into three while the Circle Line runs below. “We’re in that tricky ground condition here where we’ve got thick, soft clays. And this is an area that was actually in the sea until the 1970s.”

What this means is that the land is still consolidating, and so we needed to understand how this would affect the structure in the very long term. This means moving from small strain to large strain finite element analysis modelling, which is more complex. The team had to capture the current consolidation state of the ground over 150m sections and model historic settlement, to predict its future behaviour.

120 geotechnical and structural engineers on the team

Given that there are similar challenges across all the contracts, it is not surprising that construction involves an extensive monitoring programme. Mott MacDonald geotechnical engineers Rosemary Vong and Heidi Seah are leading this programme and explain that they are monitoring ground performance, excavation support performance and building performance.

Variables that the engineers need to consider along the 12.5km tunnel route include wall movement or deflection, lateral or vertical ground displacement, changes in pore water pressure, displacement in any existing structures and the appearance of any cracking. To do this requires an array of equipment from inclinometers and strain gauges to settlement markers and piezometers (which measure groundwater pressure). Monitoring is carried out either manually or with real-time systems, and each monitoring instrument is assigned specific values (trigger levels) predicted from the analysis for each stage of construction.

For many on the team, this is one of the most technically difficult projects they have ever been part of – and the work they are doing here brings a real sense of pride. In some areas of the project, it is world leading. On contract N106, the work of local structural engineers, supported by some of the most experienced bridge designers in the world, has led to a new and much more efficient underground tunnel that has been designed as if it were a bridge (see below).

The whole team – which includes around 50 geotechnical engineers and 70 civil structural engineers – know they are contributing to a project that is set to improve its users’ quality of life. “It was originally planned as an expressway, but NSC was reconceptualised to become Singapore’s longest transit priority corridor (TPC) and will now include dedicated, continuous bus lanes saving bus commuters between 10 and 15 minutes,” explains Nick Osborne. Traffic flow in the tunnel will significantly reduce congestion of, and add flexibility to, the surface route that is also set to host cycle and pedestrian paths. All of this will contribute to meeting the aims of Singapore’s Land Transport Masterplan 2040, to make infrastructure safer and more inclusive.

‘A strong foundation for my career ahead’

Three early career professionals who worked on the N101 contract – geotechnical engineers Heidi Seah and Rosemary Vong and structural engineer Wendy Sjamsudin – talk about what they enjoyed most about the experience:

“I am privileged to be working on the N101 project which is right beside the MM Singapore office. It’s a priceless feeling to look down from the window and watch the design come to life every day! Also, the exchange of knowledge between us consultants and the contractors is a two-way street, and everyone is learning and growing along the way. The project is definitely a challenging one but the team’s commitment in delivering it with pride is remarkable – something everyone working on the NSC projects can be proud of. I am thankful for this experience!”
Heidi Seah, geotechnical engineer, Mott MacDonald

From our Singapore office window, engineers like Heidi could look down and watch their design come to life

“The best thing about working on this project is being able to learn new things every day, and to be able to look forward and be proud of the work we’ve done when the corridor is completed. Most importantly, it is the people in this project. Support, help and guidance were always available. Thank you, Nick Osborne, Tan Rwe Yun, Wai Mon Thi and Jasper Sim for being the main drive motivating me to do better.”
Rosemary Vong, geotechnical engineer, Mott MacDonald

“The North-South Corridor (NSC) contract N101 is a big and complex project and, personally, N101 is my first underground tunnel project. I think one of the most interesting aspects of this project is the variety and complexity of work that I am involved in. The proposed tunnel is located at a congested area which crosses existing underground MRT tunnels. These site conditions make the project challenging, as construction will be ongoing while the MRT lines are in operation. In this project, I get involved in the design of diverse structures ranging from a simple single-box tunnel to complex interfaces with existing infrastructure that have to be analysed in a 3D model considering numerous constraints and challenges. Each design has taught me valuable lessons that help to hone my skills as an engineer, and I am thankful to be given this exposure as an early career professional as I believe it will set a strong foundation for my career ahead.”
Wendy Sjamsudin, structural engineer, Mott MacDonald

Inventive tunnelling: the ‘underground bridge’

Structural engineering lead Krishna Nadarajah faced a tricky problem on contract N106. The new stacked box road tunnel interfaced with several existing infrastructure, such as the Bukit Timah Canal, a highway viaduct and MRT lines. Space was tight. “The MRT tunnel is below, the road tunnel is above – and that’s why we have introduced the bridge system,” he says.

The tight layout of the N106 section

The underground bridge system uses the diaphragm walls to connect to the base slab forming the tunnel box to span across rail tunnels and supported on two locations.  This eliminates the need for the internal concrete box that was originally to have been poured insitu after installation of the temporary works.

Finite element modelling (FEM) analysis and a deep understanding of structural performance assured Krishna and the contractor that the new design was more than capable of meeting the project requirements. To explain to the local authority, the team brought in some of the world’s most experienced bridge designers from Australia and the UK.

“The main objective was to explain that the lowest of the motorways going across the rail tunnels would not put extra load onto the rail tunnels,” explains Christian Frandsen, a technical director and bridge specialist in Mott MacDonald’s Adelaide office. “The first thing I realised looking at the numbers was that actually the way they were building it would relieve the weight on the rail tunnels, which was quite a revelation.”

Demonstrating this to the local building authority meant carefully describing the behaviour of the loads being transferred from the base and roof slabs. “So what we did here in Adelaide was to analyse what was going on in a simplified manner,” says Christian. “And we boiled it down to a question the client had, which was: how do you do the shear transfer from the slabs of the tunnel, the bottom and top slabs, which is basically acting as a flange in terms of a beam? And the diaphragm was then considered the web of the beam.”

But this is not how the structure was going to perform. Each of the diaphragm wall panels was so wide and the gap between them so small, at 300mm, that it was working as a network of elements rather than as individual beams. This was clearly demonstrated in Christian’s simplified modelling. “And then comparing that with the FEM models, we could get reasonably comparable results. Where we would expect to see the high stresses, that’s where they were. So, does the model give you what you expect to see? And the answer was: yes.”

Thus the design was shown to be quite conservative and the client was convinced. “This was the happiest moment in the whole process,” says Krishna, who explains that the contractor has been able to use this design to make huge material savings – reducing both cost and carbon.

How we improved the tunnel design for the N106 contract section

Geotechnical engineer Rosemary Vong explains the responsibilities she has on the project

Structural engineer Wendy Sjamsudin explains some of the challenges of the North-South Corridor project