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High strength mats win on A13 marshes

A NEW high strength geosynthetic has been used for the first time to provide basal reinforcement along sections of the A13 road realignment project through Dagenham in Essex.

The scheme includes the construction of embankments and viaducts to carry the road over marshland of very soft and often contaminated soils alongside the River Thames. Consultant Hyder Consulting came up with the design for the embankment foundations which are piles overlain with the high strength geosynthetic reinforce- ment.

Tarmac Civil Engineering, the main contractor for the scheme, chose Comtrac P basal rein- forcement which it helped to develop with geosynthetic manufacturer Huesker Synthetic and supplier MMG Civil Engineering Systems. This geosynthetic has a high uni- directional strength up to 1,200kN/m and was seen as offering the most economical solution for reinforcing the fill above the pile caps.

'Comtrac P is formed from bundles of high tenacity polyester yarns surrounded by a polymeric coating, which provides prot- ection from both mechanical damage and chemical attack and offers a design life of 120 years,' explains Huesker's Gra- ham Thomson. Over 37,000m2 of material was supplied in strength grades ranging from 500kN/m to 1,200kN/m.

Two layers of geosynthetic reinforcement were laid above the pile caps within a compacted sand and gravel layer. The 500kN/m grade was placed parallel to the line of the road, while the higher strength grades (with grade depending on embankment height) were laid transversely in continuous strips folded over at the toe of the slope and secured by a gabion 'wrap- around'.

The embankments were built between September and October last year and the realignment is due to finish in summer 1999.

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