A new theoretical method to evaluate the upper limit of the retention ratio for the design of geotextile filters in contact with broadly granular soils

The geotextile filters design is based on the retention and the permeability criteria. In particular, the retention criterion, that a filter must satisfy in comparison to the base soil, is commonly expressed as OF D85, where OF is the geotextile characteristic opening size and D85 is the soil particle diameter corresponding to the 85% of the passing soil mass grain size distribution. As consequence, the filter should retain only the larger particles of the base soil. This criterion works if the larger particles retain the smaller particles and this condition is verified when these particles form the solid skeleton (constituted by interconnected particles that transfer the stresses). When the granular soil has a broadly grain size distribution, with uniformity coefficient greater than 3, the larger particles generally do not belong to the solid skeleton but they are “immersed” in the smaller particles matrix that constitutes the solid skeleton (Giroud, 2010). So, for broadly granular soils, if the retention criterion previously defined 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 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. The paper focuses on the development of a new theoretical method that, starting from the base soil mass grain size distribution and from its relative density, determines the upper limit value of the geotextile filter characteristic opening size, OF, to be used in the retention criterion in order to avoid the internal erosion of broadly granular base soil