Resurrected Stonehenge tunnel means new challenge for designers

Plans to revive tunnelling of the A303 adjacent to Stonehenge announced in the National Infrastructure Plan this week will see designers consider a range of approaches for tackling the complex ground conditions along the route.

Unexpectedly complex geology leading to spiralling costs was a key factor in the government's decision in late 2007 to shelve the original project which included a 2.1km tunnel and upgrades along 12.4km of the A303. At this point the cost had reached £540M from £223M when it went to public inquiry in 2004.

However in its Road Investment Strategy published this week the government resurrected the scheme and detailed a £2bn package of measures for the A303/A30/A358 corridor as initial steps in creating an “Expressway to the South West”.

This includes the construction of a twin bored tunnel at least 2.9km long where the A303 passes Stonehenge as part of a wider scheme to dual the A303 between Amesbury and Berwick Down.

Professor Rory Mortimore conducted detailed site investigations at the site for the Highways Agency in 2003. He is managing director of consultant Chalk Rock Ltd and tells Infrastructure Intelligence that the conditions were more complex than those initially predicted in the early ground investigations carried out in 2001.

The groundwater table was found to be much higher than originally expected and a complex chalk layer had loose and sandy deposits within it. These two elements meant that the initial plan, to use open face tunnel boring machines, was no longer possible, and more specific closed face methods that would provide support to the face of the excavation, needed to be used.

“During the first investigations at Stonehenge the water table was several metres below the proposed tunnel invert. By the autumn of 2002 the water table had risen 8 m to be within the tunnel profile and by January 2003 the water table was above the tunnel crown along the western part of its length,” he says.  

Furthermore the investigations revealed a layer of unusual“phosphatic chalks” containing loose fragmented materials which in some cases disintegrated to a sand when lifted out of the borehole core box. For the A303 tunnel this means that a tunnel boring mechanism which provides support to the face of the excavation must be used. The project team could also opt to vary the vertical alignment of the tunnel to keep the TBM in more competent ground.

The good news for the Highways Agency is that since the original scheme was developed the UK has developed more experience in this kind of tunnelling with the Lee Tunnel and Crossrail projects both using closed face TBMs that provide pressure to the face of the tunnel excavation. Experts say that the cost differential between open face and closed face tunnelling is now much lower than it was in 2002.

News of the scheme’s resurrection has been widely welcomed however some environmentalists are calling for the scheme to include a much longer tunnel. With the government committing to “at least 2.9km” the project characteristics remain open for debate.

Environment, growth and customers key to Roads Investment Strategy - read here

National Infrastructure Plan

Road Investment Plan:

Chalk Rock Consultants

If you would like to contact Bernadette Ballantyne about this, or any other story, please email