Effect of Unloading Confining Pressure Rates on Macroscopic and Microscopic Mechanisms in Diorite Based on PFC 3D

Xiaoxiao Duan^1*Dengke Yang¹Xuexu An^2*Wen Zhang³

• ¹School of Intelligent Science and Information Engineering, Xi'an Peihua University, Xi'an, Shanxi, China
• ²Shaanxi College of Communications Technology, Xi'an, China
• ³College of Energy Engineering, Xi'an University of Science and Technology, Xi’an, China

Underground engineering activities inevitably cause rock unloading damage. Herein, we employed the discrete element method to investigate the microscopic and macroscopic response mechanisms of deep hard diorite samples during the loading and unloading process. We performed numerical analysis at three unloading-confining-pressure rates using PFC 3D . The macroscopic mechanical characteristics, particle displacement, number of contact force chain failures, and propagation and evolution characteristics of the spatial distribution of tensile shear microcracks along the axial and radial directions of the samples during the loading and unloading process were studied.Results show that the peak strength and strain of the samples increase with decreasing loading rate. In addition, the confining pressure exhibits instantaneous fluctuation during the unloading process, indicating gradual damage and fracture evolution in the samples. The radial displacement component of the particles and the number of contact force chain failures in the samples show a nonlinear concave increase from the inside out during the loading and unloading process, especially after achieving peak strength, indicating that the damage and fracture of the samples are the most severe near the unloading surface. Furthermore, microcracks in the samples evolve from the unloading surface to their interior during the loading and unloading process. Finally, tensile microcracks outnumber the shear microcracks, and their distribution density increases with decreasing unloading confining pressure rate.