The report describes the results of slope stability analyses, laboratory tests to measure the frictional properties of various interfaces that may be used in landfills and surface impoundments, and larger-scale tests to verify the data from laboratory tests. Several cases of sloughing of cover soils on FMLs in canals and tailings ponds have occurred. Sloughing at surface impoundments has occurred during heavy rainfalls, leading to the conclusion that the sloughing resulted from a buildup of pore-water pressure in the soil which reduced stability. This effect was demonstrated in tests on a 100-square-foot test stand. Stability analyses incorporating the effect of pore water was developed. Friction angles determined by direct-shear tests, the typical method used to measure interfacial properties, were compared with friction angles from larger-scale tests. The study determined that, in general, the direct-shear method produces friction angles that are less than those measured with a larger engineering-scale system. Therefore, friction angles determined by the direct-shear method are probably conservative for use in stability analyses.