Abstract:Hoek–Brown strength criterion has been widely used in rock and rock mechanics analysis, rock slope stability analysis, rock tunnel design, and other fields, but the conventional Hoek–Brown strength criterion does not consider the influence of intermediate principal stress on rock strength, and its application cannot reflect the true triaxial mechanical properties of rock. Therefore, it is necessary to improve the Hoek–Brown strength criterion. By analyzing the true triaxial test results of rock, the influence law of rock intermediate principal stress on rock strength was clarified. There are two characteristics of strengthening and restraining its influence: when it is greater than the critical value $\sigma_2^* $, the rock strength is enhanced, and when it is less than the critical value $\sigma_2^* $, the rock strength is weakened. Based on the critical stress state of the intermediate principal stress, the intermediate principal stress coefficient β was introduced, and a two-dimensional Hoek–Brown strength criterion was improved to the true triaxial Hoek–Brown strength criterion. The parameters in the model were determined by the nonlinear fitting method. By comparing the true triaxial test data of 8 rock types, the accuracy and universality of the improved strength criterion were verified. and compared with the other three true triaxial strength criteria. The results show that the best parameter m of the improved true triaxial strength criterion can fall within the range suggested by Hoek and Brown, and can inherit the advantages of the Hoek–Brown strength criterion. The predicted root mean square difference (RMSE) values of the improved strength criterion for the eight rock types are lower than those of the other three criteria, which can accurately predict the rock strength, is suitable for most rock types, and better reflects the influence of the intermediate principal stress on the rock strength. The Pan–Hudson criterion always overestimates the rock strength value under low σ₂ conditions and underestimates the rock strength value under high σ₂ conditions. The Zhang–Zhu and Priest criteria have a strong dependence on the rock type and the description accuracy is relatively poor. The improved true triaxial Hoek–Brown strength criterion has an important application and promotion value.