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MAN THE RAMPARTS

SLOPE ENGINEERING

Nature is besieging Hastings Castle far more effectively than any army. Damon Schünmann reports on attempts to protect its future.

Hastings Castle in East Sussex is under attack by a far more persistent foe than any besieging army - the force of nature.

Part of the site where the castle's keep was believed to stand has already fallen - literally. The collapse, which happened at some point after 1800, saw the structure topple over the cliff edge that had receeded relentlessly towards its base.

From the Norman invasion of 1066 to 1800, erosion moved the cliff about 200m inland, but in the past 200 years it has eaten away a further 300m.

Between the cliff and the sea, a patch of reclaimed land houses some industrial buildings. It also hosts the site of ces of contractor Ritchies, which is working on a £400,000 project for Hastings Borough Council to protect the cliff face from further damage - and protect the buildings below it from the rock falls that periodically occur.

Edge Consultants undertook a rope access inspection to decide which areas needed reinstatement, with consultant Bureau Veritas carrying out subsequent inspections before and after remedial work.

Ritchies business development manager David Gibson says: 'The scope of the work includes removing material, some rock bolting, repairs to existing and new dentition, investigation into previous sprayed concrete cover and cliff face inspection.' The other part of the project involves installation of a Geobrugg Swiss manufactured rock fall protection system, Rocco RXi-050, rated at 500kJ.

Gibson says the barrier is certied in accordance with Swiss government guidelines as European ones have not been published yet.

Dentition involves removing 35m3 of rock using roped access and doing about 100m2 of masonry repairs in the sandstone, siltstone and mudstone cliffs.

Bureau Veritas associate Stewart Nicol says: 'The rocks here are generally weak and friable and are causing undercutting of stronger rocks above. In the past, contractors have used brick and sandstone block dentition so we're cleaning out the pointing and redoing it while identifying and removing loose material.

'As the rock is weak we can't use rock anchors, so at one end of the site we're securing Maccaferri rock netting onto the face. This is because there's a slab being undercut by erosion with a joint behind it. With the two of these together there could be a problem, ' he says.

'We were limited in what we can at this site as the castle is a national monument. As [the rock barrier site at] Roc-a-Nore Road is an SSSI [site of special scientific interest] we had to develop a method statement that English Heritage would agree to.'

The 85m catch fence has a 60year design life and is designed to capture rocks during a large fall by absorbing energy through brake rings. These looped shock absorbers contract in diameter as the steel wire rope passing through them is pulled by the weight of impact.

The rings can only be used once, so although the elasticity in the catch fence might contain a small fall without activating them, they would have to be replaced if triggered by a bigger fall. But even if the rings are activated, the barrier, the fourth of its kind in the UK, can still provide limited protection in the event of a second fall.

Gibson says: 'The barrier is designed to catch rocks if there is another rock fall after the first, in accordance with Swiss guidelines, as some systems go at at this point. This one will be at least 68% of its 3m height after the first impact.'

The fence posts are installed into one of two foundations designed by Donaldson Associates', the choice determined by the depth of concrete allowed by the bedrock prole.

Where the concrete is only 500mm deep, pairs of vertical micropiles - incorporating 25mm diameter stainless steel bars in 100mm diameter boreholes - socket 2m into the bedrock. This type of foundation also has a passive tension anchor embedded at least 4m down into the bedrock.

However, where the concrete foundations can go in deeper, from 500mm to 1000mm, the vertical piles are dispensed with in favour of a passive tension anchor.

A restraining cable attached to an upslope anchor also secures the fence and site workers are grouting the anchors with a neat CEM 1 mix.

Ritchies is covering the Rocco netting with chain link fencing to prevent smaller material getting through.

The project, which began in December, was completed at the end of February.

How brake rings work

The rock catch fence features a system of brake rings. These provide one-off shock absorption if triggered by a heavy fall that is not contained solely by the elasticity of the fence.

Lateral support ropes are guided through pipes bent into loops and held by compression sleeves. If the fall is sufcient to activate the rings, they contract and dissipate residual energy from the ring net impact, without damaging the ropes themselves.

Swiss manufacturer Geobrugg says the rope's breaking load is not diminished by activation of the brake.

Ups and downs of castle life

Hastings Castle was originally built after William of Normandy's coronation as king of England following his 1066 invasion.

Between 1171-74, Henry II built the stone keep which historians believe stood at the southern edge of the site - but this is conjecture as the area has disappeared due to cliff erosion.

King John ordered the castle be dismantled in 1216, fearing it would fall into French hands. In 1225, the structure was restored by Henry III. But severe storms hit Hastings later that century, and along with the destruction of the harbour, parts of the castle collapsed and were lost to the sea. Less than half of the castle remains.

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