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.