Static prestress-governed nonmonotonic damage evolution trends and the prediction of sandstone strength degradation under low-frequency disturbances

The degradation and stability trends of interlayer rock strata subjected to residual coal mining disturbances are governed by the evolution of internal cracks during disturbance loading. To elucidate this process, uniaxial compression tests were conducted on sandstone specimens subjected to low-frequency disturbance loading under different static prestress levels. The irreversible strain accumulation, hysteretic response, and stagewise evolution trends of the secant modulus were analysed. Furthermore, the role of static prestress in regulating the crack evolution process was examined using energy dissipation and acoustic emission data. The results indicated that the level of irreversible strain increases logarithmically with the number of disturbance cycles and that its accumulation rate is highly sensitive to the static prestress level, with a pronounced nonmonotonic dependence. An analysis of the acoustic emission b value and the shear crack proportion revealed distinct damage regimes. At low prestress levels, the repeated closure and slip of preexisting cracks resulted in the rapid accumulation of damage and significant strength degradation. At intermediate prestress levels, particularly within approximately 40–50% of the UCS, precompaction suppressed the crack reopening process, and the stable evolution of small-scale shear cracks limited the development of damage. At high prestress levels, disturbance loading accelerated the tensile crack propagation and coalescence procedures, leading to renewed macroscopic mechanical degradation. On the basis of mechanism-driven parameter selection and a sensitivity analysis, a model for predicting disturbance-induced strength degradation was developed using irreversible strain accumulation and wave velocity evolution values as the key indicators. Under the present experimental conditions, the proposed model provides an empirical basis for evaluating disturbance-induced degradation exhibited by the bearing capacities of interlayer rock strata during upward mining.