The essence of deep rock mass excavation in engineering is to subject the surrounding rock to instantaneous or rapid confining pressure unloading. Prior to excavation, the surrounding rock has been under high in situ stress for an extended period. Thus, conducting in-depth research on the damage evolution characteristics and mechanical behaviors of the rock mass during the confining pressure unloading process, after being under long-term stress, is of utmost significance for accurately evaluating the stability of the surrounding rock under excavation-induced disturbances. Taking white sandstone as the research object, a new method for determining the long-term strength stress level \(\sigma_{t}\) of rock samples under different confining pressures through conventional triaxial creep tests is proposed. Then constant axial pressure ( \(\sigma_{t}\) ) and unloading confining pressure triaxial creep tests were carried out. The results show that when the initial confining pressure is 10 MPa, there is a strain reversal phenomenon in the instantaneous axial strain; the instantaneous axial and radial strains and creep strains increase with the increase of the initial confining pressure. With the increase in the number of unloading cycles, the axial creep strain of the rock shows a semi-arc trend, while the radial creep strain keeps increasing; the axial creep strain energy dissipation rate increases in an exponential function shape, and the radial creep strain energy dissipation rate curve evolves in a concave-downward shape. The instantaneous axial and radial strains, along with the energy dissipation rate, exhibit a continuous increasing trend. Upon examination of the scanned views of the rock specimens, it was observed that when the initial confining pressure was set at 10 MPa, two principal cracks traversed the rock specimen. In contrast, when the initial confining pressure reached 16 MPa and 20 MPa, only one principal crack extended through the specimen. Overall, the failure of the rock specimens was characterized as shear failure. Significantly, as the initial confining pressure increased, a greater number of secondary fractures developed within the specimens, indicating a more substantial degree of rock damage. This research unveils the damage evolution mechanism of white sandstone under confining pressure unloading at the long-term strength stress level. The findings hold significant value for predicting and evaluating the stability and potential failure risks during the excavation and unloading processes in deep rock mass engineering projects.
