Influence of intermediate principal stress on rock strength: Insights from closed crack evolution under true triaxial compression

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It is crucial for predicting rock failure to understand the mechanisms of cracks evolution under true triaxial stress state which is frequently encountered in deep underground engineering. To elucidate the mechanisms of the intermediate principal stress effect on cracks evolution and strength, this study investigated the gypsum specimen with a through closed flaw parallel to three principal stresses, respectively, in true triaxial experiment. The acoustic emission technique and stress curve were used to determine the characteristic stress thresholds. The results demonstrated that the intermediate principal stress had a weak impact on the initiation and peak stress of the flaw parallel to the intermediate principal stress itself. While it had a great effect on the flaw parallel to the minimum principal stress. As the magnitude of the intermediate principal stress increased, both the initiation and peak stresses rose considerably. In cases where flaws were parallel to the maximum principal stress, the effect of the intermediate principal stress could be divided into two stages: initially, both the initiation and peak stresses increased with rising intermediate principal stress. However, once the intermediate principal stress exceeded certain thresholds, it triggered the crack initiation, resulting in a decrease in both initiation and peak stress. Due to the random distribution of inherent cracks in rocks, the effects of the intermediate principal stress are coupled, resulting in an initial increase followed by a decrease in the strength. Furthermore, the results of cracks modes also illustrated that substantial stress can stimulate the initiation of tensile cracks when flaws were parallel to the stress, which explained why the spalling or slabbing is more likely to occur in the surrounding rocks of deep underground tunnels.