Study on the damage characteristics of feldspar sandstone in the tongchuan area under Thermal-Wet Cycles

Rock and soil masses are significantly affected by thermal-wet cycles, and the resulting changes in their internal structure and strength poses a serious threat to cavern stability. In this study, sandstone samples from Tongchuan City, Shaanxi Province were subjected to continuous thermal-wet cycles tests until failure. The variation in physical properties and acoustic emission characteristics after different number of cycles were analyzed to investigate the damage mechanism induced by thermal-wet cycling. The research results indicate with increasing cycle numbers, the mass loss rate of sandstone first increases and then decreases, while surface roughness increases continuously from 0.043 to 0.214. The number of micropores (r<0.1 μm) decreases, mesopores (0.1 μm<r<1 μm) increase, and macropores (r>1 μm) remain almost unchanged. After 20 cycles, cracks began to appear on the sample surface and rapidly develop, penetrating the entire sample at 70 cycles. The thermal acoustic emission activity of sandstone is significantly enhanced with cycling, and the changes in acoustic emission characteristics can be divided into three stages: Stage I (0−200 s), microcracks expand slowly and stably, with few ringing counts, a gradual increase in cumulative counts, and b-values fluctuating within a limited range; Stage II (200−700 s), microcracks undergo unstable propagation, with increased acoustic emission activity and ringing counts, and a rapid increase in cumulative ringing count with frequent and significant fluctuations in acoustic emission b value. Stage III (700−900 s), sandstone failure becomes essentially stable, acoustic emission activity and ringing counts decline, cumulative counts plateau, and b-values decrease. A strong correlation is observed between mass loss rate, surface roughness, and the total number of AE signals. The research reveals the damage evolution mechanism of sandstone under cyclic thermal-humid conditions. The findings provide a scientific basis for the protection of ancient cavern rock masses.