The 3-D weakness plane model was developed to clarify the mechanisms of influences of various stress states on the strength of rocks. The model assumes numerous planes of weakness whose directions are uniformly distributed in a rock; each plane slips or opens based on the Coulomb criterion with a tension cut-off; and the rock is regarded as failed when the ratio of the failed plane number to that of all planes reaches a certain value. The equal strength parameters were assigned to all planes and no complicated statistical functions are used. The model was applied to true triaxial compression, uniaxial tension, Brazilian and extension tests. The model was very simple but the effects of a stress state, namely, nonlinear increase of peak stress with small intermediate principal stress and nonlinear decrease with large intermediate principal stress in true triaxial tests, larger tensile strength by Brazilian tests than uniaxial tensile tests and the bilinear nature of peak stress in extension tests, were simulated very well. Distributions of stress and failed planes on the Schmidt net increased the understanding of the mechanism of the stress state effects on rock strength.
