Award-winning ground investigations near Stonehenge
Stonehenge is 3 km west of Amesbury in Wiltshire, south-west England, UK. This 5000-year-old Neolithic structure is both a British cultural icon and a UNESCO World Heritage site. However, the ancient structure’s site is bisected by the A303 trunk road, the most direct route between the south-east and the west of England. English Heritage manages the prehistoric monument and the National Trust owns the surrounding land.
In 2017, Highways England, English Heritage and the National Trust announced the route for upgrading the A303 near Stonehenge; this included engineering a twin-bore tunnel. The £2.4-billion scheme aims to alleviate the traffic-choked road by widening it into a dual carriageway near the Stonehenge site before it dips underground as a 3.3-km tunnel.
Structural Soils, an RSK company, completed a high-quality ground investigation, including a novel approach to sampling the phosphatic chalk at the site. The data will help to minimise the risk from unexpected ground conditions during construction by increasing the understanding of the site’s structural and lithostratigraphical geology. Structural Soils’ engineers recovered intact phosphatic chalk cores, including fault-controlled, shallow marine transgressions and regressions, and significant lengths of uninterrupted core from the Newhaven Chalk Formation. Our drillers used a low‑aggressivity core bit and limited water use, which increased the quality and stability of the recovered material.
It was crucial to identify where the phosphatic chalk had been deposited within the Newhaven Chalk Formation to aid developing a highly detailed structural geological model. We did this by using a team of very experienced geological rock-core loggers that identified flint and chert marker beds, ranging in thickness from <1 mm up to 0.5 m, and typical fossil assemblages. The detailed logging helped to confirm the geological model and aided in determining the fault-controlled shallow marine transgressions and regressions, which was aided by precise geologging of the boreholes.
Using the data excavated by the Structural Soils team to build a 3D model of the proposed scheme, planners can make scientifically based judgements regarding building the tunnel. Owing to its outstanding work, Structural Soils won the 2020 Ground Engineering Award for the Ground Investigation Project of the Year, sharing the award with AECOM and Highways England.