Cliff face stabilisation - Nefyn, Gwynedd

REQUIREMENT

At the beginning of 2001, prolonged heavy rainfall caused a major landslide at the coastal town of Nefyn in North Wales which resulted in a fatal accident.

Gwynedd Council required a fast and reliable means of securing nearly 100m of the 25m high cliff to prevent any further collapses.

SOLUTION

To secure the unstable cliff face, main contractor and installer, Colin Jones (Rock Engineering) Ltd of Porthmadog, used a combination of biodegradable seeded matting covered by plastic coated wire mesh rock netting and mechanical ground anchors.

The matting will allow vegetation to take hold before degrading while the anchors not only retain the rock netting but also secure the cliff face to a depth of 6m.

Mechanical ground anchors were specified by Ove Arup & Partners because of their installation speed, reliability and instant loading capabilities.

Using a specialist rig on a 20m high scaffold platform, the stainless steel mechanical anchors with 16mm high yield bars were driven 6m into the stiff sand substrate. They were then locked, fitted with load plates and nuts and tensioned to a proof load of 80kN.

Together with the netting the mechanical ground anchors have now securely stabilised this section of cliff, which stands above a busy public car park at the coastal resort of Nefyn.