Abstract:Two new concepts about the principal shear stress couples and their actions are proposed through the analysis of Mohr-Coulomb criterion and the twin shear criterion. Based on these concepts,a new failure criterion is given out for geomaterial. The effect of the whole three principal shear stress couples acting on the dodecahedral element should be taken into account in the establishment of failure criterion. Theoretical analyses about the loci on the p plane,the Mohr circles on the s-t plane,the effect curves of the medial principal stress and the directions of the failure surface show that the proposed criterion can basically reflect the true failure and strength characteristics of geomaterial with the verification of experimental facts.