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Slope Engineering
Man the Ramparts
Nature is besieging Hastings Castle far more effectively
than any army. Damon Schunmann reports on attempts to protect
its future.
Hastings Castle in East Sussex is under attack by a far more
persistent foe than any besieging am1Y - 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.
Geobrugg fence iin action
in Korea (left) and installed at Hastings.
Between the cliff and the sea, a patch of reclaimed land
houses some industrial buildings. It also hosts the site offices
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 certified 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 flat 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 profile.
Where the concrete is only 5OOmm deep, pairs of vertical
micropiles - incorporating 2Smm 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 5OOmm 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.
Areas of rope access inspection
by Bureau Veritas and/
or repointing of existing dentition/new dentition by Ritchies
Article courtesy of Ground Engineering - April 2006
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