A to Z of Engineering: T is for Tunnels

Welcome to Engineer Academy where we’re exploring an A to Z of Engineering – everything from acoustics to zoos.

In each episode, we spin the wheel to find out what type of engineering we’ll be exploring with the help of Engers, our engineering expert.

You can listen to the full series of the A to Z of Engineering here.

Let’s take a look at Tunnel Engineering.

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There are thousands of tunnels across the UK, some hundreds of years old. The longest tend to be railway tunnels.

The Severn Tunnel is one of the oldest tunnels in the UK and for many years was the longest mainline tunnel. It opened in 1886 and is 4.4 miles long.

The Northern Line under London is longer, running 17 miles between East Finchley and Morden.

And the longest of all is the Channel Tunnel at 23 miles – but half is in France!

You can see the construction of the Channel Tunnel below!

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The development of the HS2 line between London and the Midlands includes the building of 5 new tunnels.

So what is a tunnel anyway? Put simply it’s a space under or through an obstacle. That could be for traffic under a busy city, a high speed rail line linking cities across the country or a way to hide utilities under the ground – things like electricity cables, or water and sewage pipes.

How one constructs a tunnel depends on its length and size, and also on the ground and groundwater conditions through which the tunnel is built.

Tunnel engineering is a specialised subset of civil engineering. Tunnel engineers need to have a deep understanding of different types of soil and rocks, how grounds behave and the interaction between the ground and structure built inside.

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There are two basic types of tunnel construction. Cut-and-cover where tunnels are constructed by digging out a shallow trench and then covering it over; and Bored tunnels. No, they’re not BORING! They’re just tunnels which are constructed by digging out without removing the ground above using equipment like tunnel boring machines – the technical version of a giant mole! We went to HS2 to find out more…

Over half of the HS2 route between London and the West Midlands will be in tunnels or cuttings, helping to reduce the visual impacts in the landscape. They are using 10 giant tunnel boring machines – or TBMs – to dig 103 kms of tunnels between London and Crewe at depths of up to 90 metres. In total, 130 million tonnes of earth will be excavated – that’s enough to fill Wembley Stadium 15 times. The rotating cutter-head at the front of the TBM bores the tunnel, installing the round concrete segments that form the tunnel walls as it goes.

Each TBM is a self-contained underground factory – up to 170m in length – that’s nearly 1.5 times the length of a football pitch, and weighing 2,000 tonnes – the equivalent of 340 African bush elephants!

They dig the tunnel, line it with concrete wall segments and grout them into place as it moves forward. A crew of 17 keeps the machines running, supported by a team of over 100 people managing logistics and maintaining the smooth progress of the tunnelling operation. Each tunnel requires tens of thousands of precision engineered, fibre-reinforced concrete wall segments – which are all are made on site.

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HS2 are also building tunnels using the cut-and-cover method of construction with trees and shrubs planted on top to become green tunnels.

First, a cutting is excavated where the green tunnel is needed. The excavated earth is kept close by as it will be needed later on. In the second stage, a concrete floor is laid and then pre-cast segments are installed forming the structure of the tunnel. The final stage occurs after the tunnels have been installed. The earth that was removed is replaced on top of the new structure, new trees and shrubs are planted and the tunnel blends into the landscape, connecting wildlife habitats along the line of route.

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Unlike TBM created tunnels which are circular, green tunnels are designed as an ‘m-shaped’ double arch, each the height of two double-decker buses. And instead of casting the concrete segments om site, they’re made in bulk in Derbyshire to speed up construction and improve efficiency. They’re slotted together to create a double arch – one central pier, two side walls and two roof slabs – the largest weighing 43 tonnes.

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Concrete and steel are some of the biggest sources of carbon emissions within the construction industry and by reducing the amount of both materials needed for the tunnel, this lighter-weight modular approach is expected to more than halve the amount of carbon embedded in the structure. It also requires less people and equipment on site, improving safety and reducing disruption for residents.

The green tunnels have specially designed ‘porous portals’ at either end to reduce the noise of trains entering and exiting the tunnel, along with small portal buildings to house safety and electrical equipment.

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The beauty of being a tunnel engineer is that you get to see the process of how the underground space is created. The scale of these mega projects is almost unimaginable, no matter how much experience you have. Tunnel projects are costly and complex. They’re mega projects that can take 5 to 10 years to build.

And that’s our take on the letter T – it’s been TERRIFIC!

If you would like to check out some other types of engineering, why not check out Telecommunications, Television, Textile or Transport engineering.

Join us again next time to spin the wheel and explore another letter in the A to Z of Engineering!