The influence of soil particle shape, confining pressure and the amount of relative displacement on the value of the interface friction angle are studied in the laboratory using a ring shear apparatus. Two quartz sands, one angular and the other rounded, with two geomembranes, one soft and the other hard, were used as the interfacing constituents. The geomembrane surface texture before and after shearing is quantified using a profilometer. Test data indicate that the value of the interface friction angle can vary significantly for each of the surfaces investigated. The smooth HDPE geomembrane exhibits distinct peak and residual interface friction angle values that range from 65 to 90% of the friction angle of the contacting sand. The rough HDPE and PVC geomembrane interfaces are shown to mobilize virtually the full friction angle of the contacting sand.