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The long-term filter perfor1nance of 6 nonwovens differing in pe1 mittivity, thickness, mass per unit area and type of polymer was studied experimentally in a set of large permeameters. The geotextile samples of 500 mm diameter sandwiched between a san_dy silt and a coarse gravel were submitted to steady-state seepage under a hydraulic gradient of i=3 for 18 months. The boundary conditions, such as the relevant properties of the test soils and the water,
were well controlled throughout the test. So the continuously taken flow-rate measurements are a reliable basis for the evaluation of the time-dependent permeation behaviour of the ·system consisting of a fine-grained soil with very low cohesion. Such systems·’are problematic with respect to erosion in contact with coarse-grained drainage material and typical nonwoven geotextile filters. The test results enable a good comparison of the behaviour of different types of
nonwoven geotextile filters under conditions which are encountered in practice in civil and geotechnical engineering. The paper presents the test set up, the measurements and a detailed description of the observations obtained during the
long-term experiment. An investigation into the soil structure immediately above the geotextiles is given, and an interpretation of the test results with respect to the filter performance.