Project Sheet: Pant Dedwydd, Cefn Mawr: landslide

image1

Client

  • Richards Moorehead & Laing

Contractor

  • Pennine Geotechnical

Works Undertaken

  • Desk Study
  • Investigation of the ground conditions
  • Slope stability assessment
  • Design of Remedial Works
  • Assisting Richards Morehead & Laing with the drainage issue

Description of Project

A landslide occurred at this site in 2007/8 resulting in both the lateral and vertical movement of a retaining wall onto Dolydd lane in Cefn Mawr.

GroundSolve Ltd were asked to investigate and analyse the landslide and carry out detailed design work for the scheme.

After inspection, it was concluded that the landslide was a classic shallow circular failure, with the movement of the bottom 1m of the slope resulting in the displacement of the wall backing onto Dolydd Lane. It had been driven by fill placed at the top of the slope.

The project involved re-grading the slope and the construction of an anchored retaining wall at the toe of the slope.

Initially an assessment of the ground conditions needed to be carried out to identify the soil composition of the slope, so that the type of anchor best suited to stabilise the site can be determined.

It was decided that anchored gabion baskets would be used as the structural support of the new retaining wall, with long cement-grouted soil nails anchors being the most suitable for the silt/clay soil observed at the site. The length of the soil nails being determined by the weak glacio lacustrine deposits.

The existing slope was re-graded to a gradient of 1 on 2. Once the slope and drainage work were complete, the anchors were installed at 1m intervals horizontally along the wall. They were then cemented in place. In front of the gabion baskets, a stone-faced wall was built to match the appearance of the existing retaining wall.


Drainage had also been an issue and possibly one of the reasons for the landslip. The slope improvement works therefore included the installation of a cut-off drain at the top of the slope. This prevents all surface run-off from Dolydd Lane, which was then diverted into the existing road drainage system further along the lane.

Work was completed with the slope being landscaped with seeded grass and willows.

Large stones from the existing wall were re-used within the stone facing wall. The remaining material was distributed and compacted within the slope to form an under-drainage layer.

The wall of anchored gabions during the construction process
The wall of anchored gabions during the construction process
Before construction: view of the unstable wall from Dolydd lane
Before construction: view of the unstable wall from Dolydd lane
Wall and slope after completion as seen from Dolydd lane
Wall and slope after completion as seen from Dolydd lane