The article presents the Mohr – Coulomb plasticity condition modified by incorporating a third parameter depending on sample axial deformation value for which sliding planes are formed and developed under triaxial compression. A relative characteristic of a sample’s work under shear strength is offered; it is determined by the ratio of the axial strain 1 to the ultimate strain 1с for which the parameters of the Mohr – Coulomb failure envelope are defined. Based on the triaxial test data it was established that for 1/1с