# 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
Figure 6d shows the variation curve of the average crack width on the soil surface in samples of medium to highly expansive soil with respect to the dry-wet cycle number. The results demonstrate that the average crack width increases with an increasing number of dry-wet cycles in both the medium and highly expansive soil samples. Specifically, the average crack width of the medium expansive soil samples increases from 0.9 mm to 1.4 mm, while the average crack width of the highly expansive soil samples increases from 0.8 mm to 1.6 mm. This phenomenon indicates that cracks on the sample surface expand continuously as a result of repeated tension and healingduring the drying-shrinkage-wetting-expansion process, leading to an increasing width of surface cracks as the cracks extend deeper. Figure 6: \(Cont.\)  
Based on the evolution of the cracks' basic morphological parameters with the number of dry-wet cycles, it is evident that the crack ratio of the expansive soil tends to stabilize as the number of dry-wet cycles increases. This observation suggests that the degree of crack development in the soil gradually decreases under the influence of moisture variations following multiple dry-wet cycles. During the heating and dehydration process of the samples, the surface soil loses water at a significantly faster rate than the interior soil, resulting in notable moisture content gradients across different parts of the samples. Consequently, this lack of coordination or constraint in the deformation between the surface and interior soil generates corresponding tensile stresses within the soil. If the tensile strength provided by the soil is smaller than the tensile stress caused by moisture loss, stress concentration is released in the surrounding area of the developing crack, leading to the occurrence of tensile cracks in the expansive soil. The repeated process of cracking-closing-cracking during multiple dry-wet cycles causes the crack to continuously expand, ultimately resulting in the formation of interconnected multiple cracks. At this point, the sample has been completely fragmented into different-sized soil blocks, and the influence of dry-wet cycles on the development of cracks in the expansive soil gradually weakens. This observation aligns with the trend demonstrated by the experimental results, where the total number of cracks, total length, and width increases with the number of dry-wet cycles. **(2) The Impact of Initial Moisture Content on Crack Development Characteristics**  
Figure 7 presents the variation curves of basic crack parameters of expansive soil specimens with different initial moisture contents under different dry-wet cycling conditions. As shown in Figure 7a, the crack ratios of different initial moisture content expansive soil specimens show an increasing trend followed by stabilization with the increase in dry-wet cycling times. The crack ratio of medium-expansion soil specimens exhibits obvious differences under different initial moisture content conditions, increasing with the increase in initial moisture content. After the first dry-wet cycle, the crack ratio of medium-expansion soil specimens with an initial moisture content of 30% is only 52.9% of that with an initial moisture content of 40%. With the increase in dry-wet cycling times, the difference between the two gradually decreases. After the fifth dry-wet cycle, the crack ratio of the latter only increases by 16% compared to the former. For high-expansion soil, the influences of different initial moisture contents on the crack ratios of specimens are relatively complex. Among them, the crack ratio variation curves of high-expansion soil specimens with an initial moisture content of 30% and 35% almost overlap, while the crack ratios of specimens  
Figure 6: Variation curves of basic morphological parameters of cracks in expansive soil under different dry–wet cycle numbers. with an initial moisture content of 40% significantly increase compared to the former two.