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<0.5 the sample is in the stage of consolidation. For 0.5 ≤ 1н/1с ≤ 0.8, deformations are localized along the shear planes and sliding surfaces are formed. It should be noted that for 1н/1с = 0.5, the formation of shear planes only begins, and for 1н/1с = 0.8, they are completely formed. Values of the d parameter are defined and samples of their particular values are drawn for each 1н/1с value during the triaxial compression. Due to the statistical processing of the data the confidence limits of the d parameter variation were determined and their mathematical simulation was performed.