Mechanical response and failure precursors of sandstone under freeze-thaw cycling with varying saturation

In cold-region rock engineering, freeze-thaw (F-T) cycles and varying saturation levels significantly influence rock mechanical behavior and failure precursors. However, the acoustic emission (AE) characteristics and critical slowing down (CSD) precursors of red sandstone under different F-T cycles and saturation conditions remain largely unexplored. In this study, uniaxial compression tests combined with AE monitoring and CSD theory were conducted to investigate the damage evolution and failure precursors of red sandstone under varying saturation levels (0%–100%) and F-T cycles (0–30 cycles). The results show that UCS decreases by approximately 22.88% as saturation increases from 0% to 100%, and by about 40.20% after 30F-T cycles. Under high saturation and repeated F-T cycles, AE activity intensifies and crack propagation becomes more pronounced. The b -value decreases markedly prior to peak stress, while the cumulative b -value increases rapidly, indicating its potential as an instability precursor. Variance analysis based on CSD theory reveals that early-warning signals appear earlier and the warning window is extended with increasing saturation and F-T cycles. Furthermore, a conceptual dynamical framework linking initial damage ( D ), system recovery rate ( λ ), and CSD is proposed, illustrating that microcrack propagation and structural degradation under coupled F-T and saturation conditions drive critical slowing down behavior. The findings provide theoretical support and a conceptual methodology for stability assessment and precursor-based early warning in cold-region rock engineering.