Abstract: The deep rock mass is in a three-dimensional stress field, and the surrounding rock of the borehole or roadway is in an obvious true triaxial stress state due to the excavation-induced disturbance. Extraction technology is widely used as an important measure for gas control in coal mines. The plastic zone around a borehole is a good channel for gas flow. Therefore, it is important to study the characteristics of the plastic zone of the surrounding rock for the optimal layout of the extraction borehole. To study the plastic zone characteristics of drilling borehole surrounding rock, we analyze the stress distribution of surrounding rock based on the generalized plane strain theory, and compare the applicability and accuracy of several commonly used strength criteria (e.g. MC, Mises, DP, MLC, SMP). The results show that the MLC criterion can better characterize the true triaxial strength of rocks. Based on the MLC criterion, we derive a formula for the radius of the surrounding rock plastic zone, and analyze the effects of deviatoric stress, intermediate principal stress, internal friction angle, cohesion and radius of drilling hole on the permeability-increasing circular area. It is found that the permeability-increasing radius increases with the increase of deviatoric stress, decreases first and then increases with the increase of the intermediate principal stress, decreases with the increase of the internal friction angle or the cohesion of rock, and increases linearly as the radius of drilling hole increases. This research results can provide an important reference for the parameter design of engineering technology such as roadway support and coal seam gas drainage in practical projects.

Key words: true triaxial stress, strength criterion, borehole, plastic zone, permeability-increasing radius