Rate- and Normal Stress-Dependent Mechanical Behavior of Rock Under Direct Shear Loading Based on a Bonded-Particle Model (BPM)

Shear failure often occurs during the construction of the geotechnical engineering projects, especially under dynamic loading condition. To investigate the effect of loading rate and normal stress on the shear responses of rock materials, a bonded-particle model (BPM) was adopted to insight the failure process from a micro-mechanical perspective using the moment tensor. Firstly, we propose a novel calibration methodology for direct shear test, which provides a possibility to effectively derive microscopic parameters of PBM for obtaining the macroscopic properties of rocks considering loading rates under direct shear. Validation results show that the simulation demonstrated good agreement with the experimental results. Then, a series of simulated direct shear tests under different normal stresses ranging from 0 to 20 MPa and loading rate ranging from 10 –3 to 10 –1 mm/s were performed. The results demonstrate that both normal stress and loading rate have a positive influence on shear strength. However, normal stress has a greater impact on the shear strength compared to loading rate, and the effect of loading rate on shear strength weakens as the normal stress level increases. Increasing the loading rate and normal stress level led to a higher distribution of AE events in the nonlinear stage. At lower loading rates and normal stress, intense AE activity was concentrated around the peak stress point. In contrast, under higher situations, the nonlinear stage is prolonged, resulting in an increased number of AE events in the nonlinear and post-peak stages. Normal stress has a greater impact on the generation and propagation of micro-cracks compared to loading rate. With higher normal stress, the failure mode shifts from a shear-dominated to a mixed mode with tensile micro-cracks in a dominant role.