Initial technical details for the proposed new 2.9km long tunnel which will take traffic away from the A303 close the Stonehenge World Heritage Site (WHS) have been released.
The details were published in the A303 Stonehenge Amesbury to Berwick Down technical appraisal report from Highways England as part of the documentation made available for the government’s consultation on the proposed routes for the tunnel which launched last Thursday.
It stated that the new highway would be a dual two-lane all-purpose expressway with a design speed of 120km/h.
At present, part of the single carriageway A303 runs as close as 165m from the monument. Congestion can cause delays of up to an hour in the August peak when motorists slow down to look at the stones.
At grade solutions to upgrade the existing road were considered too intrusive, would affect the character of the site and also cause substantial harm to the “outstanding universal value” of the site, says the report.
“The purpose of a tunnelled solution would be to remove the A303 from the most sensitive part of the WHS, thereby reducing severance and enhancing the character of the WHS,” says the report.
But concerns over the length of the 2.9km tunnel have been raised by campaign group The Stonehenge Alliance as it will still require a dualling of the existing road within the WHS. The group wants 4.5km long tunnel to take traffic outside the WHS.
“The Alliance considers the road tunnel should be at least 4.5km long (the minimum distance to avoid more road building within the WHS). Ideally, the tunnel should be longer so that all the portals are well clear of the WHS,” said the Alliance.
“Ideally, we would advocate a considerably longer tunnel of at least 6km, to allow restoration of the whole WHS and its setting.”
The report says an appropriate balance between affordability and impact must be achieved. It says a 2.9km long tunnel will deliver benefits to the setting of significant features in the Stonehenge landscape.
“Route options incorporating 4.5km tunnels were assessed as having significantly higher estimated scheme costs that were considered to be unaffordable and were not considered further in the assessment,” the report said.
At Stonehenge Bottom, the report says the minimum depth of the tunnel will be 6m to ensure the stability of the tunnel crown. It said that at this depth, disturbance to the topsoil and subgrade would be minimised, protecting archaeology and ecology in the area.
Water is also an issue for the tunnel. The report says that in permanent operational phase, the highest permeability in the dry valley is in the top ground layer. As a result cover at Stonehenge Bottom will be maximised to avoid damming the groundwater and to minimise environmental impacts.
To allow for safe excavation to commence, a minimum cover of 10m will be required at the portals.
The tunnel itself will be constructed as a twin bore tunnel to accommodate a dual carriageway highway cross section. The report said that each of the tunnel bores would have an internal diameter of between 11m and 12m.
A decision on whether the work will be done using a tunnel boring machine (TBM) or whether it will be excavated and lined with sprayed concrete lining will be made in subsequent design stages, says the report. Lining thickness has been assumed indicatively as 350mm thick for the primary and 375mm for the secondary lining, or 450mm thick for a segmental lining.
Factors which would influence the choice of construction method would be the extent and distribution of phosphatic chalk and its engineering characteristics, which have yet to be determined.
As part of the works, new structures including underpasses, viaducts and grade separated junctions will have to be built at five or six major locations depending on the route chosen.
An announcement of the preferred route is due in July and the project team expects to submit a development consent order application in mid-2018. It is expected that construction on site will start in March 2020.