Stonebarrow Lane, Charmouth

Steve Morgan

Steve Morgan

Principal Engineer

Tel: 01392 439905
Email

Services

  • Geological Desk Study
  • Geotechnical Site Investigation
  • Geotechnical Analysis and Design Recommendations

Geotechnical investigation on an active landslide complex

Set against the backdrop of one of the largest coastal landslides in the UK between Lyme Regis and Charmouth, a new residential property is proposed to be built on an active landslip on the western flank of Stonebarrow Hill. To complicate matters, a basement up to 5m deep is being incorporated to meet planning requirements and maximise residential space.

CGL initially undertook a detailed desk based review of geological information, with sources taken from the British Geological Survey, including borehole logs and LIDAR imagery, aerial photography, peer reviewed journals and from the neighbour, who happens to be a Professor in Geomorphology.

The landslide was identified as being a translational landslide activated as a subsidiary movement to the main coastal Stonebarrow landslide complex. On site, the landslide is characterised by slow moving ‘creep’ movements, evidenced by the hummocky terrain, rather than the more spectacular retrogressive landslides encountered on the coast which regularly reveal the famous fossils of the Lias Clay on Charmouth beach.

CGL went on to complete a comprehensive window sampling investigation across the sloping site to depths of up to 10m. The investigation revealed a significant thickness of loose, granular landslip deposits derived from the upslope Greensand Formation and Chert Beds.

As part of the geotechnical report, CGL undertook finite difference slope stability modelling to assess the residual strength of the landslip. Groundwater was identified as a critical factor in the long term stability of the slope, and recommendations for slope drainage were provided. CGL also produced earth pressure calculations to assist in the temporary design for the basement, and recommended use of mini-piled foundations to pin the reinforced concrete structure to stable strata at depth.

The design is being developed by the structural engineer, AKS Ward, with CGL having a continued input particularly for the temporary works. CGL will also look to retain a site presence during excavation to accurately log the ground conditions as the works progress to validate the design.