For filtration applications, geotextiles should have opening size small enough to retain particles of the adjacent base soil and voids large enough to let drained water pass through geotextile without restriction. Candidate geotextiles are selected for most applications according to index tests : the filtration opening size of the geotextile is compared to the indicative grain size of the base soil and this retention ratio must be smaller than one. For applications with silty and broadly graded soils however, the ASTM D5101 Gradient Ratio test is often reported for the evaluation of the soil/filter compatibility. The primary function of a filter is to retain base particles. In broadly graded soils with Cu > 8, filtration involves the washout of a certain amount of particles before bridging can result in a stable combination. Filtration tests on broadly graded cohesionless soils have highlighted the existence of a selffiltration zone near the interface. The washout of a certain amount of base particles finer than the opening size of the filter was observed and it is a function mainly of the retention ratio and of the shape and extent of the gradation curve of the base. Stable combinations were encountered with a threshold value of 2500 g/m2 of washed out particles above which continuous piping developed. It is proposed that the compatibility testing procedure is modified to include the measurement of the amount of particles passing through the geotextile and collected at the bottom of the permeameter.
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