Specially adapted Green Terramesh® units provide soil reinforcement solution

26/02/2004

Three famous Listening Mirrors, forerunners of today's early warning radar detection systems, have been stabilised as part of an on-going conservation programme for English Heritage overseen by consulting engineer Cameron Taylor Bedford. Specially modified Green Terramesh® units supplied by Maccaferri, specialist in retaining structures, slope reinforcement and erosion protection, have been installed to underpin the massive concrete structures.

Built on the Romney Marshes in the 1920s, the three structures comprise two circular dishes measuring 20 ft and 30 ft in diameter and a 200ft long parabolic curved wall. Over the years, gravel extraction has created a lake, leaving the mirrors marooned on a narrow peninsula, with the surrounding gravel level 2m lower than originally. On-going wave action was causing further erosion, leaving the foundations dangerously exposed.

In the first phase of the project the peninsula housing the Mirrors was cut off to create an island linked to the mainland by a swing footbridge, and Maccaferri's Green Terramesh® used to stabilise the structures. In addition, the causeway to the mainland was widened and reinforced to facilitate access to the Mirrors by both construction traffic and visitors, also using Green Terramesh®.

Green Terramesh® units are formed from a single sheet of hexagonal, double-twist woven steel mesh, Galfan (Zn 5% Al alloy) coated with an additional PVC coating for maximum durability in a water environment, which when installed forms a horizontal soil reinforcement tail, a facing section and a horizontal return top section. With a face angle of 700, each unit is 600mm in height. To stabilise the Mirrors, units have been layered up to 7 high, leaving just one unit exposed above the water line.

Working with main contractor O & L Construction, a team of divers assisted in the installation of the submerged units. Ease of installation was therefore a crucial factor in specification. Installing Green Terramesh® is quick and easy. The integral triangular rebar brackets ensure accurate installation without the need for temporary formwork. Once in place, individual units are joined together either by hand lacing or with special rings dispensed using a 'Spenax' pneumatic tool, available from Maccaferri Ltd.

Standard Green Terramesh® units are lined with coir matting, designed to retain a depth of topsoil to allow the establishment of vegetation. However, to preserve the natural environment local site-won gravel was specified for backfill and 'greening up' of the slope face was not required. Maccaferri was able to work with Cameron Taylor Bedford to design a modified version of the Terramesh Green unit that could successfully contain the 20mm gravel, whilst exposing it to view. The standard mesh opening of the hexagonal woven wire mesh is 80mm. Therefore special Green Terramesh units were manufactured and supplied by Maccaferri with the coir material replaced by Terram Grid 4/2-W geosynthetic material to achieve this.

Maccaferri Ltd specialises in the supply of gabions, geotextiles, geogrids and geomats. In addition to a full range of products for retaining structures, slope reinforcement and erosion protection, the company offers an in-house design and consultancy service.

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