Determination of geomembrane – protective geotextile friction angle: An insight into the shear rate effect

This study investigates how the shear rate can affect the geomembrane – protective geotextile friction angle. Four types of geomembranes (GMB) were considered (EPDM, HPDE, PP, and PVC) and a single nonwoven needle-punched geotextile (GTXnw) was used to make the interfaces with the geomembrane. Three shear devices were used: a large-scale inclined plane (IP), a shear box (SB), and a small-scale shear device (ssSD). The ssSD allows two shear modes to be compared: one mode involves incrementally increasing the shear stress, and the other involves imposing a constant tangential velocity at the interface. Only the PP GMB- GTXnw was tested with the SB and the ssSD. Inclined plane standardised tests show that for the three interfaces that undergoes gradual sliding (EPDM, PP and PVC GMB- GTXnw), it is shown that a step-by-step experimental procedure gives significantly lower interface friction angle than that given by the procedure from the current international standard, which is explained by the increase of interface shear stress with sliding speed. These observations are confirmed by shear box tests. One major practical result is that, following the nature of geosynthetics, the shear rate applied in large-scale shear box tests should be adapted to assess a safety value of a geosynthetic - geosynthetic interface friction angle.