In the past many workers were needed to build tunnels – but 21st century technology can make it much simpler – and quicker!

Straight and level tracks help trains run more quickly and maximise passenger comfort. But sometimes there’s an obstacle and the best way to avoid it is with a tunnel.

Basically, a tunnel is a tube hollowed through soil or stone. Sounds simple but constructing a tunnel is one of the most complex challenges facing railway engineers.

Many tunnels are considered technological masterpieces and tunnel engineers have been honoured as heroes.

Underground tunnels have been part of London for nearly 200 years since Isambard Kingdom Brunel opened the Thames Foot Tunnel in 1843.

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He devised new tunnelling shield technology to help build it, and this marked the beginning of a boom in underground construction that has continued to the present day.

How a tunnel is built depends on the material through which it passes, like hard rock, wet clay or loose sand and gravel.

Planning is important to a successful tunnel project. Do you know what is underneath the pavements you walk on? How might you find out?

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Engineers have to do lots of tests to determine the type of material they will be tunnelling through so they can be aware of any risks. More often than not, tunnels will pass through more than one type of material.

Good planning allows engineers to plan for these variations right from the beginning, decreasing the likelihood of an unexpected delay in the middle of the project.

Take Crossrail, Europe’s biggest underground construction project, building 42 kilometres of new tunnels up to 40 metres deep for the Elizabeth line – right through the heart of London, and the Northern Line extension to Battersea.

The tunnels were dug by tunnel boring machines (called TBMs for short). Each TBM is 150 metres long – the equivalent of 14 London buses end-to-end.

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Each TBM is made of a large metal cylinder with a rotating cutting head at the front, behind which is a conveyor belt that removes the earth out of the tunnel.

As each section of tunnel is cut, 7 concrete segments and a keystone create a tunnel ring that’s locked together to build a concrete tube that will last for hundreds of years. Over 200,000 concrete segments line the running tunnels.

The TBM makes use of these concrete rings to move forward – hydraulic rams push against the newly installed ring to push the cutter head forwards. And once the machine has moved sufficient distance, the next concrete ring can be installed.

A laser guidance system helps the tunnelling teams keep the TBM on course. The Elizabeth line ended ended up within a millimetre of where it needs to be.

The TBMs are operated by “tunnel gangs” of 20 people, working in shifts around the clock to construct the tunnels.

Tunnels under major cities like London have to weave their way between existing underground lines, sewers, utility tunnels and building foundations from station to station.

At Tottenham Court Road, one of London’s most congested underground locations, the route had to be built within a metre of an operational Tube platform.