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.

The stable and safe operation of highway/railway lines is largely dependent on the dynamic behavior of subgrade fillings. Clay soils are widely used in subgrade construction and are compacted at different remolding water contents and compaction degrees, depending on the field conditions. As a result, their dynamic behaviors may vary, which have not been fully investigated until now. To clarify this aspect, a series of cyclic triaxial tests were carried out in this study with three typical remolding water contents (w = 19%, 24%, and 29%), corresponding to the optimum water content as well as its dry and wet sides, and two compaction degrees (Dc = 0.8 and 0.9), which were selected according to the field-testing data. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests were also conducted on typical samples to investigate the corresponding soil fabric variations. The findings indicate the following: (a) The soil fabric at the optimum remolding water content and its dry side was characterized by a clay aggregate assembly with a bimodal pore size distribution. In contrast, the soil fabric on the wet side of the optimum water content consisted of dispersed clay particles with a unimodal pore size distribution. (b) As the compaction degree increased, to ensure the optimum water content and its dry side, large pores were compressed to make them smaller, while small pores remained unchanged. Comparatively, all the pores on the wet side were compressed to make them smaller. (c) For each compaction degree, as the remolding water content increased, a non-monotonic changing pattern was identified for both the permanent strain and resilient modulus; the permanent strain first decreased and then increased, while, for the resilient modulus, an initial increasing trend and then a decreasing trend were identified. In addition, a larger changing rate of the permanent strain (resilient modulus) was observed on the dry side, indicating a larger effect of the remolding water content. (d) For each remolding water content, as the compaction degree increased, the permanent strain exhibited a decreasing trend, but an increasing trend was identified for the resilient modulus. Moreover, the rate of change in the permanent strain (resilient modulus) on the dry side of the optimum water content was larger than that on the wet side. In contrast, the minimum rate of change was identified at the optimum water content. The obtained results allowed for the effects of the remolding water content and compaction degree on the dynamic behavior to be analyzed, and they helped guide the construction and maintenance of the subgrade.