An improved triaxial strength failure criterion for rock and its validation

The influence of intermediate principal stress (σ₂) on the strength of rocks varies greatly among different types of rocks under the true triaxial stress state, and few attentions have been paid to it. It is found that under conventional triaxial compression, rock strength exhibits nonlinear characteristics, while under true three-dimensional loading, σ₂ has a significant impact on the rock strength. Analyzing the experimental results of fourteen kinds of rocks revealed that the σ₂ effect was divided into two stages (strength promoting stage and strength weakening stage). Based on these concepts, a true triaxial strength criterion was then proposed. Finally, its accuracy and applicability were examined by comparing the predicted results with the experimental results. This criterion was also compared with the other three criteria (namely Mogi-Coulomb criterion, Drucker-Prager criterion and Chen-Chen criterion). Errors analysis shows that this proposed strength criterion is generally the best with low absolute and relative errors, followed by Mogi-Coulomb criterion, Chen-Chen criterion, and Drucker-Prager criterion. This criterion retains the advantage of the Hoek–Brown (H-B) criterion, as its parameters are selected from the traditional H-B criterion. The newly introduced parameters can be applied to judge the existence of strength weakening stage of intermediate principal stress effect and determine the critical interval of the two stages. This proposed strength criterion has certain reference value for the prediction of rock strength and the evaluation of engineering surrounding rock safety.