Experimental evaluation of multiscale damage evolution in fissured limestone under different wet–dry cycles and axial stress

Fissured limestone in the gorge-section hydro-fluctuation belt of the Three Gorges Reservoir (TGR) is continuously affected by wet-dry cycles and overburden self-weight stress, and its progressive deterioration markedly increases geohazard risk. However, the multiscale damage evolution mechanism of fissured limestone involving axial stress remains insufficiently understood. This study investigated damage evolution under coupled effects through wet-dry cycle-axial stress deterioration tests, uniaxial compression tests, and XRD, SEM, CT, and AE analyses. Based on elemental conservation, stoichiometric porosity was proposed to improve microscopic damage characterization. The results show that the deformation and failure process includes compaction, stable microcrack propagation, unstable microcrack propagation, and failure stages. With increasing wet-dry cycles and axial stress, the mass loss rate, saturated water absorption rate, and pore parameters continuously increase, while the proportion of tensile cracks decreases and the shear component increases. Under wet-dry cycle-axial stress coupling, calcite dissolution dominates mineral deterioration, grain cementation progressively weakens, the specimen surface evolves from a dense morphology to a honeycomb-like structure, and internal pore-fissure structures continue to develop. Overall, wet-dry cycling is the fundamental driver of damage deterioration, whereas axial stress promotes pore-fissure propagation and aggravates structural damage. The average difference between stoichiometric porosity and CT porosity is approximately 4.07%, indicating that the proposed index can support quantitative evaluation of microscopic damage in fractured limestone under acidic cycling. These findings deepen the understanding of damage evolution in fractured limestone within the gorge-section hydro-fluctuation belt in TGR.