WORK TO stabilise cliffs below Knaresborough Castle in North Yorkshire was due to finish as GE went to press.
The castle, first documented in a 12th century text, stands on a high sandstone cliff overlooking the River Nidd.
The weakly cemented sandstone is overlain with 0.5m to 4m of unconsolidated overburden and topsoil and has been significantly undercut by the river.
Prompted by cracking of the cliff path and retaining walls, an abseil survey revealed evidence of potential stability problems. Previous cliff failures, at the base of the unconsolidated deposits, are thought to have been exacerbated by thick topsoil and ingress of rainwater.
Harrogate Borough Council, working with English Heritage, asked contractor Can Geotechnical to tender for a design and build scheme to stabilise the slope.
Following a site investigation Can suggested using more than 100 hollow, self-drilling soil nails about 5m long to ensure the overburden was tied back to the underlying sandstone.
Can won the £82,500 contract and began work in late January.
The road below the site was closed, crash decking installed above the public footpath and a catch fence built to reduce the risk to people and traffi c passing the foot of the cliff.
Can installed the soil nails with a drill rig suspended from anchors drilled at the crest line. It then used rock netting to distribute any mobilised loads within the nails, to provide reinforcement to the cliff face and to allow revegetation.
Despite having to work from inside the castle grounds while the ancient monument remained open to the public, Can was on target to complete at the end of February.