Chris Ames looks at the archaeological and technical issues that arise from digging a tunnel at the Stonehenge World Heritage Site (WHS) and how Highways England plans to address them.
In September, Highways England announced that it was proceeding with plans to take the busy and strategically important A303 underground at Stonehenge, albeit a further 50 metres away from the historic monument itself.
Concerns about damage
However, campaign groups have continued to express concern at the damage the scheme could do both above and below ground, complaining both that the tunnel is not long enough and that it will literally undermine an area of huge archaeological importance.
The response from Highways England is to reassure people that it has taken into account both the complexity and the sensitivity of the task before it and is working closely with and taking advice from heritage groups to ensure that the route enhances and protects the Outstanding Universal Value of the WHS.
It says that while the existing road severs the WHS in two, a tunnel on the preferred route will reunite the historic landscape as a whole and remove the adverse impacts of the existing road, greatly improving the setting of Stonehenge itself.
Among the groups that remain opposed to the tunnel is Rescue: The British Archaeological Trust. It says the tunnel, both of whose portals will be within the WHS, would be too short to protect the archaeology of the WHS, where important sites are likely to exist on the trace line of the expressway outside the tunnel.
Rescue says engineering work for the approach roads, cuttings and tunnel portal areas as well as in the areas of grade separated junctions would require full excavation of any archaeological remains in situ.
It argues that while there would be full recording of the archaeology encountered, this would not be ‘under the optimum conditions of targeted and planned research projects that are expected for a heritage asset of the highest significance’.
Highways England says the preferred route has been chosen using information from ground investigations carried in the World Heritage Site over the last 50 years. It is aligned through a section of the WHS where archaeological surveys have indicated that there is no buried archaeology and through an area that has been extensively ploughed and farmed.
An archaeological evaluation strategy will be established in conjunction with heritage bodies and experts in the Stonehenge landscape to investigate thoroughly any scattered surface archaeology that may exist.
The alignment of the tunnel itself will be mostly within the chalk bedrock and well below the level of any archaeology. Rescue acknowledges this but says published geological survey work in the WHS indicates some instability in the underlying chalk that could potentially result in ground subsidence. This could have implications for archaeological remains at or near the surface.
Rescue also says there are detailed site investigation reports on geology and hydrogeology that were undertaken ‘some years ago’ but whose publication is still awaited – along with more recent reports.
Highways England highlights that information regarding geology and hydrogeological conditions are included within the Scheme Assessment Report. It says that the dissolution features within the WHS are not significant and not uncommon in chalk. Construction work, particularly the tunnel boring technique, would address the issue and ensure that instability does not occur.
Because the tunnel will be dug through chalk, water will need to be pumped out of the immediate vicinity during tunnelling works – a process known imaginatively as dewatering. Rescue says there are concerns that this could have an adverse impact on waterlogged deposits at the recently discovered Mesolithic site of Blick Mead.
Previous plans for a (shorter tunnel) required extensive hydrogeological modelling over concerns that dewatering could affect the base flows of the nearby River Avon, a designated Special Area of Conservation under the Habitats Directive. Detailed hydrogeological modelling will again be needed.
Highways England says a comprehensive hydrogeological model will demonstrate that the tunnel will not have any negative hydrogeological impacts. It says the anticipated environmental impacts will be identified within the Preliminary Environmental Information Report, which will be issued ‘in due course’.
On the subject of timelines, the Highways England webpage for a scheme at Stonehenge dates it back to 1989, under Margaret Thatcher. The arguments over a tunnel at the site look set to rumble on.