The interface shear test method is used to measure the resistance to shear between a geosynthetic and soil, between two geosynthetics, or within a geosynthetic (e.g. Geosynthetic Clay Liner) under a constant rate of deformation. Interface and internal shear testing of geosynthetics is usually required when geosynthetic materials are used on sloping ground. The test measures peak and post-peak shear strength, which allows the determination of peak and post-peak friction angles.