# Development Characteristics and Mechanism of Crack in Expansive Soil under Wet-Dry Cycling
## 3 Indoor Wet-Dry Cycle Test of Expansive Soil
### Experimental Results and Analysis
From a microscopic perspective, the binding water film between clay particles in expansive soil with a higher initial moisture content is relatively thick, and the free water content increases. During the dehydration process, more significant water loss occurs, resulting in volume shrinkage. From a macroscopic perspective, in the process of crack development caused by the dehydration of expansive soil, the increase in initial moisture content directly leads to a larger gradient difference between the surface area and the internal moisture content after water loss. This leads to more significant uneven shrinkage and confinement in the depth direction, accelerates crack development and extension, and causes more obvious crack damage in specimens. This result is consistent with previous research findings. **(3) The effect of compaction level on crack development characteristics**  
Figure 8 illustrates the variation curves of the fundamental crack morphological parameters of various compacted expansive soil specimens under different dry-wet cycling conditions. The figure indicates that increased compactness leads to a decrease in the crack ratio, total crack count, total crack length, and average crack width of the expansive soil specimens. Additionally, the range of variation in the basic crack morphological parameters decreases with increasing compactness under multiple dry-wet cycles, suggesting that compactness imposes restrictions on the shrinkage and swelling characteristics of the expansive soil (Figure 8a). For example, the crack ratio difference between specimens with 90% and 80% compactness ranges from 0.23% to 0.61%. This influence of compactness on the crack ratio is most noticeable in highly expansive soil samples, with crack ratios after the second dry-wet cycle reaching 4.47% and 3.33%, respectively, for specimens with 80% and 90% compactness, indicating a difference of over 1%. Furthermore, the impacts of different compactness levels on the crack ratio stabilize as the number of dry-wet cycles increases, primarily due to the self-expansive nature of the soil samples. Figure 8b demonstrates the variation curves of the total crack counts of different compacted expansive soil specimens with the number of dry-wet cycles. Unlike the crack ratio, compactness significantly influences the total crack count. In comparison to the specimen with 80% compactness, taken as the reference value, increasing compactness by 10% results in a reduction in the total crack count of moderately expansive soil samples varying from 37.5% to 71.4%, and the decrease is from 36.7% to 68.8% for highly expansive soil samples. With 80% compactness, the total crack count generally increases with the number of dry-wet cycles for moderately and highly expansive soil samples. In contrast, the variation in total crack count of specimens with 90% compactness after the first and second dry-wet cycles is insignificant, indicating that increasing compactness limits the generation and extension of cracks in the expansive soil samples (Figure 8c). The total crack length of moderately expansive soil samples is minimally affected by compactness, with noticeable differences observed only after multiple dry-wet cycles. In contrast, the total crack length of highly expansive soil doubles under 80% compactness compared to 90% compactness. The number of dry-wet cycles affects the average crack width of the soil samples in relation to compactness. The crack width of the 80% compactness specimen is significantly higher than that of the 90% compactness specimen, with the former being more than 1.5 times higher. As the number of dry-wet cycles increases, the difference in average crack width between the two specimens gradually grows, indicating that decreased compactness intensifies the enhancement of the shrinkage and swelling characteristics of the expansive soil during multiple water absorption and desorption processes. Figure 8: \(Cont\).