Study on instability mechanism of anisotropic structural planes of coal and rock under unloading

Currently, the research on the slip instability of rock mass structural planes has not considered the unloading effect during heading. There is relatively little research on the anisotropic structural planes of coal and rock. In order to explore the conditions and influencing factors that trigger the slip of anisotropic structural planes of coal and rock, a mechanical model of anisotropic structural planes is established. A criterion for unlocking slip of anisotropic structural planes under unloading is theoretically derived. A smooth structural plane numerical model is established using universal distinct element code (UDEC) to verify the accuracy of theoretical analysis of the triggering conditions for unlocking the slip of anisotropic structural planes. The influencing factors of unlocking slip of anisotropic structural planes are analyzed. The research results indicate that unlocking slip of anisotropic structural planes of coal and rock is related to the inclination angle of structural planes, internal friction angle, and the ratio of horizontal stress to axial stress. When the horizontal stress is equal to the axial stress, the anisotropic structural plane is always in a locked state without slipping. Increase of axial pressure and horizontal pressure and decrease of internal friction angle will increase the difficulty of unlocking slip on anisotropic structural planes. For downward unlocking slip, when the inclination angle of the structural plane is less than $45^\circ + \dfrac{{{\varphi _{\rm{f}}}}}{2}$ ($\varphi _{\rm{f}} $ is internal friction action), its increase will increase the difficulty of unlocking slip. When it is more than $45^\circ + \dfrac{{{\varphi _{\rm{f}}}}}{2}$, its increase will reduce the difficulty of unlocking slip. For upward unlocking slip, when the inclination angle of the structural plane is less than $45^\circ - \dfrac{{{\varphi _{\rm{f}}}}}{2}$ , its increase will increase the difficulty of unlocking slip. When it is more than $45^\circ - \dfrac{{{\varphi _{\rm{f}}}}}{2}$, its increase will reduce the difficulty of unlocking slip. For the locked state of structural plane, when the inclination angle of the structural plane is no more than 30°, if the axial stress is greater than the compressive strength, the brittle failure will occur in coal rock combination.