Cracks evolution and micro mechanism of compacted clay under wet-dry cycles and wet-dry-freeze-thaw cycles

Desiccation crack is a significant factor contributing to numerous engineering and environmental disasters. Under complex climate conditions, soils undergo not only seasonal wet-dry (WD) cycle, but also freeze-thaw (FT) cycle, which can alter the structural characteristics of soil, and influence the formation and development of cracks. In this study, the laboratory WD cycle and wet-dry-freeze-thaw (WDFT) cycle tests were carried out on the compacted lean clay samples which were dried to different dryness degrees to analyze the formation and development of cracks under different conditions and to discuss the mechanisms involved from a microstructural perspective. A high-resolution camera and scanning electron microscope were utilized to capture pictures of the macroscopic cracks on the sample surface and microscopic pores within the samples at each stage, respectively. The quantitative analyses were then conducted on the macroscopic cracks and microscopic pores. The results indicate that, with higher dryness degree, the crack ratio and crack width of the sample decrease with consecutive WD cycles. The samples subjected to WDFT cycles exhibit a higher crack ratio, wider cracks, and a greater proportion of primary cracks compared to the samples subjected solely to WD cycles. With lower dryness degree, the crack ratio and crack width remain relatively unchanged after each WD cycle, mainly consisting of small, shallow, and densely distributed sub-cracks. The overall crack ratio of samples subjected to WDFT cycles is lower than that of samples subjected solely to WD cycles. This is attributed to the aggregation of aggregate particles during the drying process and the dispersion promoted by the wetting process. Additionally, under high drying degrees, the FT process stabilizes the aggregate structure, while under low drying degrees, the FT process promotes the dispersion, rearrangement, and even fragmentation of aggregates, resulting in differences in crack development between WD and WDFT at different dryness degrees. These research findings provide valuable insights into the study of soil crack evolution and its mechanisms under complex environmental conditions.