Filter drains are extensively used in highway works, both for removal of surface water and to ensure the stability of earthworks, pavements and retaining structures. This Report describes a study of the present condition and effectiveness of granular filter drains in the verges of the M20 Motorway at Maidstone, in cuttings of Gault Clay. Examination and trial excavations showed that the filter drains were in good condition after 20 years, with the exception of partial blockage by calcareous deposits at one location. The drains were effective in lowering the water table in their immediate vicinity, but did not prevent shallow slips in the adjacent cuttings. It is doubtful if the drains had any substantial effect on pore pressures in the sub-base or clay formation. Road detritus was retained on the surface of the drain, and this severely impeded surface water drainage. The results of the observations are discussed in terms of the design and specification, construction, maintenance and long-term performance of filter drains.

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