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COAST IS CLEAR

SLOPE ENGINEERING

Unstable cliffs above railway tracks in south Devon are being stabilised with lightweight soil nails.

Damon Schünmann reports.

On a stormy day, the waves on the Teignmouth coast in south Devon crash over the top of the sea wall. On a really bad day they hit not only the trains that run along the coastal line metres from the water's edge, but the cliffs which in places are only 3m from the tracks.

This makes life tough for the drillers handling the 750kg down-the-hole hammer rigs working up and down the face of the cliffs to prevent more of the many slides that periodically close the rail line.

'There was a storm here yesterday and it was like being grit blasted, ' says Tim Nixon, site supervisor for subcontractor CAN Geotechnical on the day GE visited.

Difficulties have plagued the 6.5km section of railway line between Dawlish and Teignmouth since it opened in the mid 1800s.

Wave action is the least of the problems. Surface water overtopping the cliffs causes washouts, high pore water pressure triggers rock falls and rapid surface weathering has also played a part in cliff instability.

Repeated blasting to trim back the cliffs from the line in its early days has also possibly weakened them, causing cracking and opening up discontinuities (GE June 04).

Some of the collapses have been quite large.

Brian Wilson, general foreman for main contractor Dean and Dyball, says after one recent slide 500t to 700t of material had to be removed each week for three months - at the end of which there was another similar size slide.

Network Rail is the client for the £6.4M contract for all work at the site.The 15-month job is due for completion in April.

CAN has been brought in to stabilise the cliffs in the 1.7km stretch between Smugglers Cove in the east to Eastcliff bridge in the west.

Most of this work involves installation of soil nails and inclined drains in material comprised of weak to moderately weak cemented breccia.

Rig crews have had to contend with difficult conditions on the uneven surfaces of the cliffs that CAN operations director Andy Wingfield says are between 45infinity and vertical and up to 100m high.

More than 2,500 glass reinforced plastic (GRP) soil nails are being grouted into 60mm boreholes in 'close centre' areas (nails at 2m to 3m spacings), generally in the upper reaches of the cliffs, and open centre groupings (nails at 10m to 20m spacings), mostly in lower areas.

In the close centre regions, 22mm diameter, 2m and 4m nails are being used as well as 25mm diameter 10m nails. The nails are manufactured by Weldgrip whose general manager Steve Jackson says they have a tensile strength greater than steel: 470kN and 500kN for the 22mm and 25mm nails respectively. But this maximum is reduced because of the thread at the top of the nails, so for insitu load testing purposes steel nails are being used.

'We're testing 5% of the nails for the ability of the grout to adhere to the sides of the borehole, 'Wingfield explains.

The close centre nails are a preventive measure and are fixing rock fall netting, interwoven with three-dimensional geomat to create Maccaferri MacMat-R.With the stabilising action of the nails, this active protection system is intended to hold the soil in position, preventing collapse of the cliffs, in contrast to the open centre work which is a containment measure to catch slides.

The nails used for the open centre work are all 25mm diameter and 10m long, and are attached to rock fall netting which does not include the geomat.The nail's plate washers are connected via 500mm strops to 16mm steel down-cables that have had the slack taken up in them, to spread the load should a slide occur.

In total, 17,535m 2of rock fall netting is being used on the job but hydroseeding and natural grow-back will disguise it once complete.

The benefits of using GRP nails at the site are readily apparent. 'They are light, flexible and don't corrode, ' says Wingfield. There are also cost benefits because of the time saved by the drillers who are using hand-jammers to reascend the cliffs.

'It's much more efficient if you can take a 10m GRP nail down the face in one section instead of a steel nail in three sections, ' Wingfield adds. 'The choice of GRP has greatly helped our ability to get the nails on to a very difficult face.'

It is not just a weight issue. Jackson explains that the 10m lengths have a 2.5m bend radius.

This curvature means the flexible nails can be easily fed into boreholes when working on steep faces.

The inclined drains being installed 15m into the cliff face have been fixed in position using an expanding foam in the last 2m before the surface. 'Fixing the drains with the foam has helped working in a very adverse environment, ' Wingfield says.

The 64, 60mm diameter perforated pipes which lie at 20infinity to the horizontal will help reduce the groundwater to below slide trigger levels. Dean and Dyball site engineer Chris Egan says they are designed to reduce hydrostatic pressure close to the face of the cliffs.

It is hoped the combination of soil nailing and inclined drains will ensure the stability of the cliffs, but as future slides cannot be ruled out, further contingency measures have been necessary.

Dean and Dyball has built almost 2km of up to 5.5m high rock trap fencing at the foot of the cliffs to prevent material getting on to the railway. A 2m deep drainage trench has been constructed in the cess at the foot of the cliffs to take surface water coming down off the face and any water from the track ballast.

Within it lies a 300mm perforated pipe on a 20mm clean stone base, the outlet for which runs to the edge of the sea wall. A geotextile runs down the sides of the trench and over the top of the pipe, allowing water through but catching the fines.More clean stone is backfilled over this to ground level.

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