Nonlinear Failure Envelope for Soils

The reasons for nonlinearity of failure envelopes for soils are examined and a simple expression for the variation of the angle of shearing strength with stress level is proposed. The nonlinear failure envelope in terms of effective stresses for most soil and rock types can be described by a simple four-parameter expression of hyperbolic type. The parameters are the basic angle of friction, the maximum angle difference, the median angle normal stress, and, conditionally, cohesion. The cohesion term vanishes in the case of noncemented soils, and applies mostly to te peak strength of rock. Peak and residual failure envelopes for soils ranging from crushed rock, gravel, and sand to clay and clay minerals can be described in terms of the proposed parameters with good agreement and generality. The proposed description simplifies interpretation of shear test results. The nonlinear description in the proposed form can be incorporated in limit equilibrium methods and stress-strain analysis.