Pre-critical deformation properties and dilatancy behaviors of Beishan granite under different stress paths

Accurately understanding the mechanical laws and mechanisms of micro-damage to the near-field surrounding rock in high-level radioactive waste geological disposal repository is of great significance for achieving the disposal encirclement function. Given that the mechanical behavior and response characteristics of rocks are related to stress paths, the mechanical tests were conducted on three typical stress paths, including loading axial pressure-fixing confining pressure, loading axial pressure-unloading confining pressure, and unloading axial pressure-unloading confining pressure. The Beishan granite in the pre-selected area for high-level radioactive waste geological disposal in China was taken as the research object, and the effects of stress path and initial confining pressure on the pre-critical deformation properties and dilatancy behavior of which under high stress levels were systematically studied. Firstly, taking the brittle stress-drop as the critical state, for the controllable and comparable stress path bifurcation stage, a unified quantitative comparison of macro deformation parameter evolution process and characteristics based on deviatoric stress ratio is proposed. It is found that when unloading confining pressure at high stress levels, axial pressure unloading actually exacerbates lateral deformation and dilatancy compared to loading; The secant Poisson's ratio curves of secants between different paths are dispersed and clustered, with significant differences in evolutionary characteristics, but the confining pressure effect is not obvious; The relationship between the volumetric and radial strain increment follows a quadratic function, but there are significant differences in gradient and curvature of curves under different paths. Secondly, the plastic theory is used to describe the nonlinear deformation characteristics of rocks, and variables such as plastic strain compliance and radial axial plastic strain increment ratio (proportional coefficient) are defined. The results show that the greater the confining pressure, the more significant the inhibitory effect on lateral deformation and dilatancy, and the stronger the ability to withstand plastic deformation. Compared with axial compression loading, unloading delays the failure, making the development of plastic deformation more complete; Although initial confining pressure has a suppressive effect on rock deformation and failure, it has a greater promoting effect on plastic strain sensitivity and development, and has a greater impact on axial deformation. Finally, it is found that the relationship between the volumetric and axial plastic strain increment under each stress path follow an exponential distribution relationship. Based on this, a unified theoretical relationship between the apparent dilatancy angle and dilation angle before the critical point was constructed, and the influence of initial confining pressure was examined through the dilatancy index. The results show that the apparent dilatancy angle and the dilation angle of loading axial pressure-fixing confining pressure are the smallest, and the confining pressure effect of the dilatancy index is the most obvious; The increase gradient of dilation angle of loading axial pressure-unloading confining pressure is the largest, and the dilatancy trend is the most prominent; The apparent dilatancy angle and dilatation angle of unloading axial-unloading confining pressure are the largest, the dilatancy index is least affected by the initial confining pressure, and the dilatancy property is the most significant. The research results reveal the correlation between the pre-critical deformation evolution and dilatancy properties of Beishan granite under high stress level with stress path, indicating that the development of surrounding rock dilatancy micro-damage in multi-directional unloading area need be paid close attention to in the engineering practice and theoretical research of high-level radioactive waste geological disposal.