A comparison study of interface shear strength parameters of textured high density polyethylene (HDPE) geomembranes and linear low density polyethylene (LLDPE) geomembranes sheared against two geotextiles and one geosynthetic clay liner commonly used in South African landfill lining systems is presented. Tests were performed using the 305 x 305 mm large direct shear box over a range of normal pressures of 25, 50, 100, 150, 200 and 300 kPa. A shear rate of 0.1 mm/min was used for geomembrane-GCL interfaces and a rate of 1 mm/min for geomembrane-geotextile interfaces. It was found that LLDPE and HDPE geomembranes produced different friction characteristics when sheared against different geosynthetics. LLDPE geomembrane/ geotextile interfaces showed that the conventional linear failure envelopes did not always give the best regression relationship between shear stress and normal stress parameters for sheared interfaces. These geomembrane shear strength envelopes could be described more accurately as bilinear failure envelopes. When a linear failure envelope was considered, at normal stresses less than 150kPa, LLDPE geomembrane peak interface shear stresses were higher than HDPE geomembrane peak shear stresses. Normal stresses greater than 150kPa indicated that HDPE geomembranes had higher peak interface shear stresses when compared to those produced by LLDPE geomembranes. From these observations, applications where low normal stresses (<150kPa) would be applied, such as in landfill capping systems, it was proposed to select LLDPE geomembranes and to use HDPE geomembranes where large normal stresses would be experienced, such as along the base and slopes of a landfill.
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