Preparation for the arrival of HS2’s first tunnelling machines has passed a milestone with the completion of the 17m high headwall and ground reinforcement at what will become the south portal of the 10-mile long Chiltern tunnel.

Tunnel facing wall (c) HS2

The site, just inside the M25 in North-West London has been excavated down so that the tunnel boring machine can slope downwards as it starts digging under the M25 and then on towards Birmingham.

In order to reinforce the ground behind the headwall, more than 636 ‘soil nails’ – some up to 20m long – were driven into the wall and connected to the concrete lining. This reinforcement is required to hold the ground and the surface of the wall in place as the TBMs begin to break through.

Tunnel facing wall (c) HS2

The two tunnel boring machines are due to arrive on site later this year and will launch early next year.

The ‘twin-bore’ Chiltern tunnels will be the longest and deepest tunnel bores on the route, with separate northbound and southbound tunnels and five ventilation shafts. Tunnelling is expected to take around 3 years to complete.

Once work is complete, the whole construction site will be landscaped with material excavated from the tunnels and trees planted in order to blend it in with the surrounding countryside. The huge tunnel facing wall will vanish underground and likely never be seen again.


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  1. dave says:

    where about is this please? is it accessible to view?

  2. MilesT says:

    Chalfont Lane…near West Hyde or “behind” Chiltern Open Air Museum? (I hope not the latter…)

  3. Steve Ehrlicher says:

    Of course, utilising the existing railway from Old Oak Common to Denham area and then cutting across to pick up the M40 corridor all the way to outskirts of Birmingham was too easy.

    • ianvisits says:

      How many hundreds of houses would need to be demolished to widen the railway between Perival and Ruislip.

  4. Wayne says:

    All this for a shopping centre.

  5. Wayne says:

    Ian, the brevity of the comment was a concise wrapping of my opinion .
    I am grateful for sites such as this, and the work done by people in producing them, however that does not mean I find the construction righteous.
    I saw HS1s rise and now fall. Construction engineering is indeed mathematically complex & awe inspiring, however the aesthetic consequences last beyond the vision of the slide rule. I grew up next to the last part of the M25 to be opened, a necessary evil, and maybe a saviour to the economy argued by many; its still a conduit though.
    The walls have been built , tunnelling commencing soon; portals admired by the drivers of speeding trains as consumers travel. Developers need more.
    Shopping centres are a metaphor, not necessarily a criticism, an observation from someone who sees so many cranes in London and who drove trains and watched the O2, Wembley , Stratford, HS1 et al all being built next to lines I was on, and the irony; only the shopping centres produce consistent revenues for the railway companies.
    Keep up the good work.

  6. George Storrow says:

    Will the ‘Donor Box’ also help pay for the extra £485 million that the the tunnel is costing compared to the original HS2 Project?

  7. Richard Clifford says:

    I’ve put a drone video of the area on YouTube:

    • Bob Robertson says:

      Great footage. That massive headwall is almost completely lost in the vastness of the site.

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