True triaxial shear creep behaviors of sandstone under constant normal load and constant normal stiffness conditions

The long-term stability of rocks is crucial for ensuring safety in deep engineering, where the prolonged influence of shear loading is a key factor in delayed engineering disasters. Despite its significance, research on time-dependent shear failures under true triaxial stress to reflect in situ stress conditions remains limited. This study presents laboratory shear creep measurements on intact sandstone samples under constant normal load (CNL) and constant normal stiffness (CNS) conditions, which are typical of shallow and deep engineering cases, respectively. Our investigation focuses on the effects of various lateral stresses and boundary conditions on the mechanical behaviors and failure modes of the rock samples. Results indicate that lateral stress significantly reduces shear creep deformation and decreases creep rates. Without lateral stress constraints, the samples are prone to lateral tensile fractures leading to macroscopic spalling, likely due to “shear-induced tensile” stress. This failure behavior is mitigated under lateral stress constraints. Additionally, compared to CNL condition, samples under CNS condition demonstrate enhanced long-term shear resistance, reduced shear creep rates, and rougher shear failure surfaces. These findings suggest the need to improve our understanding of rock mass stability and to develop effective disaster prevention and mitigation strategies in engineering applications.