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Soft Rocks stand up to Mersey currents

Following the cutting of a new river channel to re-route the UK's River Mersey, an embankment was needed to block off the old channel. Design and construction was awarded by client, Manchester Ship Canal Company to Edge Consultants.

The plan was to re-use sand which had been excavated during the construction of the Manchester Ship Canal in the 19th century to provide the fill for Naue Fasertechnik's Soft Rocks system.

The Mersey at this point has strong, fast currents, and the design had to ensure the design of the embankment was resistant to wash-out of the sand and to scour.

A second embankment further around the meander was to provide a deposit area to infill the channel and ultimately reclaim the land. The embankments would also serve as filters, relieving the build-up of pore water pressure.

A temporary site access road across the Mersey at the location of the upstream embankment was constructed to assess the suitability of Soft Rocks. Both woven and non-woven types were used in the trial with the non-woven option the one chosen. Jones Brothers, Ruthin was the contractor for both embankments.

Scour protection was provided by Terrafix B 813 sandmat lining the base of the embankment. This has a layer of sand ballast between two layers of needle-punched geotextile (one a standard Terrafix 813). Placed underwater on the bed of the river, the sand ballast causes it to sink, displacing the air trapped within the 2M to 3M fibre bridges in each square metre.

A hopper was set up on the riverbank near to the sand deposits. The Soft Rocks were then filled to 80% of capacity by a Komatsu excavator taking sand from the deposit and feeding it into the hopper. The bags were stitched with a handheld sewing machine.

Once there was a sufficient quantity of Soft Rocks for a complete layer across the river, the excavator exchanged its bucket for a modified grab to place them.

The last step was to protect the side slope with a layer of Terrafix 813, and a layer of rip rap.

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