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Looking for support Technological advances and the ever improving quality of plastics mean that the geosynthetic sector is trying for a bigger share of the stability market. Mike Walter reports.

Advances in polymer technology mean that geosynthetic materials are becoming stronger and more durable, say the manufacturers and suppliers of geotextile products. Geotextile use as a reinforcement or separation medium in erosion control and road construction is growing thanks to increases in proven reliability.

The producers are aware that their products need to have their profile raised, however, so that the benefits they provide get universally recognised.

'Not enough people are aware of the advantages of using the latest materials,' says Synthetic Textiles plant manager Wilson Yates. 'Some civil engineers are still coy about using the products, although certain applications have a very strong following.'

One such use is polyethylene coated polyester strips to reinforce eroding soil embankments. Terram ParaGrid, marketed by Maccaferri, reinforces slopes where a design life of up to 120 years is required.

The grids are rolled out on to compacted soil up to 10m into the embankment from the slope. Soil is backfilled on top before another layer of the geotextile reinforcement is placed, to give the bank longitudinal and lateral support of up to 100kN/m. It is often used in conjunction with wire gabions placed on the exposed face to give the embankment additional structural support.

Maccaferri marketing manager Colin Evans sees distinct reasons for using the plastic in preference to steel mesh, for example. 'The soil fill does not have to be of a high granular quality, as the ParaGrid has good frictional qualities and acts as a reinforcement even in marginal soils. The product is also relatively cheap to manufacture.'

The road construction sector is increasingly using geosynthetic grids to improve a road's resistance to deformation, although demand for any sort of reinforcement here is reducing as a result of cuts to the roads programme.

Geosynthetic grids can be placed either between or beneath bituminous layers in the road formation, depending on the level of stress exerted on the road. The grids increase road load bearing capacity and reduce the chance of permanent deformation over a longer period.

Despite the number of new-build road contracts slowing up, existing motorway widening work continues. The M25 in Surrey is one example where embankments are receiving ever steepened sides which need to be reinforced. 'Slopes often become steeper after road widening, in order to reduce the amount of land which has to be taken from nearby areas. Because of this, the use of reinforcement instead of angle of repose slope gradients has become very important,' says managing director of Huesker Synthetic, Graham Thompson.

Hydraulic engineering on coastal shores is one area which has greatly benefited from developments in fibre technology. For rock armour protection to work most effectively, large boulders to dissipate the eroding force of waves are placed on top of a fabric. This acts as a separator between the rocks and the sand, allowing sea water to pass through the membrane while adequately supporting the large rocks placed on top. Here, lighter materials are carrying heavier loads than ever before.

Geofabrics, manufacturer of one type of coastal membrane, sees such advances in product design as keeping the industry afloat. 'Our products have undergone considerable technical improvement recently, to enhance fabric puncture resistance and increase the strength of the material, without increasing its weight,' says managing director Bob Warwick.

New 'greener' materials and uses are always being tried and tested such as biodegradable soil mats, marketed as a way of preventing local erosion and to encourage vegetation growth. 'The natural fibres are especially popular as a lot of engineering consultants are looking at the environmental aspects of their work. Increasing the use of natural material is the next step,' said managing director of Hytex UK Mike Hyder.

Such developments are fine for the prevention of rockfall on gentle slopes, but a product which will entirely supersede plastic in reinforcing steeper embankments is still a long way off. 'Biodegradable fibres are not nearly as strong as plastic as it is very difficult to ensure that each strand can stretch and support material at the same rate. Fibre mats are normally used on the surface of a slope rather than being part of a permanent structure,' said Hyder.

According to Huesker Synthetics Graham Thompson, the relatively slow take-up of geofabrics lies with some engineers who still perceive plastic, let alone biodegradable products as unreliable. 'We must take the lead of the Japanese industry, where many engineers have been convinced that plastic reinforced structures stand up better to seismic loadings. Although not a frequent occurrence over here, localised landslips can be prevented much more effectively with the use of geotextile materials,' said Thompson.

This article was produced for NCE by Barrett, Byrd Associates.

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