Detailed plans for the proposed £1.6bn, 2.9km long, tunnel which will take traffic away from the A303 close the Stonehenge World Heritage Site (WHS) have been released. The scheme now awaits a decision on its development consent order (DCO) application.
More than 300 documents have been logged as part of the application submitted to the National Planning Inspectorate by Highways England.
The scheme will upgrade the existing single carriageway A303 between Amesbury and Berwick Down, to a new dual two-lane carriageway and includes a 2.9km long twin-bore tunnel through the World Heritage Site (WHS) with portals inside its boundaries.
The new tunnel roughly follows the current road alignment which is 165m from the Stonehenge monument.
Under the newly released plans, 85m of the tunnels at the eastern end and 200m at the western end will be constructed using cut and cover, with the remaining section bored with a tunnel boring machine (TBM) at a maximum depth of 40m.
Concerns have previously been raised by environmental groups that tunnel construction would adversely affect the Outstanding Universal Value (OUV) of the site. This has prompted Highways England to say it wanted the scheme to be an “invisible” road.
To minimise impact during construction, tunnelling is to start and finish at the western portal to minimise the transport of material through the WHS. TBMs will be deployed from the western portal and then either turned around to create the second bore or transported back to the western end to repeat the process.
In both cases, processing plant for excavated material will be located to the west of the western portal.
Stonehenge tunnel central section plan
Around 900,000m3 of spoil is expected to be excavated during the construction of the two 14m diameter tunnels. Around 400,000m3 of this has been earmarked for the construction of embankments on the Winterbourne Stoke bypass. The remaining 500,000m3 will be used elsewhere. The planning submission states it would be “preferable on landscape and engineering grounds” to keep the site for storing excavated material to a single 25ha area with a maximum height of 2m.
A preliminary assessment of the vertical ground movement from the tunnelling is anticipated to be 20mm to 30mm based on the expected construction methods.
On the western portal approach around 1.4km of retaining wall with a maximum height of 23m will be built, although said some of its length will have a “soft top” – a landscaped slope at the head.
Last year, Highways England said while some contractors had shown a “strong appetite” for the work, the issue of risk allocation, particularly around the archaeology risk, was one of the sticking points.
Initial ground investigation works for the scheme in the vicinity of the eastern portal have revealed a early Bronze Age round barrows and a possible ring ditch and linear anomalies “likely to be associated with former field boundaries”. At the western portal and approach cutting, however, it said it passed through an area where surveys indicated that there was “limited archaeological survival” within its footprint, although it said there were substantial groups of known monuments surrounding the site to the northwest, southwest, south, southeast and east.
Proposed further ground investigations will include boreholes advanced by rotary coring and rotary open hole techniques and insitu geotechnical testing including self-boring pressuremeter testing. The investigations will be targeted at the western portal to provide “greater understanding” of the distribution of fault lines and potential ground movements.
A decision by the Planning Inspectorate on whether to accept the plans is expected in mid- to late November.
If accepted, it will undertake a detailed examination of the application, in which stakeholders and the public can participate, before making a recommendation on whether it should go ahead to the transport secretary. Work on site is scheduled to start in 2021.
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