A £1.4 billion plan proposes replacing much of the A303 route that runs past Stonehenge with a new route that includes a 2.9km (1.8 miles), deep-bored tunnel, just a few hundred meters south of the monument.
Running up to 40m beneath the ground and containing four lanes of traffic, the tunnel will pass to the south of the stones. In tunnelling terms, the project is relatively straightforward. With the exception of a higher-permeability area at the lowest point of the tunnel route – a region known as "Stonehenge Bottom" – the ground consists basically of chalk. But the Stonehenge tunnel faces a different challenge, arising from the fact that its two proposed entrances/exits lie within the world heritage site.
The project is still at its early days, since construction can only start once the current proposal has gone through a four-year regulatory process. In the meantime, the team behind the project is working on the technical challenges of driving a tunnel through such a sensitive archaeological area.
It is not the first time a similar scheme has been suggested, but previous proposals had been withdrawn in the face of intense opposition. This time, however, many of those involved believe the project is closer to happening than ever before.
According to Highways England structural engineer Derek Parody that leads the effort, 2.9 km is the maximum length a tunnel can have before it becomes necessary to install ventilation shafts along its length. That is prohibitive in such a sensitive archaeological site – and so is the huge amount of surface infrastructure typically required for a tunnelling project of that length.
The precise method of tunnelling hasn't yet been determined, but it's a choice between two techniques: a conventional mining or excavation process that uses a concrete spray to form the tunnel lining, or the use of a boring machine that lays the lining as it moves through the ground.
The sprayed concrete lining method uses a dewatering process, whereby powerful pumps are used to remove groundwater from the area around the tunnel face to prevent it from collapsing. The advantage of this approach is that the excavated material is dry and relatively easy to process. Given that the project is expected to remove more than one million cubic meters of chalk, this is an important consideration. The disadvantage is that dewatering requires significant – though temporary – surface infrastructure. On the other hand, tunnel boring machine techniques – which use either slurry or a pressurized system to maintain the tunnel face – don't require dewatering but do produce a waste product which is more difficult to handle.
Ensuring that the tunnel construction doesn't destroy or interfere with any important archaeology consists a greater technical challenge, though. The Stonehenge team hopes that there will be no surprises once construction begins. "The perfect result for a scheme like this is that they avoid great archaeology rather than dig it up," says Phil McMahon, inspector of ancient monuments for Historic England, the body that advises the government on Stonehenge.
Archaeologists working on the project are currently using a range of geophysical tools, backed up by test digs, to probe the ground along the length of the route. The primary techniques being deployed are magnetometry, which uses sensors to detect variations within the Earth's magnetic field caused by buried features, and ground penetrating radar, which fires electromagnetic signals into the ground and detects reflected signals from structures that lie beneath. According to McMahon, these teams have already come up with a number of findings that have been fed back into the plans, including the discovery of a pair of Neolithic long barrows and a small henge along the route that runs to the west of the tunnel.
In such a well-studied area, however, running onto great archaeological discoveries is not considered probable. A far bigger priority is ensuring that the context of the wider landscape is preserved, such as that the sightlines between the area's various monuments and barrows – thought to have been deliberately designed by the Neolithic engineers of ancient Britain – are left intact.
Archaeology experts have already expressed their objections regarding the project, claiming that research about Stonehenge still has a long way to go, before revealing all of the monument's secrets. According to the University of Bradford archaeologist, Professor Vince Gaffney, leader of the Stonehenge Hidden Landscapes initiative, "The landscape is structured around the monument – you shouldn't be buggering around with the astronomic alignment and impacting on how people will experience it."
Overcoming the objections of experts like Professor Gaffney may be the biggest challenge the project faces. But McMahon is still optimistic that this can be achieved. "What is on the table at the moment – although it requires significant improvement in areas – is a generational opportunity to finally sort out the A303 at Stonehenge," he says. "This is a jewel in the crown of heritage. Being able to achieve an infrastructure scheme within it that protects all of its precious parts really would be a global exemplar."
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