Macro-micro damage and energy release rates of fractured sandstone subjected to dry-wet cycles

The deterioration of rock mass in the Three Gorges reservoir area results from the coupled damage effects of macro-micro cracks and dry-wet cycles, and the coupled damage progression can be characterized by energy release rate. In this study, a series of dry-wet cycle uniaxial compression tests was conducted on fractured sandstone, and a method was developed for calculating macro-micro damage ( D R ) and energy release rates ( Y R ) of fractured sandstone subjected to dry-wet cycles by considering energy release rate, dry-wet damage and macro-micro damage. Therewith, the damage mechanisms and complex microcrack propagation patterns of rocks were investigated. Research indicates that sandstone degradation after a limited cycle count primarily exhibits exsolution of internal fillers, progressing to grain skeleton alteration and erosion with increased cycles. Compared with conventional methods, the D R and Y R methodologies exhibit heightened sensitivity to microcrack closure during compaction and abrupt energy release at the point of failure. Based on D R and Y R , the failure process of fractured sandstone can be classified into six stages: stress adjustment (I), microcracks equal closure (II), nonlinear slow closure (III), low-speed extension (IV), rapid extension (V), and macroscopic main fracture emergence (VI). The abrupt change in damage energy release rate during stage V may serve as a reliable precursor for inducing failure. The stage-based classification may enhance traditional methods by tracking damage progression and accurately identifying rock failure precursors. The findings are expected to provide a scientific basis for understanding damage mechanisms and enabling early warning of reservoir-bank slope failure.