Experimental Study on Mechanical Properties and Stability Analysis of Structural Plane under Unloading Normal Stress

Excavation in rock induces the unloading of stress in excavation-disturbed zones and can cause the structural plane to reach shear failure due to the unloading of normal stress. Unloading normal stress tests of a regular sawtooth structural plane were conducted in this study to investigate the influence of the unloading velocities (v) and asperity angles (theta) on mechanical properties and the stability of the structural plane. The average value of normal displacement (D-u(1)) and the shear displacement of unloading (D-su) gradually increases with an increase in theta and D-u(1) accounts for 59.70%, 31.81%, and 18.60% of the height of a single asperity under different theta. However, D-u(2), D-s, and the unloading capacity (Delta sigma(n)) gradually decreased. Moreover, D-s account for 24.52%, 11.61%, and 7.4% of the length of a single asperity, respectively. With an increase in theta and v, three-dimensional (3D) morphology parameters and normal deformation energy (U-n) decreased gradually, indicating that the damage degree of the structural plane increases. The analysis of the evolution of the gap width of the structural plane at the initial point, unloading point, and instability point indicates that the greater the theta or the smaller the v, the lower would be the likelihood for the structural plane to be damaged. Concurrently, its instability is more sudden and the impact tendency is stronger.