i) First, look at what the National Trust, English Heritage and Highways (via a consultant) say in their January 2017 assessment of impacts along the eastern stretch of the tunnel route -
"Our assumptions to provide further detail for assessment of impact are:
9. At the portal face, the total width of the roads etc. will be 45m; a 30m cut and cover section back from the face of the portal is likely to be required for construction reasons before the required depth of cover is attained above the bores; 10. The depth of the road surface at the tunnel portals will be at least c.10m below the current ground level in those locations. This should allow sufficient height above the carriageway within the tunnel and for sufficient cover above the tunnel below present ground levels; 11. From the tunnel portal at each end there will be a partial cutting until the surrounding ground levels have dropped to grade for the road. The length of this section of cutting/ partial cutting will depend on the local contours; 12. Outwith these short sections of cutting, the road will run at grade except where it has to be embanked on the approach to an overbridge. P13
From Countess Roundabout to the east tunnel portal The new road will cross Countess Road (A345) on a flyover with grade separated junction. The road deck of the bridge will be at 8m above the highest point of the roundabout below. From there the road follows the existing route through the cutting past Vespasian’s Camp and then runs to the new portal position to the north of the existing road and c.100m east of the line of the Avenue as it runs across the flank of King Barrow Ridge towards the River Avon in West Amesbury. This is c.200m east of the eastern portal position assessed in the 2014 report, and around 400m east of the portal position for the 2.1km tunnel scheme considered in 2004. This also means that the road threshold at the new east portal position should be around 10m lower than in the options assessed in 2014. This has considerable implications for the visibility of the surface stretch of road between the tunnel portal and the existing cutting past Vespasian’s Camp... East of that cutting, the principal impact will be that of the flyover and the grade separated junction of which it will be part.. while this will be very intrusive in the local landscape, the surrounding topography means that it will be screened from direct view from the identified key groups of attributes of OUV (Note Blick Mead was not assessed as of ‘Outstanding Universal Value in this report, as only site of Neolithic or Bronze Age date within the WHS were. Blick Mead does not even feature on the map of the area on P 10 DJ) . Its impact on the OUV of the World Heritage property and its attributes will therefore be minimal. Our calculations suggest that the road level exiting the east portal of the tunnel should be c.85m Above Ordnance Datum (AOD) though this will need reviewing when more detailed development of a scheme is available from Highways England. The road is likely to be partially in cutting for c.250m before running out across the dry valley west of Vespasian’s Camp on an embankment which could be lower than the existing one, depending on the treatment of the farm access road which passes under the A303 at this point." (pp 15 and 16).
To summarise the above, this means that an 8m high flyover will be built by Blick Mead and Countess Farm (which shares the same terrace as Blick Mead and has a Mesolithic findspot, plus waterlogged conditions similar to BM) on a road c.45m wide. There will be a cutting just before the Eastern portal which will be be 10m deep and around 30m long (this is a massive cutting). This is a huge amount of extra infrastructure to put in so close to the floodplain which is presently protecting Blick Mead and its environs.
We are concerned that the compaction caused by the huge amount of additional weight on the road as a result of the 8m flyover along a road width of 45m, which extends 800m from the roundabout to the Eastern Portal, as well as the massive deep cutting by the Portal, which is about 500m west of the site, will result in a drop in the water table that will destroy the organics at Blick Mead. This is compounded by the fact that Highways has done zero assessment of the water table and at Blick Mead, despite the consultation period finishing in less than one month (6/4/18). Highways representatives have asserted that there will be "no impact on Blick Mead" , but they must be basing these remarks on general models of water flow in the broader area. This is not good enough, particularly as such arguments were used in the case of Star Carr, see paragraphs 3-5 here - Lessons from Star Carr on the vulnerability of organic archaeological remains to environmental change
Further, bore hole surveys between the mid 1950's and the early 21st century show a steep drop in peat measurements in Blick Mead-Countess area. For example, Foundation Engineering took a bore hole sample just east of Countess Roundabout (so close to Blick Mead) on the 20th of April 1965 (SU14se51) and the peat there was measured at exactly 3 feet, around 90cm. On 17th of December the British Geological Society took a bore hole sample from almost exactly the same place (1415519.03E142020.76N) and no peat was found. Peat requires waterlogged conditions to persist and is therefore a sound indicator of the water table dropping.
Highways England's Preliminary Environmental Information Report (PEIR) gives an assessment of a range of issues including air quality, cultural heritage, biodiversity, and ‘road drainage and the water environment.
Potential permanent impacts arising from the construction of the proposed scheme identified in the PEIR include "impacts caused by lengths of the tunnel below the groundwater level in the chalk interfering with groundwater flow". It notes: "There are a number of springs in the area down hydraulic gradient of the tunnel including the spring system around Amesbury Abbey, which could be affected."
However, the document’s list of potential mitigation measures does not appear to contain any specific measure relevant to such an impact - "In this particular case, our assessment to date indicates that there will be not be any adverse impact that requires any mitigation above that provided by the design of the proposed scheme." This suggests that Highways England is confident that an assessment that it has yet to complete will not identify impacts that cannot be mitigated. This is also a cause for concern.
Highways England say it plans to submit a development consent application in the autumn of 2018, by which time many of its current assumptions will need to have been thoroughly tested.
So, to conclude with regards to the impact of construction on the ground water level, we do not know if any scientific data exists (e.g. a commissioned geotechnical / hydrogeological survey). Nevertheless, any impact that will cause a reduction in the ground water level in the Avon valley will have a major impact on the environmental and archaeological resource. The reduction in water levels will cause desiccation (oxidation) of sedimentary sequences, especially those that are organic rich, leading to shrinkage and further compaction. The preserved organic (sub-fossil biological) remains that are now known to exist at Blick Mead will be oxidised leading to the potential loss of information. Professors Tony Brown, Nick Branch, Peter Rowley-Conwy and Dr Barry Bishop all co signed my letter to the Times in January 2017 raising the alarm about this.
(*) Image from the book on the excavation:
Blick Mead: Exploring the 'first place' in the Stonehenge landscape
Archaeological excavations at Blick Mead, Amesbury, Wiltshire 2005–2016
Series: Studies in the British Mesolithic and Neolithic
David Jacques, Tom Phillips and Tom Lyons