Investigating the geological and engineering characteristics of the chalk strata underlying the A303 and the Stonehenge World Heritage Site
Written by Michael Kerr, Senior Engineer
Introduction by Dave Cage
With input from others
Concepts of Heritage and Legacy are not unfamiliar to engineering geologists and geotechnical engineers, who are taught throughout their career to use their skills and knowledge to research and analyse the ground to provide unbiased, accurate data and advice.
Working at the interface between the natural ground and our urban construct presents many challenges, none more so than balancing geological and human societal heritage needs.
This awareness was at the forefront of our minds when, in Spring 2019, Highways England commissioned Geotechnics to undertake a site investigation to inform a capacity upgrade along a 7-mile section of the A303 in Wiltshire, within the landscape of the Stonehenge World Heritage Site.
Being given the opportunity to work on such a high profile scheme was an honour, and our specialist team faced tremendous challenges, both in the coordination and implementation of a ground investigation within one of the world’s most well-known World Heritage Sites, but also in delivering a large-scale investigation that required the formation of numerous deep boreholes and highly specialist testing within the unique Late Cretaceous phosphatic chalk underlying the site.
KEY POINTS:
Client; AECOM and Highways England
1800m of drilling
49 boreholes to depths between 20m – 75m below ground level
5 drilling rigs working concurrently, a combination of rotary and cable percussion
Highly unusual and unique Late Cretaceous Phosphatic Chalk strata
Highly specialised in-situ testing; downhole geophysics, packer and High Pressure Dilatometer testing
Within the Stonehenge World Heritage Site
Data to advise appraisal for proposed 2.9km tunnel scheme
How do you undertake a large ground investigation alongside one of the country’s busiest roads, within a World Heritage Site?
With careful consideration and planning!
At planning stage we drew on expertise and skills from across the company when putting our project team together. Once on-site we used industry-leading suppliers, and we developed a close working relationship with the consulting engineer, AECOM.
From day one, daily briefings were held with all site staff and AECOM. This provided a forum to plan the day’s work, for people to raise any issues and to disseminate information to the whole team. These meetings were invaluable as everyone started the shift with a clear idea of their objectives, and an overall sense of the progress of the project.
Caring for people and the environment
The area has a rich archaeological history, and it was important that this was taken into consideration at all stages of the investigation. The chalk downs of Wiltshire are also a critical habitat, so ecological factors also had to be taken into account. This was before we had even put a spade in the ground.
Understanding these issues, and the effect they could have on the programme, meant we were well prepared going into the start of site works. We didn’t bring any rigs to site for the first week, giving ourselves plenty of breathing space to complete the initial surveys. This meant that when the rigs did mobilise, we were well prepared.
We knew from the start of the project that protecting the environment on the surface and the aquifer at depth was vital. We were able to mobilise hundreds of ground protection boards to provide safe access, keep our work areas clean, and protect the underlying landscape.
Valuing geological heritage and legacy
Phosphatic chalks are rare in Europe, generally occurring as small outcrops or subcrops as local channel-fills in southern England and northern France (e.g. Jarvis, 1980, 1992, 2006; Mortimore and Pomerol, 1987). It was generally considered as a surprise to encounter the strata on previous investigations in the area.
The data from all the investigations, including this one, will become part of the ‘commonwealth of data’, enabling future generations access a very unique geological model helping to significantly mitigate risks to future construction projects, as well as inform archaeological potential.
The most challenging specialist testing during this investigation was the packer and HPD due to the fractured rock. Drilling the required pockets for the tests was difficult used a huge volume of water.
Logging the chalk was technically challenging due to the level of detail required. As part of the preparation for the project we asked esteemed geologist Rory Mortimore to hold a training course in our offices. This greatly helped our on-site team of specialist geologists, who throughout the project worked incredibly hard and produced some of the most detailed logs we’ve ever seen.
Delivering excellence
We established on-site dedicated teams to manage the workload. Rig engineers and an enabling crew took care of the drilling and site work, while at the site compound a team of geologists supported by technicians logged and processed the core. This approach meant everyone on site had a clearly defined role and responsibility, leading to an efficient and co-ordinated team. Our careful approach and collaborative attitude produced fantastic results, with the investigation being completed on time and within budget.
CREDITS:
Rotary drilling was undertaken by Rotadrill UK
Cable Percussive drilling was undertaken by Dave Cowling
Specialist in-situ testing was undertaken by In-Situ Ltd
Photography by Matthew Nichol