# 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
This phenomenon indicates that the influence of initial moisture content on the crack ratio of expansive soil under multiple dry-wet cycling conditions is affected by its own expansibility. For medium-expansion soil, an increase in initial moisture content will enhance the soil's shrinkage and swelling characteristics. In contrast, for high-expansion soil, the influence of initial moisture content on its shrinkage and swelling characteristics can only be observed when the initial moisture content is above 35%. Figure 7b shows the variation curves of the total number of cracks of different initial moisture content expansive soil specimens under different dry-wet cycling conditions. The total number of cracks of both medium- and high-expansion soil specimens increases with the increase in dry-wet cycling times. Among them, the total number of cracks in medium-expansion soil specimens is not strongly correlated with the initial moisture content, and the total number of cracks is basically the same under different initial moisture content conditions. The total number of cracks in high-expansion soil specimens is more sensitive to changes in initial moisture content. After the first dry-wet cycle, the total number of cracks in high-expansion soil specimens with initial moisture contents of 30% and 40% are 23 and 36, respectively. After the fifth dry-wet cycle, the total numbers of cracks in the two specimens become 42 and 80. This phenomenon indicates that the higher the initial moisture content, the greater the total number of cracks, and the difference in the total number of cracks becomes more significant with the increase in cycling times. Figure 7: \(Cont\). Figure 7c shows the variation curve of the total length of cracks of different initial moisture content expansive soil specimens under different dry-wet cycling conditions. The total length of cracks in both medium- and high-expansion soil specimens generally increases with the increase in initial moisture content. The difference in the influence of initial moisture content on the total length of cracks in medium-expansion soil specimens gradually decreases with the increase in dry-wet cycling times. The total lengths of cracks in high-expansion soil specimens under initial moisture content conditions of 35% and 30% are relatively similar, and tend to stabilize after the second dry-wet cycle. The total length of cracks in high-expansion soil specimens with an initial moisture content of 40% is significantly higher compared to the other two groups of specimens, and continues to increase with the increase in dry-wet cycling times. Figure 7d shows the variation curve of the average width of cracks of different initial moisture content expansive soil specimens under different dry-wet cycling conditions. The results indicate that the average width of cracks in expansive soil specimens is significantly influenced by the initial moisture content. When the initial moisture content is higher than 35%, the average width of cracks is significantly increased compared to specimens with an initial moisture content of 30%. The average width of cracks in high-expansion soil specimens is more sensitive to changes in dry-wet cycling times, and shows an overall increasing trend. Under multiple dry-wet cycling conditions, the average width of cracks in medium-expansion soil specimens  
Figure 7: Variation curves of basic morphological parameters of cracks in expansive soil under different initial moisture content conditions. changes relatively less, especially for specimens with an initial moisture content of 30%, maintaining a range of 0.47-0.74 mm. Based on the evolution of basic crack parameters of medium- and high-expansion soil specimens with different initial moisture contents under multiple dry-wet cycling conditions, it can be concluded that as the initial moisture content decreases, the crack distribution of specimens tends to become simpler, and the crack ratio, quantity, and geometric dimensions decrease.