The most important physical property of the soil, with respect to slope stability, is its strength. Among the several methods used to measure the strength of a soil, engineering practice has demonstrated that the relevant measure of resistance to sliding is the residual strength (Skempton, 1964). Residual strength is commonly measured in a direct- or ringshear device that accumulates large displacements on a welldeveloped failure surface. The residual strength is the minimum strength measured under fully drained conditions for at least three different normal loads.