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Soft Rock system used on Mersey

GERMAN GEOSYNTHETICS manufacturer Naue Fasertechnik has supplied Secutex Soft Rocks non-woven geotextile sand bags and Terrafix scour protection geotextile for a new embankment on the River Mersey.

The embankment was built to block off a meander of the river which had been bypassed by a new channel. A second embankment built further round the meander downstream formed a reclamation area that is being filled with sand. Both embankments were designed as filters, to relieve the build-up of pore water pressure, while retaining the fill between them.

Ruthin-based contractor Jones Brothers, working for client Manchester Ship Canal Company, built the upstream embankment, which was designed by consulting engineer Edge Consultants, which also supervised construction.

The structure was formed using local sand excavated during construction of the Manchester Ship Canal at the end of the 19th century. Because this section of the Mersey is fast-flowing, the embankment had to be capable of resisting wash-out of the sand fill behind.Potential underscour also had to be prevented.

Trials at the end of 1999, involving construction of a temporary site access road across the river at the proposed embankment location, assessed the suitability of Naue Fasertechnik's Soft Rock system to build the embankment.

The system basically comprises 1m 3,2t geotextile sand bags. Both woven and non-woven geotextile bags were tested and non-woven Secutex bags were chosen for the final embankment structure.

Naue Fasertechnik also supplied a scour protection geotextile for the base of the embankment. The Terrafix B813 sandmat consists of a layer of sand ballast sandwiched between needle-punched geotextile layers. The ballast gives the geotextile extra weight, which allows it to be placed underwater without having to use rock ballast.

Local sand was used to fill the Soft Rock bags to about 80% of their capacity and they were stitched with a handheld sewing machine. Once enough bags were ready, an excavator was used to place the geotextile scour protection filter across the river and on the riverbanks and the bags placed on top.

The embankment was finished off by protecting the side slopes with Terrafix 813, a double layer filter geotextile that allows water through without clogging. This was covered with a rip-rap layer.

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