Global and local sand–geosynthetic interface behaviour

The influence of overall particle shape, mean particle size, initial relative density and effective normal stress on the global and local soil–geosynthetic interaction is investigated here through an extensive series of direct shear interface tests between six sands in contact with one multifilament woven geotextile, one non-woven geotextile and one geomembrane. Analysis of the soil–geosynthetic data highlights mobilisation of higher peak friction angles with increases in initial relative density and angularity of sand particles and decrease in the effective normal stress. The average frictional efficiencies of the sand–geosynthetic interfaces are 0·95 and 0·79, respectively, for the sand–geotextile and sand–geomembrane interfaces. It is shown that the interface slip increases with the initial relative density of sand and regularity of particles, and decreases with increasing the mean particle size and effective normal stress. Analysis of the local particle movements by means of digital particle image velocimetry revealed that the average thickness of the shear zone for the sand–geosynthetic interfaces tested ranges from 3·8 to 11·9 (with an average of 7·0) times the mean particle size of the contact sand. The experimental data indicate that thicknesses of the shear and dilation zones are almost equal for the sand–geosynthetics interfaces tested.