Repair of earthwork failures involving landslips represents significant expenditure for Highways England across the national network each year. A variety of engineering solutions conventionally used involve significant earthworks and import of new construction materials, resulting in the generation of waste and a high carbon footprint.

The area is geologically and geomorphologically complex and includes a long history of mining and quarrying. The road itself is formed on a 3.5m high sidelong embankment comprising of poorly engineered fill up to 30° overlying a shallow slope of weak soils and deeply weathered mudstones with a locally high water table and a face angle of 16°. The latest event in an extended history of failures and defects occurred in winter 2014 when movement on a deep circular failure resulted in severe cracking and displacement of the road pavement over a length of 25m with an additional 50m also affected.

The EKG slope stabilisation system was proposed by Amey Consulting with Electrokinetic providing specialist subconsultancy services for the geotechnical design. Siteworks were carried out for Gloucester County Council by Envex Limited as the main contractor and Amey LG Gloucestershire as project manager.

The design comprised:

• 3 zones to optimise the design to the differing ground conditions and reduce costs
• 96 heavy and standard duty anodes ranging from 6m – 9m
• 69 flexible cathodes ranging from 7m – 12m
• A filter drain incorporated along the road to intercept surface water from the slope above the landslip

Construction summary:

• Prior to construction removal of vegetation was planned and undertaken by the Forestry Commission
• Prevailing and widespread very soft ground conditions presented difficulties even for a small, lightweight slope climbing rig. These were overcome by the use of stiff polymer geogrid which acted as temporary matting over the soft ground. Construction was completed in 10 weeks based on:
• Electrode installation required 30 days (2 man crew) with a small slope climbing rig; no earthworks
• Active treatment was staged across the 3 treatment zones such that the active design treatment period of 7 weeks per zone was spread across a total of 10 weeks
• Rebuilding of road sub base and surface commenced during EKG treatment
• Driving of electrodes yielded valuable additional information which allowed the ground model to be modified and the design to be refined during construction to accommodate variations in ground conditions.

The delivered benefits of the EKG system included:

• Project ran on time and to budget despite very difficult ground conditions during installation
• Slope remediated in existing condition, requiring no earthworks and produced zero waste
• Use of the observational method during the construction phase in order to reduce material installation and time on site
• Final construction costs for EKG system were in accord with the budgeted estimate, which was 31% lower than the granular replacement option and 27% lower than the bored pile wall option.