Specialist highlights Stonehenge Tunnel ‘sinkhole’ concerns

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Hydrogeological specialist George Reeves is part of campaign group the Stonehenge Alliance, which opposes National Highways’ £1.7bn A303 Amesbury to Berwick Down scheme past Stonehenge. According to Reeve, the tunnel could suffer the same issues as the HS2 ones because of numerous naturally occurring fissures in the surrounding landscape.

The current proposal for the scheme involves the construction of a new 12.8km two-lane dual carriageway, with a 3.2km tunnel that would remove traffic passing through the Stonehenge World Heritage Site.

Reeves gave evidence for the Stonehenge Alliance at the examination of the scheme in 2019, where he warned of the presence of weak, unstable, often phosphatic Chalk and major facture zones along the proposed tunnel route.

In 2019, Reeves said that tunnel boring machines (TBM) used on the project could potentially encounter existing voids and fissures in the Chalk rock, which, aggravated by a water table varying over several metres, could likely give rise to incidents of subsidence.

“Unless ground stabilisation techniques, such as a bentonite shield-based tunnel boring machine and/or grouting of fractured bedrock are used, there is a high risk of tunnel face collapse, void migration, and potential sinkhole creation upwards from the tunnel crown,” he said at the 2019 examination.

Reeves has now raised fresh concerns, first reported by NCE’s sister title Ground Engineering, about the scheme following the recent ground collapse above HS2’s tunnelling operations in Buckinghamshire.

HS2 Ltd has blamed the collapse on the ground conditions that the TBMs are passing through under the Chilterns, which are “quite porous with chalk predominating” and feature pre-existing dissolution features or fissures.

In a letter shared with Ground Engineering written by Reeves and Save Stonehenge World Heritage Site (SSWHS) honorary secretary and archaeologist Kate Fielden, they said that a similar incident could happen above the Stonehenge tunnel.

The letter said: “Engineers may be able to ensure tunnelling is possible under ‘pre-existing’ bedrock conditions in the Chilterns – and at Stonehenge – but critical at Stonehenge is the presence of weak phosphatic chalk and the potential loss of or damage to significant archaeological remains owing to subsidence in this formation.

“This could happen anywhere along the 3km twin road tunnel route but would be especially damaging where tunnelling is close to the surface and the archaeology is particularly sensitive.”

But British Geological Survey (BGS) geologist Andrew Farrant noted that the geology at Stonehenge is “subtly different” from the geology where the ground collapse formed in Buckinghamshire.

While there are some “minor faults” present in the landscape around Stonehenge and “natural dissolution features may be present”, Farrant said that the location of the proposed tunnel at Stonehenge is “not conducive to the formation of large dissolution pipes or open karstic cavities, so the potential for encountering large, open or sediment filled voids is likely to be low”.

He added that there are no known chalk mines in the area around Stonehenge.

Despite this, Farrant said that there is “potential for zones of brecciated ground around the fault zones, areas of weak phosphatic Chalk within the syn-sedimentary channel and weathered, destructured Chalk close to the surface associated with periglacial weathering”.

He continued: “In these areas, tunnelling needs to proceed with caution to avoid a similar type of collapse to that which occurred at HS2.

“Collapses and the formation of sinkholes or crown holes are a potential hazard for any tunnelling project, including that at Stonehenge, but the risks can be mitigated by a good understanding of the geology derived from a detailed ground investigation. The tunnelling contractors would need to bear these geological hazards in mind and choose the appropriate tunnelling methodology and equipment for the expected ground conditions.”

National Highways disputes the claim that the tunnel construction method for the A303 Stonehenge scheme would damage the landscape in any way.

National Highways A303 Stonehenge scheme project director Derek Parody said that the TBM to be used on the scheme will be “specially designed for the particular chalk ground conditions within the World Heritage Site”.

The design of the TBM is informed by extensive ground investigations, including geotechnical and geophysical surveys, and groundwater monitoring, National Highways said.

All this information has been included in the environmental impact assessment that National Highways submitted as part of the development consent order (DCO) application and public examination for the scheme.

The planning application for the road scheme is still pending redetermination by the secretary of state for transport, following the quashing of the decision to grant the DCO in 2021.

National Highway has nevertheless progressed with awarding contracts for the scheme to ensure programme timescales are maintained. It signed a contract with the More joint venture for the £1.25bn main works contract in October of last year. Webuild has a 42.5%, FCC Construcción a 42.5% and Bemo Tunnelling UK a 15% stake in the consortium.

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