Shear strength of geomembrane–soil interface under unsaturated conditions

Interface shear tests on non-textured geomembrane–soil interfaces were performed using a modified direct shear apparatus installed with a miniature pore pressure transducer (PPT). Using the PPT, pore-water pressure changes in the vicinity of geomembrane–soil interface were measured during shear, thus making it possible to analyse test results in terms of effective stresses. When plotted in terms of total stresses, interface shear strength envelopes, consistent with those published in the literature, were obtained. However, the results were difficult to interpret using effective stresses, particularly those obtained at high normal stresses. It was found that at low normal stress values the shear strength of a geomembrane–soil interface in terms of effective stresses could be predicted using unsaturated soil mechanics concepts. At high normal stresses, it was found that the failure mechanism changed from soil particles sliding at the surface of the geomembrane to soil particles becoming embedded into the geomembrane and plowing trenches along the direction of shear. A plowing failure mechanism resulted in the mobilization of significantly higher shear strength at the geomembrane–soil interface. It was found that at higher placement water content for the soil, near saturated conditions prevail at the geomembrane–soil interface, resulting in lower effective stresses, a shallower plowing mechanism and lower values of mobilized interface shear strength.