For engineers, boring a tunnel can be fairly straightforward. But when that tunnel goes beneath one of the world’s most heavily protected ancient sites, it becomes a lot more complicated.

It’s one of the world’s most famous ancient monuments: an instantly recognisable icon from a forgotten world, a place for quiet wonder and contemplation.

And yet for many people, the first and perhaps only glimpse they get of Stonehenge is from a traffic jam on the A303, one of the main routes between London and southwest England.

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This could be about to change. A bold £1.4bn plan proposes ripping up much of the existing road and replacing it with a new route that includes a 2.9km (1.8 miles), deep-bored tunnel just a few hundred metres south of Stonehenge.

It’s still early days: construction can only start once the current proposal has gone through a four-year regulatory process. But in the meantime, the team behind the project has begun grappling with the technical challenges of driving a tunnel through such a sensitive area, while avoiding the risk of turning one of the world’s most heavily protected monuments into one of the world’s most controversial construction sites.

It is not the first time a similar scheme has been suggested. Previous proposals, which have included a cut-and-cover tunnel, foundered in the face of intense opposition. But despite opposition this time around – perhaps most notably from the International Council on Monuments and Sites, which advises Unesco on its world heritage sites – many of those involved believe the project is closer to happening now than ever before.

Indeed, the National Trust and English Heritage, which between them manage both Stonehenge and the land around it, have cautiously welcomed a proposal that they say will restore tranquillity to the site and – by reconnecting its southern and northern parts – reopen ancient processional routes that haven’t been walked for around 5,000 years.

Chalk it up

The person responsible for leading this effort is Highways England structural engineer Derek Parody, an experienced road-builder who now finds himself mixing the technical vocabulary of the engineer with talk of “ley lines” and “astronomical alignments”. It is, he says, a project unlike any other. “I’ve been engaged in major road schemes almost all my life, but building a tunnel through a world heritage site is a new one for just about everyone,” he says.

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 land here is basically chalk.

In that sense it’s a far cry from the UK’s most high-profile tunnelling project of recent years, London’s Crossrail, where tunnel-boring machines had to excavate their way through different clays, soils and even geological faults. But the Stonehenge tunnel faces a different challenge, one largely arising from the fact that its two proposed entrances/exits lie within the world heritage site.

According to Parody, 2.9km is the maximum length a tunnel can have before it becomes necessary to install ventilation shafts along its length. That is a no-go 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 chief attraction of the sprayed concrete lining method is that it 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 metres of chalk, this is an important consideration. The disadvantage is that dewatering requires significant – albeit temporary – surface infrastructure.

On the other hand, tunnel boring machine techniques – which use either slurry or a pressurised system to maintain the tunnel face – don’t require dewatering but do produce a waste product which is more difficult to handle.

Arguably a far bigger technical challenge, though, is ensuring that the construction of the route doesn’t destroy or interfere with any important archaeology. According to Parody, the process required to do this also represents a golden opportunity to add to the knowledge of this much-studied site. It will end up with an area of “effectively examined archaeology in a level of microscopic examination that not many road schemes have,” he says.

Unlike Crossrail, where much was made of the unexpected discoveries unearthed during the tunnelling process, 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. Both techniques are, says McMahon, ideal for the area. “We’re quite lucky in the Stonehenge landscape,” he says. “There are very few features more than a couple of metres deep and you’re looking mostly at upper chalk, which is a really good reflector for archaeology when you’re looking at negative features, i.e. things that have been cut into the bedrock.”

These teams have already made a number of important finds that have been fed back into the plans, says McMahon, 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.

Cut lines

But in such a well-studied area these kinds of discoveries are rare. 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.

For instance, a key area of concern with the current proposal, and something that McMahon and others hope to be able to persuade Highways England to address, is the positioning of the western entrance. “It’s too close to one of the major funerary monuments in the landscape called the Normanton down barrow cemetery,” says McMahon, “and the road coming out also sits for part of its route on the same astronomical alignment as the midwinter setting Sun.”

Alongside ongoing studies, there’s also a wealth of existing research for the team to draw on, not least the recently completed Stonehenge Hidden Landscapes initiative, which resulted in the most detailed archaeological map of the site ever produced. Although previous explorations had focused on the Stonehenge monument itself, this project, led by University of Bradford archaeologist Professor Vince Gaffney, used mobile arrays of sensing equipment to collect data from around the site. These tools were linked directly to GPS systems that recorded the precise location of every measurement taken.

This project revolutionised the use of technology in archaeology and led to a number of key discoveries, including the remains of a giant stone circle surrounding Durrington Walls, a 1.5km circumference “superhenge” just a short distance from Stonehenge.

But despite his project’s celebrated success, Gaffney says that technology has not yet evolved to the point where it can uncover all of Stonehenge’s secrets. “The work that we did was invaluable, but the landscape is not the sum of the things that you dig and build. How would you tell that thousands of people would have been at Stonehenge in the Neolithic period? All they dropped was stone and we can’t see it because it’s under grass. Yet that might be the most important part of the archaeology,” he says.

With so much still to learn, Gaffney is opposed to the current proposal, which he fears could destroy some of these secrets forever. “Something should be done but I’m not entirely sure about this response,” he says. “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.”

Ultimately, overcoming the objections of experts like 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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