Mechanisms of crack development and strength deterioration in compacted expansive soils under controlled wetting-drying conditions

Expansive soils are extremely susceptible to environmental changes. Under conditions of drought or heavy rainfall, the soil tends to develop cracks, thereby affecting its mechanical properties. This study aims to investigate the formation of cracks and the mechanisms of strength degradation in compacted expansive soils under different wetting-drying circumstances. Seven wettingdrying (W-D) cycles were conducted at various temperatures, relative humidity (RH) levels, and water content ranges. Scanning electron microscopy (SEM), image processing techniques, and direct shear testing were integrated to analyze the soil structure deterioration at micro, meso, and macro scales, respectively. Experimental results indicate that the crack parameters exhibit an initial growth followed by stabilization with increasing W-D cycles, while the shear strength gradually decreases. The conditions of higher temperature, lower RH, and larger W-D ranges correspond to the greater initial growth rate of the crack parameters, along with a higher cohesion decay rate. The internal friction angle tends to fluctuate within a narrow range, so the reduction in cohesion is the primary cause of shear strength degradation during W-D cycles. Under distinct overburden stress conditions, the shear strength exhibits non-linear characteristics. The measured values of cohesion under low-stress conditions are lower than theoretical values, while the internal friction angle demonstrates the opposite trend. Correlation analysis shows that the crack rate is more closely associated with the cohesion degradation rate, presenting a certain linear relationship. The slope and intercept of this relationship are related to environmental temperature and relative humidity. Overall, the macroscopic strength degradation is due to the coupled effects of microscopic structural damage and mesoscopic crack development. In a cyclic W-D environment, the imbalance of the tensile stress field leads to the accumulation of microscopic fatigue damage. It promotes the formation of mesoscopic cracks, creates weak areas, and damages soil integrity, ultimately manifesting as deterioration in macroscopic mechanical properties. This study provides valuable insights into the cognitive understanding of the mechanical structure damage evolution of expansive soil under extreme environmental factors.