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The HS2 Tunnel Boring Machines (TBMs)

What is a tunnel boring machine?

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. Each TBM is made up of thousands of parts including:
  • a rotating cutterhead;
  • a screw conveyor;
  • conveyor belts or extraction pipes; and
  • a tunnel segment erector.
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 can dig up to 15 metres a day!
Cross section of a TBM with labels showing the various parts.

HS2 tunnel boring machines

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 64 miles of 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.

Our TBMS will be 170 metres long

and dig up to 90 metres below ground.

Florence and Cecilia

The first of these TBMs, Florence, was launched in May 2021 and will dig the 10-mile-long Chiltern tunnels – operating 24/7 for the next 3 years. Cecilia, the second of the 2000 tonne TBMs measuring over 170 metres each will be launched shortly at the same site to bore a parallel tunnel going underneath the Chilterns in Buckinghamshire.

These two TBMs will be operated by HS2’s main works contractor, Align – a joint venture formed of three international infrastructure companies: Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.

Both machines are specifically designed for the mix of chalk and flint they will encounter under the Chilterns. Operating a ‘continuous boring’ technique, they are expected to take around 3 years to excavate the 9.1m metre diameter tunnels which will be lined with concrete as they go. The TBMs will convert the spoil into slurry which will be pumped back to the Align main site where it will be treated before being used for landscaping on-site, removing the need for additional HGVs on local roads.

Find out how Florence and Cecilia got their names

The story so far

Florence and Cecilia are our first TBMs. The enormous 2,000 tonne machines were built in a factory by world-leading German tunnelling specialists Herrenknecht. After completion the machines were disassembled before beginning their long journey to England. They were shipped in pieces to Align’s main site to the west of London just inside the M25.

On arrival at site they were assembled by the entrance to the tunnels (see timelapse video above), where a specially prepared head wall allows them to safely begin their excavation.

Florence has now started to dig and build the first tunnel and Cecilia will shortly follow her to build the second of the twin bore tunnels in the Chilterns.

Tunnel boring machines factory assembly

Boring statistics

  • The TBMs are 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.
  • When they start they will run non-stop for 3.5 years.
  • The 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 TBM cutterhead which will bore the tunnels 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 9.1m, 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.
  • 112,000 of these concrete segments will be required to complete both tunnels.