Large displacement interface shear behavior of landfill cover materials

Landfill cover materials have stability and large displacement problems, therefore it is important to determine the interface shear behavior of these materials. In this study, large displacement direct shear and interface shear tests were conducted to determine shear strength and interface shear strength parameters of the soil/geosyntetic interfaces. For soil, crushed stone material was used. For geosynthetic materials, HDPE geomembranes (smooth and textured) and non-woven geotextiles (polyester) were used. Direct shear tests of crushed stone and interface shear tests of crushed stone to geomembrane/geotextile composite were conducted by a specially designed, large-scale, large-displacement, constant-contact-area direct shear test device under low normal stress levels. The experiments were conducted with large displacements up to 230 mm. Additionally, in one interface test, contact stresses were mapped. The direct shear tests of crushed stone resulted with lower friction angles comparing with the data in the literature. For the interface shear tests, the strength parameters were obtained slightly higher than those presented in the literature. For both smooth and textured geomembrane types, the interface friction angle values exceed 40° even for displacements up to 230 mm. Contact stress maps show that stress concentrations can exceed 600 kPa even for a 6 kPa normal loaded test. It is deduced that, these excessive stress concentrations may be linked with the interlocking behavior between soil and geosynthetic materials.