DOWN TO EARTH

This article shows how bio-engineered reinforced soil retaining banks are suddenly becoming acceptable practice in UK motorway widening. After their recent first use on the M4, kilometres of them are now being built along the M25, instead of concrete walls. Before their acceptance by the Department of Transport (DOT), they had been used in the UK only for minor retaining structures to access roads and car parks. Athough they were conceived as an attractive environmental alternative to hard retaining structures, their adoption on the M25 is motivated by savings in cost and time. Within a month of starting work on the northbound side of the M40 to M4 section of the M25, long stretches of reinforced soil embankments were completed. A 25mm mesh geogrid was used, in addition to the non-woven material normal in European applications. In determining the banks' design, overall stability, direct sliding, and internal slips had to be analysed. Topsoil is placed on top of the banks, as soon as they approach their full height of up to 4.5. Reinforcement is obtained from a Fortrac geotextile grid, which is wrapped round to hold the banks' 60 degree face with the help of a seed mat. Dense undergrowth, including willow cuttings, will be planted on top of the banks.