Effect of Loading and Unloading Rates on Sandstone Deformation and Dilatancy under True Triaxial Condition

After conducting true triaxial tests on sandstone in a laboratory setting, this study aims to determine the safe tunnelling rate of the roadway by examining the instability and failure characteristics of surrounding rock under different disturbance stresses in deep underground roadway excavation. Results showed that the mechanical properties, deformation, and failure characteristics of sandstone differed under different loading and unloading rates. Specifically, as the loading rate increased, the crack initiation stress increased while the damage stress remained unchanged, and the deformation anisotropy of the rock decreased. In contrast, as the unloading rate increased, the residual stress of the rock decreased, the brittleness increased, and the deformation anisotropy of the rock increased. Additionally, the expansion of the rock went through three critical stages: (1) A–B: a sharp increase in the dilatancy of sandstone ( M ) in a short period, accompanied by a large number of cracks, (2) B–C: a weakened stage of expansion ability, in which M continued to decrease over time, albeit at a slower rate, and (3) C–D: a stage of enhanced expansion ability, during which M began to increase again, albeit at a slower rate than its rate of decrease. This final stage was the longest.