The use of geotextile filters is very common in engineering applications, such as the reinforced soil walls. The main aim is to prevent the movement of fine particles from the base soil, allowing the liquid to flow as freely as possible. As a result, the geotextile filter design is based on the retention and the permeability criteria. Generally, the retention criterion that a geotextile filter must satisfy is expressed in terms of geotextile characteristic opening size and of an indicative soil particle diameter. The criterion works if the filter larger pores retain the smaller particles of base soil. 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 under the dragging hydraulic flow. In the paper, the existing geotextile filter design criteria are shown for the retention criterion and the permeability criterion; the influence of the main factors affecting the filtration design are illustrated and the recent methods to evaluate the internal stability of granular soils are analysed.
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