# 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
During the preparation of expansive soil samples with varying compaction levels, increased compaction results in the greater fragmentation of larger soil particles, thereby compromising the soil's structural integrity. The resultant fine particles from this fragmentation efficiently fill soil pores, thereby diminishing the sample's porosity. Microscopically, this reduction in porosity constricts the pathways for free water flow within the soil, curtailing the capacity for volumetric expansion. Macroscopically, the lowered porosity impedes water infiltration during absorption and wet cycling, thereby dampening the soil's propensity to absorb water and swell. Additionally, it diminishes the gradient in moisture content from the sample's surface to its core, leading to a more uniform distribution of tensile stresses throughout the soil mass. This uniformity in stress distribution, in turn, restricts the propagation and expansion of cracks. **(4) The impact of thickness on the characteristics of crack development**  
To investigate the effect of thickness on crack development in expansive soil, specimens of medium and highly expansive soil were prepared with thicknesses of 1 cm, 2 cm, 3 cm, and 4 cm. The crack development characteristics of the specimens were analyzed after multiple dry-wet cycles. Figure 9 illustrates the curves of the basic morphological parameters of the cracks. Figure 9 displays the crack ratio variation with the number of dry-wet cycles for different thicknesses of expansive soil specimens. The results demonstrate that thicker specimens exhibit higher crack ratios. The influence of thickness on the crack ratio is relatively small under different dry-wet cycle conditions. The crack ratio of the highly expansive soil specimens remains between 5.65% and 6.3%, while for the medium expansive soil specimens, it ranges from 4.2% to 5%. After five dry-wet cycles, the final crack ratio of the 4 cm-thick specimens is 6.3% (for the highly expansive soil) and 5% (for the medium expansive soil). In contrast, the final crack ratio of the 1 cm-thick specimens is 5.65% (for the highly expansive soil) and 4.2% (for the medium expansive soil). This represents an increase of 10.3% and 16% in crack ratio, respectively. The crack ratio represents the ratio of the crack area to the total area of the specimen. Analyses of experimental data reveal that thickness does influence the development of cracks in expansive soil; however, the exact nature and extent of this influence are not readily apparent. This phenomenon is primarily attributed to the specimen size effect and the imposed maximum thickness of 4 cm under experimental conditions. Figure 8: Variation curves of basic morphological parameters of expansive soil cracks under different compaction conditions. Figure 9b,c depict the variations of total crack length and total number of cracks with the number of dry-wet cycles for different thicknesses. The results indicate that, with increasing thickness, the total crack length decreases while the total number of cracks increases. The final total crack lengths of the 1 cm-thick specimens of medium and highly expansive soil are 341.4 mm and 634 mm, respectively. On the other hand, the final total crack lengths of the 4 cm-thick specimens of medium and highly expansive soil are 160.8 mm and 193 mm, respectively. The final total numbers of cracks in the 1 cm-thick specimens of medium and highly expansive soil are 51 and 98, respectively. In contrast, the final total numbers of cracks for the 4 cm-thick specimens of medium and highly expansive soils are 8 and 28, respectively. This phenomenon suggests that samples with smaller thickness have more divided soil blocks and larger total crack lengths during the dry-wet cycle process. Figure 9d shows the variation in average crack width with the number of dry-wet cycles for different specimen thicknesses. The average crack width reflects the extent of crack opening in expansive soil during the influence of dry-wet cycles.