Tunnel Boring Machines or TBMs are giant machines used to build tunnels. The TBM is an amazing piece of machinery. It can tunnel under a town or city without disturbing anyone above ground, and in rural areas this tunnelling helps to reduce the impact on the countryside.

How does it work?

Each TBM is made up of thousands of parts including:

• a rotating cutterhead
• a screw conveyor
• conveyor belts or extraction pipes
• a tunnel segment erecto

These help the TBM carry out its two main functions of digging the tunnel and building the tunnel.

To dig the tunnels, giant cutterheads at the front of the TBM rotate, cutting away at the earth. When digging, the earth is carried up the screw conveyor and out of the TBM within a slurry pipe or on a conveyor belt. On average they will dig up to 15 metres a day.

Building the HS2 tunnels

We will use TBMs to excavate tunnels through a variety of soil and rock in dense urban areas and to reduce environmental impacts in rural areas.

In total 10 giant TBMs will excavate new tunnels on the first phase of the high speed railway between London and the West Midlands.

Each machine operates as a self-contained underground factory, which as well as digging the tunnel, will also line it with concrete wall segments and grout them into place as it moves forward at a speed of around 15 metres a day. A crew of 17 people will operate each TBM, working in shifts to keep the machines running 24/7. They will be supported by people on the surface, managing the logistics and maintaining the smooth progress of the tunnelling operation.

Boring statistics – HS2 tunnelling in numbers

• HS2 will use ten giant tunnelling machines to construct new high speed rail tunnels for the London to West Midlands (Phase One) section of the route.
• The TBMs are up to 170m in length – nearly 1.5 times the length of a football pitch.
• Each one weighs roughly 2000 tonnes – the equivalent of 340 African bush elephants.
• The HS2 tunnels will go as deep as 90 metres (m) below the ground – ensuring communities and countryside above are not impacted by the railway.
• The size of the largest HS2 TBM cutterhead is 10.26m, roughly the height of two giraffes standing on top of one another.
• The internal diameter of the tunnels in which the trains will pass through will be around 9m, slightly larger than two London buses stacked on top of one another.
• The tunnels will be lined with concrete segments that will be 2m x 4m and weigh on average 8.5 tonnes each.