# 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  
**(1) The Influence of Dry-Wet Cycle Number on Crack Development Characteristics**  
Expansive soil exhibits significant shrinkage and swelling effects during repeated cycles of water absorption and loss, resulting in noticeable volume changes and the continuous extension of cracks on the soil surface due to uneven deformation. For this experiment, a total of five dry-wet cycles were performed to examine the changes in fundamental crack characteristics in samples of moderately to highly expansive soil. Taking the crack development characteristics of samples of medium to highly expansive soil with 35% initial moisture content and an 80% compaction degree as examples, the influence of the dry-wet cycle number on crack development characteristics in expansive soil has been analyzed. Figure 6a shows the variation curve of the ratio of crack area to sample area in samples of medium to highly expansive soil with respect to the dry-wet cycle number. The results indicate that, for different types of expansive soil, the crack ratio initially increases and then reaches stability with an increasing number of dry-wet cycles. After the first cycle, the crack ratios for the samples of medium and highly expansive soil are only 0.7% and 0.8%, respectively, indicating the absence of observable cracks or damage on the soil surface during the initial cycle. After the second cycle, the crack ratio for the samples of medium and highly expansive soil rapidly rises to 3.8% and 4.5%, respectively. Following the fifth dry-wet cycle, the crack ratios for the samples of medium expansive soils reach 4.6%, representing a 20% increase compared to the end of the second cycle. In contrast, the crack ratio for the samples of highly expansive soil exhibits a more pronounced response to the dry-wet cycle number, reaching 6.08% after the fifth cycle, which corresponds to a 35.9% increase compared to the end of the second cycle. Compared to the highly expansive soil samples, crack development in the medium expansive soil is less sensitive to the dry-wet cycle number, indicating that the influence of cycle number on crack development gradually diminishes after the third cycle. Under the same cycle conditions, the crack ratio for the highly expansive soil continues to increase in subsequent cycles, but with a decreasing rate. Since the crack ratio represents the ratio of total crack area to sample area, it can yield the same result for different combinations of crack numbers, lengths, and widths. Therefore, the variations of the total crack numbers, lengths, and average widths on the soil surface in samples of medium to highly expansive soil under different dry-wet cycle numbers were monitored and analyzed. Figure 6b shows the variation curves of the total crack numbers in medium to highly expansive soil samples with respect to the dry-wet cycle number. The results demonstrate that the surface crack numbers of the medium expansive soil samples remain stable after the first and second cycles, then increase rapidly to 44 after the fourth cycle and decrease to 35 after the fifth dry-wet cycle. In contrast, the highly expansive soil samples accumulate 32 cracks after the initial dry-wet cycle, which number steadily increases with the cycle number, reaching 60 by the end of the fifth cycle. Figure 6c presents the variation curves of the total crack lengths on the soil surface in samples of medium to highly expansive soil with respect to the dry-wet cycle number. The results show an overall increasing trend in the total length of cracks on the soil surface with the dry-wet cycle number. Specifically, the total crack length of the medium expansive soil samples rapidly increases after the third cycle, rising from 99.9 mm to 290 mm after the fifth cycle. In contrast, the total crack length of the highly expansive soil samples remains relatively constant after the second and third cycles, but continues to increase in subsequent dry-wet cycles, reaching 442 mm.