Study on the mechanical properties and fracture mechanism of hollow‑cylinder sandstone under lateral unloading conditions

To analyze the impact of excavation unloading on the stability of the surrounding rock in existing deeply buried tunnels，the study utilized conventional triaxial compression tests and lateral unloading tests on hollow-cylinder sandstone specimens，simulating the mechanical behavior and fracture mechanisms of circular tunnels under complex stress conditions. Initially，stress-strain curves of hollow-cylinder sandstone under various confining pressures obtained from indoor mechanical tests were analyzed to assess the impact of confining pressure on mechanical properties. Subsequently，the effects of unloading rate and stress level on the strength deformation characteristics and energy evolution laws of hollow-cylinder sandstone were detailed，based on the stress-strain curves of rock samples under lateral unloading. Finally，the multi-scale fracture mechanisms of hollow-cylinder sandstone under lateral unloading were discussed through observations using cameras and micron-scale industrial CT on the failed specimens. The results showed that both the peak strength and elastic modulus of hollow-cylinder sandstone increased with confining pressure，with peak strength being more sensitive to confining pressure than the elastic modulus. As the unloading rate increased，the peak strength and damage threshold of hollow-cylinder sandstone showed a non-linear decreasing trend，while the Young's modulus remained essentially unchanged，and the elastic energy and dissipated energy at the peak stress decreased. With increasing unloading stress levels，the peak strength and damage threshold of hollow-cylinder sandstone showed non-linear and linear growth，respectively，with no significant change in Young's modulus，but an increase in elastic energy and dissipated energy at the peak stress. Under conventional triaxial compression，hollow-cylinder sandstone exhibited shear failure，and with increased confining pressure，the fracture angle decreased while the degree of fracturing for internal wall increased. Under the same confining pressure，as the unloading rate increased or the unloading stress level decreased，the surface fracturing of the hollow-cylinder sandstone was severe while the fracturing damage of internal wall weakened. The crack rate in the failed hollow-cylinder sandstone was directly proportional to both the unloading rate and the stress level. The study’s conclusions provide insights into the failure and instability mechanisms of the surrounding rock in deeply buried tunnels/roadways under lateral unloading conditions.