How can Singapore extend its road network in its dense urban landscape? Mott MacDonald is helping the country’s ambitious project to bring a 21.5km underground highway to life.
Mott MacDonald is working on five of the 11 civil contracts in Singapore’s North South Corridor (NSC) project. $8 Billion in contracts have been awarded to take a massive three lane highway underground and leave the surface open to public transport, greenery, and cycle routes.
It’s part of Singapore’s goal to be car-lite. The complex project will be primarily a cut and cover tunnel interconnected with roadways above ground.
“It’s complicated from an engineering perspective. There are a series of tunnels all connecting into each other, in addition to flyovers passing over. There are also underground MRT lines and a deeper cable tunnel underneath that as well,” says Tom Critchfield, a Principal Engineering Geologist at Mott MacDonald.
The tunnels will run through a wide range of Singapore’s complex geology from their coastal starting point, characterised by reclamation fill over soft marine clays, to the central igneous core of the island where the tunnel will cut into strong and abrasive granitic rocks with a highly undulating rockhead, overlain with a deep tropical weathering profile.
Refining layers of geology with layers of data
“It’s really important in such a densely populated environment to understand the ground risks, and in turn the risks to any other infrastructure items,” explains Alistair Green, a Principal Engineering Geologist at Mott MacDonald.
“The area has complex geology with a number of key risks associated with it. It’s important to use Leapfrog Works to visualise the underlying ground conditions throughout all stages of the design.”
Local policy adds to the complexity. Singapore’s regulations require boreholes or probe holes every 12 metres. It’s a lot of data, but the commonly used wash boring technique with sometimes widely spaced sampling means that data is not necessarily high quality and can create a simplistic view of the layers below.
The weathered nature of the rock can often further complicate the interpretation of the borehole data. As the drilling process itself breaks up the weakened, but otherwise intact, samples it makes it hard to distinguish saprolite and boulders from the in situ solid rock (that is often so critical for deep foundations and embedment of earth retaining systems such as the diaphragm walls used to support the NSC tunnels).
One additional complication, unique to the section around Little India, is the presence of Singapore’s most poorly understood and often most problematic geological deposit – the Fort Canning Boulder Bed.
“The source material for the Fort Canning Boulder Bed material, a historic colluvial deposit, is very close by. So, the difference between a piece of tropically weathered but in situ sandstone and a boulder rich soil which has moved or slipped is quite subtle,” says Tom.
“It is very obvious when you work on a really big tunnel because then you start to find huge boulders, but it’s fairly tricky from boreholes.”
To increase their understanding of the layers, the Mott MacDonald team is doing additional testing and integrating that data into their models.
“We’ve done our best job to model with boreholes, but we’ve also requested CPTs [Cone Penetration Tests] as well, which we’ve also used in the Leapfrog model to try to improve the resolution where we have complex interbeds in the recent quaternary deposits,” says Alistair.
These recent deposits contain the most compressive deposits, such as the organic estuarine silts and marine clays, as well as the most permeable interbeds of fluvial sand.
These layers are often the most critical for the engineering design as they are the most susceptible to settlement and potentially even collapse. Unfortunately, the standard boreholes with wider spaced samples can miss critical layers but CPTs allow centimetre-level resolution.
“We also put in the soil behaviour type (SBT). By putting all the information in, we’re able to come up with quite a nice understanding of these layers.”
The CPT and SBT profile the layers of soil to a finer degree, which gives the team more confidence in how they’ve defined them.
Building on a solid foundation
Solid granite rockhead can hold a cut and cover tunnel securely in place and luckily Mott MacDonald’s contracts contain a lot of it. However, the geology surrounding the rockhead is quite complex, including those layers of soil, loose boulders, and even a buried ancient landslide.
Secure placement of a tunnel’s diaphragm walls is vital to not only safety but costs on the project. If a wall panel is placed on an unstable surface, moving it could double its cost.
That’s why the Mott MacDonald team is also adding rock mass parameters and permeability data to their Leapfrog Works model, finding the simple addition of Rock Quality Designation (RQD) to be particularly valuable to inform the design.
“Low RQD indicates areas of high fracturing with lower rock mass strength and could potentially indicate areas of higher permeability,” says Tom.
Knowing the strong and weak points of the geology below helps the engineers’ design and placement of wall panels. The team also uses their Leapfrog models to select target areas to do additional investigation or testing, including the pile load test, as well as target zones for grouting.
Having a model that incorporates CPT, SPT, pile test and RQD data gives a more precise picture of the ground conditions that can inform important design decisions.
“We’ll use the Leapfrog model to ensure that the contractor selects a nice representative, weathering sequence and to show that the pile test will be valid for a lot of the contract,” says Tom.
Connecting geology and engineering design
While geological models can assist engineering design by decreasing ground risk, engineering design can also help guide the geological model.
“We’re importing the BIM models into our Leapfrog model, so we can see the tunnel with relation to the geology,” says Tom.
“When the engineers want something very quickly, we just cut cross-sections using Leapfrog. Sometimes we cut them, quickly using the serial section tool, at five-metre intervals perpendicular to the tunnel so they can slice their way through it.”
By quickly sharing new insights between their teams, everyone can base their next step on the latest information.
“We’ve been working with the contractor, as it’s on a very, very rapid design schedule to meet the construction needs,” explains Andrew Forsythe, who leads Mott MacDonald’s Engineering Geology Team in Singapore.
“If you think about what we used to do – mostly just 2D sections with only the contour plots of the rockhead and other key surfaces providing any sort of 3D aspect – it was a long workflow because it wasn’t dynamic. Whereas, now if we get new borehole data, we can just stick it straight into our model. This is where Leapfrog really shines.”
As new geology and BIM data come in, the teams can update their geological model in near real-time and then share it quickly – eliminating common information silos.
Communicating confidence to the client
As well as sharing insights within their teams, Singapore’s North South Corridor is a government project and design decisions must be evaluated by officials. A challenge is ensuring that the highly technical, specialised information behind Mott MacDonald’s models is understood by everyone.
“We’ve used Leapfrog on many of these occasions to show our understanding and to show the information that’s available,” says Andrew.
“One of the strengths of Leapfrog is to be able to present that to people who don’t necessarily know much about the ground conditions, and do so in a highly visual manner that anyone can understand with the right explanation. They’re very well received.”
As noted, one of the major considerations for this project – for safety and cost – is the placement of the tunnel wall foundations. Government officials must scrutinize the models thoroughly to ensure risks are minimised before they approve a project phase.
“If we’re with the contractor presenting to the government, often questions are raised about the adequacy of the foundations that we’re placing the diaphragm walls on. So, if we’ve got all the information at our fingertips, we’re able to show the quality of the rock using Leapfrog,” says Andrew.
A massive project, moving fast
The North South Corridor is a design-build project. This means that the Mott MacDonald design team will still be involved in the build process instead of handing it off to a separate contractor.
“The dynamic aspects of Leapfrog allow us to update things by continuously bringing in the BIM model,” says Tom.
“We’re probably about 80% complete on a lot of the site investigations and now it’s quite exciting in the sense that we’re moving forward into construction. We can see how our predictive models will fit into what’s found when we do the excavations.”
Their models will continue to evolve as construction unearths new data. The team will discover if their model matches reality and, if it doesn’t, the build team can react quickly to the new information.
As Singapore moves rapidly to extend its road network, Mott MacDonald’s geological models will be able to keep up.