17 Jan Stonehenge Tunnel: Protecting our Heritage and Environment

After years of study, consultation and analysis, the A303 road tunnel past Stonehenge has been given the go ahead.

 

ESI provided important technical inputs to the original hydrogeological studies, which have proved it was more than just the tranquillity of the site at stake – and will present themselves again. We take a look at our role in modelling the issues.

 

Stonehenge Tunnel: Protecting our Heritage and Environment The A303 trunk road, a vital strategic route connecting London and the South West, has always had an Achilles heel. Dualled for much of its length, vehicles in growing numbers have crawled past the UNESCO World Heritage site on a single carriageway shattering the serenity of the unique environment.

Now, a public consultation has been launched on the proposed 1.8-mile (2.9 km) dual-carriageway tunnel, designed to reclaim the open fields around the stones, while adding a by-pass for the long suffering village of Winterbourne Stoke.

The tunnel forms part of a £2bn government scheme to upgrade all remaining sections of the road between the M3 and M5.

Plans for a tunnel and assessment of the habitat near the Stonehenge site have had a long and chequered history.

Original proposals for a bored tunnel were first raised in 1995 but converted by the government to  2.5 mile (4km) “cut and covered” tunnel in 1999. The National Trust heavily criticised this approach as having the greatest damage to the environment.

Revised plans for a bored tunnel of 1.3 miles (2.1 km) in length were proposed, but detailed assessment was required on the groundwater sensitivity, as construction plans could affect a nearby designated Habitats Directive Site.

We were brought in to devise modelling on the proposed dewatering on the site that would be required as part of the tunnel plans.

As the tunnel would be constructed through the Chalk, the accepted method for boring the rock would require dry working conditions.  This would require pumping from the Chalk aquifer to temporarily lower groundwater levels below the deepest point of the tunnel with discharge of the pumped groundwater back into the Chalk aquifer locally.

The Environment Agency (EA) and English Nature were rightly concerned that the proposed dewatering plans for the tunnel construction might have the potential to affect the base flows of the nearby River Avon (designated Special Area of Conservation under the Habitats Directive).

ESI was commissioned to model the effect of dewatering on flows to the River Avon. The model showed that base flow to the Avon would be very slightly reduced in the summer months, but that this was not significant in terms of the normal flow. Both the Environment Agency and English Nature were satisfied with the results and accepted that the pumping scheme would not adversely affect the base flow regime of the river.

The latest proposed scheme will continue to raise interest because of the international importance of Stonehenge. However, understanding the mechanics of what is happening below the surface, not just for working conditions, but for the integrity of the tunnel and its impact on the sensitive eco-system surrounding, will need to be carefully modelled once more.

As we accelerate investment in infrastructure builds, this and many other equally ambitious tunnelling projects will require detailed assessment on the need for an impact of dewatering in scheme design.

ESI has a long and rich history of groundwater modelling excellence that places the consultancy in prime position to add value to lead engineers and project managers alike.

Given the high degree of sensitivity of large scale civil engineering near to a World Heritage Site and other designated sites, it will be essential that all parties within the appointed project team understand the environmental setting, the impact of excavation on the ecosystem and how the Directives in place could affect delivery.

ESI prides itself on excellence in collaboration with all contractors within a consortium, as well as with the Regulator in managing environmental impact. As issues present themselves, we are expert at adapting to the situation, delivering valuable data and guidance as needed to mitigate risks through the project timeline.

Given our past experience, we look forward once again to the opportunity to take an active part in the successful delivery of the tunnel scheme under this most precious jewel in our nation’s heritage.

 

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