The geotextile filter design is based on the retention and the permeability criteria. Generally, the retention criterion is expressed in terms of geotextile characteristic opening size and of an indicative soil particle diameter. The criterion works if the larger particles retain the smaller particles and this happens when these particles form the solid skeleton. In some granular soils, internally unstable or with a broadly graded grain size distribution if the retention criterion is satisfied, this condition does not guarantee that the whole base soil is retained. In fact, the base soil could be subjected to an internal erosion phenomenon if the geotextile filter characteristic opening size is too large and if the larger particles retained by the filter are not able to retain the smaller particles of the base soil and a hydraulic flow of dragging exists. In these conditions, the knowledge of the internal stability of granular soils is a key-factor in the design of geotextile filters. Therefore, in order to evaluate the internal stability of granular soils, different methods are generally available and the results of the application of these methods on the same soil can lead to different internal stability evaluations. In this lecture, the most recent methods to evaluate the internal stability of granular soils are analyzed. Moreover, a method that allows determining the upper limit value of the geotextile filter characteristic opening size to be used in the retention criterion in order to avoid the internal erosion of broadly graded granular base soils is also shown.
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