Abstract: During the excavation and unloading process of deep cross-strata roadways, the stiffness degradation of surrounding rock is a key factor inducing dynamic disasters. To reveal the coupling effect of loading rate on rock failure mechanisms under low- stiffness constraints, multi-rate uniaxial compression tests on sandstone under high- and low-stiffness conditions were conducted using a self-developed variable-stiffness testing system. The mechanical behaviors and the rate-effect mechanisms of the stress-strain curves of sandstone under different stiffness constraints were comparatively analyzed, with a primary focus on the instability failure characteristics and energy evolution laws of sandstone under low-stiffness constraints. The results indicate that: (1) Low-stiffness constraints alter the conventional rate-dependent behavior of rock, with the transient compensatory effect of elastic energy accumulated in the system dominating the failure process, thereby enhancing the sensitivity of mechanical parameters, such as peak strength, to the loading rate. (2) With the increase in loading rate, the rock failure mode transitions from progressive step-like fracturing to transient instability at 0.15 mm/min, leading to intensified macroscopic tensile failure and an elevated proportion of mesoscopic tensile events. (3) The energy evolution law controlled by stiffness-rate coupling is elucidated. The low stiffness determines the macroscopic baseline for the forced release of post-peak elastic energy, while the increase in loading rate dynamically alters the post-peak energy dissipation path, driving the transition of energy release from step-wise dissipation to transient accumulation. The research findings reveal the essence of dynamic instability in rock masses under deep low-stiffness environments, providing theoretical support for the prevention and control of dynamic disasters in deep engineering.

Key words: rock mechanics, uniaxial compression, loading rate, low-stiffness loading, crack propagation