Kids’ Guide to Building Tunnels

How do we build tunnels?

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Digging tunnels involves some very special engineers. And how they dig tunnels depends on what they have to tunnel through. If a tunnel is going through hard rock, explosives can be used to blast a way through.

Tunnels through hard rock are often rigid enough to keep their shape and don’t need additional support to be built. But the ground under London is much softer, made of stuff like clay, gravel and chalk. And around the River Thames, it contains quite a lot of water too! Explosives just wouldn’t be right for the job. So in London, Tunnel Boring Machines are used.

Tunnel Boring

They are 150 metres long, and have a seven metre circular plate at one end that’s covered in cutting teeth. As the circular plate slowly rotates, these disk cutters slice into the ground and carve out a tunnel.

The clever part about these machines is that they carry the waste away behind them along conveyor belts back to the surface. And as they move forward, hydraulic equipment pushes rings of concrete into place to create the tunnel, section by section as the whole machine moves forward. It’s these rings that stop the newly dug tunnel from collapsing. These machines are like whole factories underground!

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