The two tunnel boring machines that set off from just inside the M25 to dig two HS2 railway tunnels have reached the halfway mark in their progress northwards.

The two tunnel boring machines (TMBs) set off 18 months ago excavating the twin tunnels between the M25 and South Heath in Buckinghamshire and are now 5 miles into their 10 mile tunnel digging efforts.

Chiltern tunnel curving out of sight November 2022 (c) HS2

As well as digging and lining the tunnels, engineers have also completed the excavation of five shafts that will provide ventilation and emergency access near Chalfont St Peter, Chalfont St Giles, Amersham, Little Missenden and Chesham Road.

They also recently completed the first breakthrough from a cross passageway to the shaft at Chalfont St Peter.

Chiltern tunnel with cross passageway visible to the right November 2022 (c) HS2

More than 1.3 million cubic metres of chalk and flint has been excavated and will be used as part of a chalk grassland restoration project at the south portal of the tunnel. This will see the creation of 127 hectares of new landscaping, wildlife habitat and chalk grasslands.

Martyn Noak, HS2’s Head of Tunnel Engineering, said: “It’s great to be able to celebrate the half-way point of this 10-mile long tunnel. The whole team has put in a fantastic effort over the last year-and-a-half and I’d like to congratulate the crews of both Florence and Cecilia for passing this important tunnelling milestone and thank them for all their hard work”

HS2’s main works contractor, Align – a joint venture formed of three international infrastructure companies: Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick is responsible for the Chiltern tunnel and nearby Colne Valley Viaduct.

Designed specifically for the mix of chalk and flints under the Chilterns, the two identical TBMs are excavating separate tunnels for north and southbound trains. The excavated material is mixed with water to form a slurry, before being pumped back to the surface.

Each machine operates as a self-contained underground factory – digging the tunnel, lining it with concrete wall segments and grouting them into place at a speed of around 15 meters a day. More than 56,000 precision-engineered, fibre-reinforced segments have been installed to create the walls of the tunnels – with each one made on site in a dedicated temporary pre-cast factory.

A crew of 17 people operate each TBM, working in shifts to keep the machines running 24/7. They are supported by over 100 people on the surface, managing the logistics and maintaining the smooth progress of the tunnelling operation.

As well as the Chiltern tunnels, HS2 also recently launched two TBMs to begin the project’s London tunnels and marked the first breakthrough with the completion of the first of two tunnels under Long Itchington Wood in Warwickshire.


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  1. Ian says:

    I agree. Absolutely a waste of hard working tax payers money. Are the general public going to be able to pay the expensive ticket price? Has anyone worked in London recently, its dead compared to pre pandemic times. So whats the use of HS2… no one works in London now. Its all home working.

    • ianVisits says:

      If you think no one works in London now — then you’ve just shown that you don’t know London.

    • ChrisC says:

      HS2 isn’t being built for now but to meet transport needs in the decades to come.

      You’re like the people saying new opffice blocks aren’t needed but again they are being built for the future not what’s needed today.

  2. TACJ says:

    It’s good to see progress being made on this important national project.

    I am confident that future governments will reboot the various cancelled sections of HS2 and commit to building a “national metro” of high speed rail, criss-crossing the country.

  3. Brian Field says:

    There are many many people that want to stay
    rooted to today’s world, or even in yesteryear.
    that simply do not want to move into the future.
    The UK needs more infrastructure, especially rail to keep up with a modern world. At present we only have 1 high speed route HS1 , London down to Kent . When you travel to mainland Europe the rail network of TGVs is a wonder to behold.
    Progressive governments have let us down badly
    regarding building a modern rail network, shame on them.we need this railway and more. The contractors make such a great job of putting the environment back together when there done.
    Can you just imagine what IK Brunel went through
    when he first tried to build the great west railway from Paddington to Bristol and all the non believers he had to win round. What a diplomat he must have been. They called him the little giant.
    He truly was .

  4. Nicholas Bennett says:

    If we are to achieve net zero and improve air quality encouraging more passenger and freight traffic onto rail is essential. The west coast mainline is already at capacity and HS2 will provide a fast alternative. Eventually (but not in my lifetime) the vision should be a network of high speed trains connecting the four countries of the UK with a similar network on the continent.

    • Sykobee says:


      This is why the Green Party oppose HS2, and would rather have more cars on the road, more flights, and more roads being built (because there has to be a solution to the problem being fixed, you can’t just not have it built).

      Just yesterday they were moaning again about a few trees being cut down. Yes, it’s sad. But it’s a London Parklet – hardly that natural in the first place, and new ones will surely be planted after the works are done. Probably more.

      Yes, in the future we will have high-speed trains connecting England to Wales, and some adjoining countries.

  5. Winstonio says:

    @Brian Field-Great minds think alike, I totally agree with you.
    Shame many politicians don’t.

  6. Matthew Crump says:

    I differ. So much of my money has been spent on runways and motorways, and it’s just assumed that I approve of that. Now, some of my hard earned money is being invested in a new railway project. I’m glad.

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