Project Sheet: Llanrwst Catchfence

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Client

  • Denbighshire County Council

Contractor

  • NWTRA

Works Undertaken

  • Survey of the existing wall
  • Site Investigation
  • Letter report providing geotechnical options 
  • Design of remedial works.

Description of Project

As part of the A470 Penloyn to Tan Lan road scheme, a new cutting had been excavated. However, soon after the road had opened, about 10m of the rock face had fractured and small deposits of scree had been observed on the road. There was a danger of rocks falling onto the highway so GroundSolve Ltd were contacted to assess the problem and design remedial works.

Before the completion of the carriageway, an inspection was carried out by NWTRA. It was suggested that the 5m high rock face may be slightly unstable. Therefore, the contractors were instructed to cast PVC sleeves into the retaining wall at 2m centres to accommodate a future catchfence. The cold winter that followed resulted in the loosening of material, which could potentially result in debris falling onto the carriageway. This confirmed the need for the catchfence.

GroundSolve Ltd were required to design the fence for the rock face along this section of wall. They were also asked to provide drawings and a specification for the contractors.

The contractors were planning on completing their works and demobilising from site just three months after the initial involvement of GroundSolve. Therefore, the design work needed to be undertaken quickly.

The analysis was undertaken using the program “Rocfal” to determine the impact velocities of falling blocks on the fence.

GroundSolve proposed to cast Hot Finished, steel, galvanised poles some 450mm into the sleeves. It was then decided that Maccaferri mesh would be used for the infill between the posts.

Two end restraints were fitted onto the end posts for additional support. They were tilted at a 45º angle, inserted into concrete foundations below the ground surface and secured in place by base plates and anchors.


The mesh is a double twist, hexagonal woven steel mesh. Additional strands of steel rope were also intertwined with the mesh and threaded through galvanised eyelets to provide strength and rigidity to resist any impact by the rocks.

The mesh is held down at the base of the wall by a series of hooked anchors to prevent debris passing beneath it. The anchors are driven about 300mm into the ground at a 1m interval along the rock face.

Unstable section of rock face as seen from the A470
Unstable section of rock face as seen from the A470
Loose material falling towards the carriageway
Loose material falling towards the carriageway