# Development Characteristics and Mechanism of Crack in Expansive Soil under Wet-Dry Cycling
## 4 Experimental Study on Mechanical Properties of Expansive Soil under Wetting-Drying Cycles
### Experimental Results and Analysis  
Figure 10 presents the variation curves depicting the cohesive strength (\(c\)) and internal friction angle (\(\varphi\)) of expansive soil specimens with different initial water contents subjected to dry-wet cycles. As the number of dry-wet cycles increases, the cohesive strength of the expansive soil demonstrates a significant nonlinear decrease. For instance, considering a moderately expansive soil specimen with an initial water content of 35%, the cohesive strength decreases from the initial value of 41.2 kPa to 20.3 kPa after the fifth cycle, representing a 50% reduction. In contrast, a strongly expansive soil specimen with the same initial water content of 35% exhibits a relatively lower cohesive strength of 5.45 kPa, which corresponds to approximately 30% of its initial value. Conversely, the internal friction angle of the expansive soil shows a relatively low sensitivity to dry-wet cycles. The internal friction angle of the moderately expansive soil specimen decreases steadily as the number of dry-wet cycles increases, reaching its minimum value of 33.5\({}^{\circ}\) after the third cycle. In comparison, the internal friction angle of the strongly expansive soil specimen remains relatively constant, ranging between 26.6\({}^{\circ}\) and 30.1\({}^{\circ}\) (Figure 10b). The influence of the initial moisture content on the strength degradation of expansive soil is of significance. To quantitatively describe the degradation law of strength parameters for varying initial moisture contents, the cohesion index of the initial state of the expansive soil sample is used as the baseline. The cohesion degradation rate index is determined by calculating the ratio of the difference between cohesion after cyclic wetting and drying to the baseline value. Figure 11 presents the variation curve of the cohesion degradation rate for strongly expansive soil under different numbers of cyclic wetting and drying and different initial moisture contents. The results indicate that increasing the initial moisture content enhances the degree of cohesion degradation for both medium and strong expansive soil samples. After five cycles of wetting and drying, the cohesion of the medium expansive soil sample with an initial moisture content of 40% decreased from 34.9 kPa to 14.6 kPa, corresponding to a decrease of 58.2%. However, under the same conditions, the cohesion of the medium expansive soil sample with an initial moisture content of 35% only decreased by 50.7%. With an increase in soil expansiveness, the influence of initial moisture content on strength degradation gradually diminishes. Based on the results of indoor wetting-drying cycle tests on expansive soil, cracks of varying numbers and widths are observed on the surfaces of medium and highly expansive soil samples during the cyclic drying-wetting process. These cracks continuously expand  
Figure 11: Shear strength decay rate curve of expansive soil under different wetting and drying cycles. Figure 10: Variation curve of shear strength index of expansive soil under different drying and wetting cycles. and connect, resulting in alterations to the soil structure and leading to strength degradation. Thus, the strength degradation exhibited by expansive soil during the wetting-drying cycle can be seen as the mechanical response of the soil to the volumetric deformation caused by drying shrinkage and wetting expansion. This section aims to establish a link between the development characteristics of cracks in expansive soil and strength indicators, providing a deeper understanding of the influence of crack evolution on soil strength under wetting-drying cycle conditions. This serves as an important foundation for using crack indicators to predict changes in soil strength, and offers practical assistance for engineering applications.