Crack evolution patterns and closing mechanisms in expansive soils with different compaction degrees under wet–dry cycles

To study the crack evolution patterns in expansive soils under wetting–drying cycles, a series tests were conducted on the expansive soil from a canal side slope in the South-to-North Water Diversion Middle Route Project. Six indoor wet–dry cycle tests were performed on the samples with compaction degrees of 97%, 88%, and 79%. The crack image processing system by using Python was developed for quantitative analysis of crack ratios the expansive soil samples. Furthermore, PIV (particle image velocimetry) technology was also utilized to monitor the entire process of crack development. Results show that the evolution of crack ratios over time in the expansive soil samples can be divided into four stages, crack formation, crack development, crack closing, and crack stabilization stages. The higher the compaction degree of an expansive soil sample, the shorter its duration of the crack formation stage, and the shorter the time required for the crack ratio to reach its peak. The stress and displacement field nephograms of the samples can effectively reflect the crack evolution process on their surfaces. In addition, closing ratio was proposed to studied the crack closing capacity in expansive soil samples. The crack closing ratio decrease with the increase of the number of wet-dry cycles, as well as the compaction degree decreases. The primary cause of crack closing in compacted expansive soil is uneven shrinkage in the vertical direction, which arises from differing evaporation rates between the upper and lower parts of the sample.