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WASHING LINE

SUBSIDENCE - During floods, rivers more commonly flow into roads, but occasionally the reverse happens. Damon Schünmann went to the very north of England to see environmentally sensitive reinstatement work.

Slip sliding away', sang folkrock mega-selling New Yorker Paul Simon. Although his sentiments at the time of writing were undoubtedly pretty far from the B6263 near Wetheral in Cumbria, had he driven along it over the past 10 months he might have felt it worth a hum.

In January 2004, the worst flooding since at least 1822 hit the area near the Scottish border. In just 24 hours, a month's worth of rain overtopped defences and inundated about 3000 homes. And when the fl oods receded they took part of the road with them, says Cumbria Highways project engineer Kevin Crawley.

Cumbria Highways put down boreholes last summer to locate the slip plane. 'We suspect it's a series of small failures rather than just a single big one; We don't think it is a deep seated failure, ' says Crawley.

'The challenge was coming up with something that satisfied the Environment Agency's requirements of minimum interference with the River Eden [which runs parallel just metres from the road] and the embankment supporting the road as these constitute a Special Area of Conservation and a Site of Special Scientific Interest, ' he says.

Subcontractor May Gurney area manager Nick Sharp says: 'There must be a weak spot that has caused the failure and not just the river, as the embankment has bellied out.

It has slumped due to subsidence and the river hasn't eroded the belly area.

'Water permeating through the ground must be having an effect by undermining the road and will be running through cobbles between the sandstone and clay layers.' The boreholes showed the stratum to be fi ll over 5m of sand with gravel and clay, over 3m of clay, 3m of gravel and sandstone.

To support the embankment, May Gurney is putting in a retaining wall as part of a £113,000 contract, with 11.5m to 12m sheet piles keying about 0.5m into the sandstone. It is vibrating the piles in using a 700N air hammer.

Sharp says: 'We're putting in the wall to have a minimum environmental impact. It will protect the road and existing embankment from further undermining.' He explains that for the moment there are more cracks in the road as hammering the sheets in consolidates the overburden. Cumbria Highways will make the necessary repairs towards the end of the project.

The embankment reinstatement includes building a Terram geomembrane lined filter drain that will sit behind the retaining wall. This will connect into an existing drain to carry the groundwater into the river.

Crawley says: 'We spoke to contractors to see if we needed piling at the top of the embankment or the bottom. We went for the top as we can reopen the road sooner. Piling at the bottom would have meant working in the river and that can't be done at this time of year as it's an internationally renowned salmon river and they are spawning.

'We had a momentary heart stopper when the first pile hit an obstruction at 2m but we went through it, ' he adds.

The scheme's designers have planned for future flooding and erosion of the embankment with a series of ground anchors to tie back the piles.

'The ground anchors are to cater for a worst case scenario of the river washing away the top of the embankment in front of the wall [meaning the tops of the piles would otherwise be unsupported on the river's side], ' says Crawley.

The 13.5m long anchors will go in at 45° to the horizontal under the road and found in medium to dense gravels. They will spread their loads via a waling beam running along the top of the piles.

These have a 120 year design life in aggressive ground with the piles having 100 years.

'The waling beam is basically there to act as a strong back to anchor the whole wall so they are not acting individually on each of the Larssen 605 piles, ' says Sharp.

Once anchor stress testing has completed, Cumbria Highways will put in a new road sub-base and surface and then connect the drain to the existing outfall.

The project including road reinstatement was scheduled to finish by Christmas. A festive season resolution, meaning Paul Simon would no longer 'Believe we're gliding down the highway, when in fact we're slip sliding away'.

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