Crack evolution of soft red-bed rock under drying-wetting cycles

Softening of soft red-bed rocks subjected to rainfall-evaporation cycles is commonly characterized by rapid disintegration and is often accompanied by cracking, resulting in degradation of the mechanical properties of the rock, which can lead to slope instability or rockfalls. The microstructural changes in soft red-bed rocks after immersion were imaged, and two-dimensional (2D) images of cracks under water absorption-evaporation conditions were obtained. The dynamics, fractal characteristics, and geometry of the cracks were analyzed using digital image processing and analysis based on morphological algorithms. The results indicate that the face–face particle bonds become point–face bonds with numerous micropores with sizes of 1–5 μm. The evolution of cracks generated after water absorption can be divided into four stages: edge crack initiation, crack propagation, crack coalescence forming the main crack, and subcrack segmentation. The evolution of the dynamic characteristics of cracks during water absorption and drying cycles can be effectively described by the crack intensity factor, crack density, and average width. The fractal dimension increases to a stable value with increasing soaking time, whereas drying increases the crack complexity, resulting in fractal dimensions ranging from 1.106 to 1.126. The geometry results indicate that the crack directions are mainly at angles of 30°–70° after soaking and primarily in the range of 50°–60° after 10 drying cycles. The transition of the crack intersection angle from a bimodal to a unimodal distribution suggests that water absorption and drying processes tend to form Y-shaped and T-shaped cracks, respectively. Finally, the evolution of the water–rock interface induced by particle dissolution, ion exchange, expansion force, and liquid surface tension was used to explain the mechanism of crack evolution related to water entry and evaporation. These results provide a theoretical basis for evaluating the cracking behavior of soft red-bed rocks.