Paul-Mohr-Coulomb failure criterion for geomaterials

Paul-Mohr-Coulomb (PMC) failure criterion provides enhanced representations of pyramidal failure surfaces, with recognizable material parameters, by considering all three principal stresses. PMC exhibits traits of more complex failure criteria through piecewise linear approximations to a curve failure surface, in changing both mean stress and Lode angle. A new least-squares fitting approach is developed to determine three PMC material parameters: two friction angles, one for compression and one for extension, and the theoretical isotropic tensile strength (the vertex of the pyramid). Experimental data from axisymmetric compression and extension tests are used to construct a six-sided pyramidal failure surface, revealing that the friction angle in extension is larger than the friction angle in compression for isotropic geomaterials, a manifestation of the intermediate stress effect. To enhance the description of the failure surface, multiaxial test data are added, and six parameters are determined by fitting two planes independently with four friction angles and two different vertices. The six-parameter PMC model is able to approximately capture the nonlinear nature of the failure surface for both rock and soil.