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Toe nails keep WCML feet dry

SOIL NAILING has replaced raking piles as part of railway embankment stabilisation works on a section of the West Coast Main Line in Staffordshire.

The 6m high embankment runs for 40m along the River Trent at Armitage before crossing it on a viaduct.The embankment's toe, which lies between 3m and 0.5m from the riverbank, had been affected by scour erosion and had to be stabilised.

Consultant Hyder's original design for the contiguous bored pile retaining wall solution called for raking piles to resist horizontal deflection. But this would have meant extending the piling platform further into the river, so it was decided to replace the raking piles with soil nails, which could be installed using a smaller rig and therefore a smaller platform.

Five site investigation boreholes were sunk followed by insitu testing and the installation of inclinometers to monitor movement within the embankment. Geology of the site comprises alluvium overlying terrace sands and gravels, followed by Bromsgrove Sandstone.

The 50m long retaining wall comprises 340mm diameter, 8m long piles installed at 490mm centres by contractor Systems Geotechnique from a 4m wide platform built partly in the river and partly on a berm at the embankment's toe.

A lightweight rig installed the piles into the sandstone. The piles were temporarily cased and poured using high strength grout mixed on site and placed via a 50mm tremmie pipe. The reinforcement was then placed and the casings withdrawn, with more grout added as necessary.

Systems Geotechnique also carried out soil nailing. The 18m long Ischebeck Titan grade 30/11 soil nails were installed in 90mm diameter boreholes at 25infinity at 3m centres. A preliminary soil nail was tested to 2.3 times the working load with additional tests carried out to 1.5 times the working load.

Load is transferred from the nails to the pile capping beam via steel plates and nuts at the nail heads. The nails were tensioned to a load of 10kN, their heads covered with removable steel caps and the embankment regraded.

All exposed components were fully galvanised and given a bituminous coating for a 60 year design life. The nails have a sacrificial layer to allow for 60 years of corrosion, ignoring any protection provided by the grout annulus surrounding them.

Birse Rail co-ordinated the project for client Railtrack with the rest of the work subcontracted to Mainland Contractors. The project was carried out as part of Railtrack's Structure Framework contract.

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