Project Sheet: Llanddona Landslide

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Client

  • Anglesey County Council

Works Undertaken

  • Site Walkover
  • Stability Assessment
  • Geotechnical recommendations of remedial solutions

Description of Project

landslip occurred on a section of the Anglesey coast path near a village called Llanddona in December 2012. Approximately 10m of the access track had been lost, with only 1m remaining at its narrowest section. This resulted in the closure of this section of the footpath.

GroundSolve Ltd was appointed by Anglesey County Council to offer geotechnical advice

Following a site walkover, it was evident that this section of the coast path is an active coastal landslip complex which is likely to have suffered from both major and minor landslips since the end of the last glacial period. The main areas of concern were the recent landslide and a nearby tension crack which is likely to result in a further landslip in the near future.

It was believed that the landslide occurred due to a build-up in pore water pressure in the glacial deposits of the slope. This increase in pressure resulted in a reduction of the effective strength of the soils. Debris from the landslide had flowed downslope for over 50m, and the remaining features suggest it had flowed in a fluid state at high speed (bottom right image).

The tension crack to the west of the landslide is on the edge of an access track. The track at this point has overlying concrete with backscars visible on either side of the track. It is believed that another large landslide could occur in this area.

GroundSolve Ltd provided two remedial solutions:

  • Diverting the coast path through an adjacent field to avoid the backscar
  • Reinstating the access track by constructing a retaining wall and backfilling

The first option was by far the cheapest however, vehicular access would no longer be possible. The second option also came with the risk of potentially triggering another landslide further along this section of vulnerable coast line.

The remedial solution for the stabilisation of the backscar involved the installation of soil nails and covering the slope with a rockmesh facing.

It was decided that the footpath was to be diverted through the adjacent field, along the base of the landslide complex.

Steps leading up to Number 2 Glaslyn cottage from the A498
Steps leading up to Number 2 Glaslyn cottage from the A498
Evidence of viscous material flowing downslope
Evidence of viscous material flowing downslope