Filter drains play an important role in highway works, in removing sub-surface water and ensuring the stability of earthworks, pavements and retaining structures. This Report, the third in a series, describes a study of the present condition and effectiveness of filter drains incorporating both granular materials and geotextiles at Swanley, Kent. The drains were originally constructed about 1962, but proved to be inadequate to deal with surface water. The worst affected areas were therefore reconstructed from 1981, by replacing the granular filter material and incorporating a geotextile filter. Trial excavation and tests showed that the granular filter material in the original drains was gap-graded. In the reconstructed lengths, the geotextile was effective in retaining detritus and passing surface water, although there was evidence of some damage during installation. Detritus was found to be accumulating in the upper part of the drain to an extent which would eventually impede drainage. The implications for granular and geotextile filter materials are discussed. (A)

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