Research on the Macroscopic and Microscopic Failure Mechanisms and Damage Deterioration Patterns of Granite under Unloading Paths

Accurate analysis of the deformation characteristics and the damage destruction mechanism of a rock mass is a prerequisite for the evaluation of the stability of the surrounding rock in tunnel engineering. This paper proposes a combination of numerical simulation techniques based on microstructure analysis and physical model experimental methods, which allows for the microscale interpretation of macroscale experimental phenomena and provides new insights for further summarizing the instability and failure patterns of rocks under unloading paths. To investigate the macroscopic and microscopic failure mechanisms as well as the damage deterioration patterns of granite under unloading conditions, physical model tests were conducted using stress paths of conventional triaxial and constant axial pressure unloading confining pressure. The experiments encompassed unloading paths, and the associated mechanical responses were finely simulated using the particle flow code method coupled with digital image processing techniques. The results reveal that at lower unloading rates, granite predominantly undergoes shear failure, with the destabilizing mechanism attributed to the formation of an “X”-shaped conjugate failure surface under the influence of tension–shear coupling. As the unloading rate increases, tensile forces progressively take precedence, leading to more pronounced brittle failure characteristics in granite. The ratio of the confining pressure reduction to the initial confining pressure at the point of specimen failure increases with the unloading rate and decreases with the initial confining pressure. Faster unloading rates correspond to a more rapid increase in Poisson’s ratio, and the unloading path primarily influences the lateral strain variation during the initial stages of unloading. Additionally, under unloading conditions, the internal friction angle of granite increases, while the cohesion decreases. The impact of unloading rate and path on cohesion becomes more pronounced. The findings of this research have certain reference value for further optimizing the methods for assessing the stability of rock masses surrounding tunnels.