# Development Characteristics and Mechanism of Crack in Expansive Soil under Wet-Dry Cycling
## 5 Analyzing the Mechanism of Crack Evolution in Expansive Soil under Dry-Wet Cycles  
Numerous factors affect crack development in expansive soil, as indicated by existing research. However, the relationship between surface tension generated during the water loss process and soil tensile strength is the crucial factor in determining initial cracking from a stress perspective. Early cracking in expansive soil is mainly due to surface tension failure. Tensile failure occurs in the soil when the surface tension value \(\sigma_{T}\) exceeds the soil's tensile strength value \([\sigma_{T}]\). The magnitude of tensile stress depends on the capillary suction of the soil matrix, which is influenced by factors such as soil particle size, the radius of the liquid shrinkage film, and the tension on the film. This relationship can be represented by the following equation [28]. \[\mu_{a}-\mu_{w}=2T_{s}\cos\theta/r=2T_{s}/R_{s}. \tag{3}\]  
In the equation, \(\mu_{a}-\mu_{w}\) represents the capillary suction of the matrix, \(T_{s}\) stands for the tensile stress on the liquid shrinkage film, \(r\) denotes the particle pore size, and \(R_{s}\) represents the curvature radius of the shrinkage film. When the tensile stress of the soil approaches its limit state, close to the tensile strength, the increased contact area between the soil surface and air hastens the evaporation of soil moisture. However, the moisture in the deeper layers must first travel upwards through the soil, reaching the surface of the specimen before it can evaporate. As a result, the rate at which water is lost from the soil near the bottom of the specimen is considerably slower compared to the surface soil. This disparity in water loss rates due to soil thickness inevitably leads to the formation of water content gradients, which in turn cause variations in shrinkage at different soil depths. The surface soil experiences the greatest shrinkage, while the lower soil layers experience smaller or no shrinkage. As the lower soil layers have yet to undergo shrinkage, the surface soil is constrained by these layers during the drying shrinkage process, leading to a continuous increase in tensile stress on the surface soil (shown in Stage (1) in Figure 13). Once the tensile stress \(\sigma_{T}\) surpasses the soil's maximum capability, the soil structure is damaged, and visible macroscopic tensile cracks emerge (shown in Stage (2) in Figure 13). The repeated process of cracking, closing, and cracking caused by multiple wet-dry cycles contributes to the widening and lengthening of cracks in both lateral and longitudinal directions, accelerating the contact between the bottom soil of the specimen and the air. This process also results in water absorption expansion and water loss shrinkage under cyclic wet-dry conditions. Consequently, the friction at the bottom of the specimen restricts the deformation of the bottom soil and steadily transmits this restraint, further enhancing crack development within the soil. Under the repeated influence of uneven internal expansion forces, the soil undergoes a series of cracking-closing-cracking, with cracks continuously extending and expanding, ultimately forming interconnected cracks (shown in Stage (3) in Figure 13). This aligns with the observed trend in the experiment, where the number, length, and width of cracks in expansive soil samples initially increase and then stabilizes with the number of wet-dry cycles. Simultaneously, as the number and length of cracks increase, multiple cracks intersect and create primary cracks that traverse the soil. At this stage, the tensile strength of expansive soil steadily rises with decreasing moisture content, with its tensile strength [\(\sigma_{T}\)] surpassing the tensile stress \(\sigma_{T}\). After multiple wet-dry cycles, the internal stress of the soil reaches a relatively balanced state.