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What lies beneath: Meet LTA's Digging Machine

Posted on | 28 Jan 2022

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Deep in the heart of the majestic Swiss Alps lies an astounding engineering feat. At 57km long and nearly 2.5km deep, the Gotthard Base Tunnel is the longest and deepest railway tunnel in the world.

This massive concrete structure, which was completed in 2016 after 12 years, could not have been done without the Tunnel Boring Machine (TBM) that helped bore through a seemingly impregnable wall of alpine rock.

In Singapore, TBMs are also used for constructing our public transport rail lines underground, as seen most recently with the Thomson-East Coast Line and Circle Line. This is one mean machine: it devours rocks effortlessly, handles all sorts of ground conditions and has created an underground labyrinth of tunnels here across the past four decades since our first MRT station opened in 1986.

While its name may sound well, boring, it is arguably one of the coolest pieces of machinery that the Land Transport Authority (LTA) uses. We did some digging to unearth some interesting facts about this incredible machine!

Infographic showing the tunnel boring process to excavate and ring-build
Did you know?

Singapore’s geology is definitely not boring. While Bukit Timah may be chock full of solid granite, Kallang is packed with softer sand and toothpaste-like marine clay. Before a tunnelling project begins, tunnel engineers will carefully evaluate and select the appropriate machine most suitable for the ground conditions.

The type of ground determines the speed of tunnelling. Digging through 4m of rock – about half the length of a bus – in 24 hours is considered a good day. On the other hand, the TBM can burrow through 20m of marine clay within the same period – just like squeezing toothpaste through a tube.

Image of workers on ground, with different soil types underground

What’s in a name

Have you heard of Slurry Shield? No, it is not a new beverage or superhero gadget, but one of three types of TBM.

Image of a slurry shield tunnel boring machine
Slurry Shield TBM: 

This machine is like a powerful blender. The cutterhead is filled with bentonite slurry, which serves as a medium to stabilise the tunnel face and remove the excavated material. It is ideal for tunnelling through various ground conditions, from sand to gravel and boulders. 

Image of an Earth Pressure Balance tunnel boring machine
Earth Pressure Balance TBM: 

This machine’s name comes from its method of excavation, where the materials dug out are used to support the tunnel face. The material is later transported out via a conveyor belt onto muck wagons. 

Image of a rectangular tunnel boring machine
Rectangular TBM: 

This machine uses a mechanised shield tunnelling method to construct underground pedestrian and vehicle crossings. The RTBM features a rectangular shield box jacking technique which makes use of the principles of an Earth Pressure Balance Machine during excavation. 

Fun Fact: Naming the three Circle Line 6 (CCL6) TBMs
Image of tunnel boring machine ready for bored tunneling at Keppel station launching shaft

We jointly organised a naming exercise with the main contractors of the Keppel and Prince Edward Road stations, for the three TBMs used in the construction of the MRT tunnels connecting HarbourFront, Keppel, Cantonment and Prince Edward Road stations on the CCL6.  

The themes for the naming exercise were “Japanese God / Warrior”’ and “Chinese Warrior”, intended to symbolise the strong ability of the three TBMs in overcoming challenges to achieve a smooth and timely completion of the underground tunneling works.

Conducted in March 2019, students from Cantonment Primary School were invited to vote from a list of shortlisted names based on the above themes.

“FUJIN” and “RAIJIN” were eventually selected for the two TBMs that would go on to build the tunnels connecting HarbourFront, Keppel and Cantonment stations. These names symbolised the fearsome Japanese God of Wind and God of Thunder that embody speed, power, force and protection.

“SUN WU” was chosen for the TBM that would construct the tunnels between Prince Edward Road and Cantonment stations. “SUN WU” was the birth name of the famous Chinese strategic thinker, Sun Tzu. The name represented leadership, strategic thinking, power, patience and strong organisational ability – qualities that were essential for the TBM to overcome obstacles during tunnelling and reach a smooth and timely completion.

The final tunnel breakthrough for Keppel, Cantonment and Prince Edward Road stations took place on 12 January 2022, marking the completion of all tunneling works along our Circle Line 6! Watch this video on “Completing the Circle Line” to see how far we’ve journeyed. 

Graphic of underground train with cars on road above
Finding space for tunnels

The land beneath Singapore’s surface is starting to get crowded, with electricity cables, pipes and building foundations all jostling for space. This is why LTA’s tunnelling teams have to take extra care not to rock the boat with their work. One solution is to build tunnels under roads where there are typically no foundations. But if absolutely necessary, LTA will also construct tunnels under existing structures, ensuring safety at all times. This helps to optimise land use above and below ground.

Did you know?

The Circle Line that tunnels between Cantonment station and Prince Edward Road station required under-crossing Singapore’s former Tanjong Pagar Railway Station.

To protect this historical building, LTA and its contractors carried out careful planning and protection works before undertaking the under-crossing works. This included conducting extensive foundation investigations to make sure the piles of the building were not within the tunnel alignment, thereby ensuring its structural safety, and installing over 600 instruments to monitor the building during the tunnelling works.

While under-crossing the building, the TBM crow was just 6.7m below the piles of the building. Watch the “Circle Line 6 (CCL6): Overcoming Tunnelling Challenges” video below.

Graphic of underground train with cars on road above
Safe shelter in the cool tunnels

Tunnelling is tough work, but it is literally a cool job – as the tunnels may even be air-conditioned using chiller units! Tunnelling was once traditionally dusty, sweaty and risky work, but things have since improved. Today, the tunnels are well ventilated with fresh air pumped in from the surface.

When it comes to safety, the TBM leaves no stone unturned. It is specifically designed to safely shelter workers whose task is to assemble and secure the concrete rings needed to complete the tunnel.

Image showing lowering of concrete segments

Each concrete segment goes through rounds of rigorous checks, from the segment fabrication yard to the construction site, until they are lowered and installed as rings underground.

As Singapore continues to expand its rail network, our tunnelling teams will continue to dig deep and pave the way for the future of public transport.

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