For many years the London to Portsmouth motorist would often have been delayed in tail-backs where the A3 wound its way up the closely packed contours of the Devil’s Punchbowl, a site of special scientific interest. But not so any more thanks to a twin-bore tunnel which sends the A3 beneath this stunning part of the Surrey landscape.
Here are a few things for the curious motorcar driver to know on as he or she whizzes 65m under the landscape:
- The tunnels were designed by Mott MacDonald and the lead contractor for the construction was Balfour Beatty.
- Construction time for the tunnels was just over three years and followed by 22 years of planning, consultation and design.
- At the northern end, 179 reptiles were trapped and relocated to the other side of the Punchbowl. Clearance of woodland took place over a year to allow local dormice to move out.
- The two tunnels are roughly 1.8km long. They were built using a sprayed concrete method. Using this method each tunnel is built in two halves: the top heading (above) and the bench heading (below). The top heading, which supports the ground above the tunnel, is built first. An excavator is used to dig out 0.5m to 2.0m of ground at the face of the tunnel. The sides of this newly excavated section are then sprayed with quick-setting concrete, to stabilise the newly excavated roof and walls. Whereas concrete is usually reinforced with steel bars, this sprayed-on concrete is reinforced with tiny fibres, which can be mixed in with the wet concrete and sprayed in place very quickly. This sprayed concrete is 200 mm thick. The construction of the bench heading follows the construction of the top heading (for more information on this construction technique see NATM in the links section below).
- The construction team achieved excavation rates of up to 53m/week.
- The traffic rushing through the tunnels keeps them ventilated (this piston effect is also how the London Underground is ventilated). When traffic is slow, a set of fans kick in to keep the air moving.
- The tunnels are fitted with a radar accident detection system that detects slow or stopped vehicles, pedestrians, and animals and large debris.
- The tunnel scheme cost £371m.
- The original A3 carriage way through the Devil’s Punchbowl has been returned to nature.
Directions for an engineering detour
If you are already driving down the A3 then no detour is necessary to view the Hindhead tunnel. By rail the nearest station is Haslemere.
Related posts on this blog
Further reading and references
- Going under the Devil’s Punch Bowl: the story of the A3 Hindhead tunnel, UK, Proceedings of the ICE – Civil Engineering, Volume 165, Issue 4, 01 November 2012 , pages 162 –170 , ISSN: 0965-089X, E-ISSN: 1751-7672 – this article inspired this post.
- Read the National Trust page about this project
- Watch this video of someone riding through the tunnel on their motorbike!
- A video explaining the New Austrian Tunnelling Method (NATM)
- Technical information on the construction of the tunnel from Tunnel Talk
- See the excellent Highways Agency page about this project, which includes project data and diagrams.
- A3 Hindhead Tunnel blog – contains some great photo slide shows of the construction. Note that many of the images in the main body of the blog no-longer appear to link to anything, but the slideshows on the left-hand side do work.
- The Devil’s Punchbowl and the Hindhead Tunnel – an in-depth article from the Wey Valley website that describes in some detail the various stages in the construction and the local reaction.