Project Sheet: Aberfrwdd Highway Support

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Client

  • Ceredigion County Council

Contractor

  • TSS Wales

Works Undertaken

  • Slope stability assessment
  • Design of the anchors
  • Designers risk assessment
  • Specifications and drawings for the work

Description of Project

Following a landslip which took away part of an access road and was threatening to undermine a gabion wall which supported the highway, TTS Wales appointed GroundSolve Ltd to design a soil nailed solution to provide support for the highway.

A site investigation reported that the ground conditions comprised of glacial till overlying a layer of mudstone bedrock. The mudstone bedrock below was described as weak, and the appeared failure mechanism observed by an engineered geologist from GroundSolve Ltd was a shallow surface slip within the overly steep glacial till.

The proposed solution was to install soil nails and a reinforced concrete beam to support the edge of the access track. On the slope below, more soil nails would be installed, covered with mesh and matting to increase the stability of the slope and reduce erosion.

There were 4 rows of anchors installed in total, with differing lengths and inclinations to maximise stability. Prior to the installation of the top row of anchors, the depth of the gabion wall had to be determined by excavating two hand dug trial pits.

A soil aggressivity assessment was undertaken concluding that the soil condition was mildly aggressive, meaning that the anchors had to be galvanised for the full length. It was specified that the anchors would have a design life of 120 years - 110 years protection to be provided by the nails themselves (made of sacrificial steel) and a further 10 years by galvanising them.
The mesh covering the slope was a plastic coated, galvanised, double-twist mesh which was laid on top of a coir matt. Securing the mesh are 30cm x 30cm plates.

To prevent ground loss, a reinforced concrete beam was inserted down through the access road reaching 2m in depth so that it extended below the base of the gabions. The top row of anchors was then installed at the mid-height of the beam.

Following the design work carried out by GroundSolve Ltd, specialised contractors were asked to complete the work.

Site work was completed in July 2011.

A close up image of the landslip below the access road
A close up image of the landslip below the access road
Before construction: the landslip below the gabion wall supporting the access road above
Before construction: the landslip below the gabion wall supporting the access road above
The stabilised slope after construction
The stabilised slope after construction