Abstract: The combined effect of high temperature and alternating rain and heat significantly accelerates the disintegration and degradation of red-bed soft rocks, frequently triggering geological disasters such as slope cracking and deformation, collapse and landslide, as well as water and mud inrushes. This poses a severe challenge to the disaster prevention system in red-bed areas. Investigating the evolution patterns of rock fractures under hydro-thermal cycles plays a pivotal role in preventing and controlling disasters in red-bed regions. To quantitatively analyze the propagation of red-bed soft rock fractures under hydro-thermal cycles, digital image processing technology was adopted alongside a fracture aperture calculation method based on axial transformation. The evolution patterns of fracture apertures and the morphological changes in contour lines of red-layer soft rock fractures were systematically analyzed across a temperature range from room temperature to 100 ℃ were systematically analyzed. The results show: (1) Under hydro-thermal cycle action, red-bed soft rock fractures exhibit significant expansion and evolution characteristics, and the increase in temperature significantly intensifies the fracture expansion effect. At 100 ℃, the fracture apertures increases by 1.96 to 10.54 times compared to the initial value, markedly higher than the 2.25 to 7.93 times observed at room temperature. Further research reveals that fractures with smaller initial apertures are more sensitive to hydro-thermal cycles, and this sensitivity becomes more pronounced with increasing temperature. (2) Spatially and temporally, the analysis of fractures aperture evolution reveals that aperture differences between adjacent measurement points increase significantly with cycling, indicating non-uniform fractures propagation. Higher temperatures significantly promote soft rock crack propagation. (3) The sand and gravel debris at fracture edges, being highly resistant to hydrothermal erosion, develops pronounced resistance differences compared to surrounding clay minerals undergoing continuous erosion. This leads to progressively larger aperture differences between adjacent points. Once clay matrix erosion reaches a critical level, the debris undergoes structural detachment. Synchronous observations reveal that despite local fluctuations in fracture contour roughness, it exhibits an overall significant decline during cycling, indicating sustained abrasion of fracture walls due to hydro-thermal coupling. (4) Hydrothermal cycles exacerbate soft rock damage through the combined effects of thermal expansion/contraction stresses and pore water phase transitions. Temperature variations promote microcrack propagation and enhance permeability while inducing capillary tension via evaporation-condensation cycles. Repeated hydration-dehydration cycles in clay minerals further induce tensile fracturing, resulting in significantly amplified cumulative damage.

Key words: red-bed soft rock, crack propagation, hydro-thermal cycle, aperture, contour line, fractal dimension