The mechanics of surficial failure in soil slopes

It is not safe to employ the classical infinite slope failure analysis procedure in which the Coulomb failure criterion is used, because a very large portion of the factor of safety is assigned to the effective cohesion which is not present in the soil. Part of the problem arises from the normal stresses used in the drained direct shear tests, which are high relative to the normal stresses prevailing in surficial failures. The real effective strength envelope is curved, and it is proposed to model it by a power function whose parameter values may be determined from the usual shear tests performed at the normal stress magnitudes usually employed. Based on the factor of safety calculations from the curved failure envelope and observations from field rainfall infiltration experiments, the mechanics of surficial failure of slopes is explained.