## 3 Damage Characteristics
### Macroscale Damage  
Previous research has shown [33; 34] that water content significantly influences the results of direct shear tests. Therefore, consistent control of water content is essential prior to conducting direct shear tests to minimize experimental errors caused by variations in water content. The samples, after undergoing the prescribed \(N\), were humidified to achieve a water content of 18.1% (natural water content) and then placed in a thermostatically controlled and humidifying glass jar for 12 h to ensure sufficient water migration. In this study, the direct shear test was conducted according to the indoor rapid shear test method at a constant shear rate of 0.8 mm/min. The tests were performed on samples subjected to different \(N\) under varying vertical pressures of 50, 100, 200, and 300 kPa. Shearing was terminated upon observing a significant decrease in the pressure gauge reading or when the shear displacement exceeded 6 mm.  
As depicted in Figure 8a,b, with increasing \(N\), the shear strength of sandy red clay undergoes significant alterations. After five wet-dry cycles, the shear strength under varying vertical stress exhibits varying degrees of attenuation. The shear strength shows the most notable variation at a vertical stress of 50 kPa, with a peak reduction of 41.3%. Taking a vertical stress of 50 kPa as an example for analysis, after the first wet-dry cycle, the shear strength of the sample exhibited a modest decrease, with reductions of 7.5%, accounting for 18.1% of the overall reduction. During the second and third wet-dry cycles, the shear strength of the sample experienced the most substantial decreases. After these two cycles, the reductions in shear strength amounted to 18.9% and 13.3%, respectively, making up 66.4% of the total reduction. That is to say, during the first three wet-dry cycles, the decrease in shear strength accounted for 85% of the total decrease. The subsequent wet-dry cycling has a comparatively minor impact on shear strength. The effects of the fourth and fifth wet-dry cycles had a lesser influence on shear strength, resulting in a decrease in within 10%. The amplitude of shear strength variation differs under various vertical stresses, yet the overall trend of variation remains consistent.  
Overall, it is evident that the effect of wet-dry cycling on shear strength. However, additional analysis is required to identify the main cause of the reduction in shear strength. Figure 8c shows the cohesion and angle of internal friction at different values of \(N\) calculated from the shear strength at different vertical stresses. It is evident that the repetitive wet-dry cycling leads to alterations in both cohesion and the angle of internal friction. The cohesion was assessed across various \(N\) values. Five wet-dry cycles led to a 49.6% reduction in cohesion. Following the initial wet-dry cycle, cohesion diminished by 7.9%, constituting 15.6% of the overall decrease. The second and third wet-dry cycles exhibited the most pronounced decrease in specimen cohesion, with reductions of 17.5% and 21.7%, respectively, accounting for 65.8% of the total decrease. Subsequent wet-dry cycling had a diminished impact on cohesion, with the cohesion beginning to stabilize after the fourth wet-dry cycle. The internal friction angle of the samples exhibits minor influence from \(N\) throughout the entire process. Over the five cycles, the internal friction angle remains relatively stable, ranging from 23.46\({}^{\circ}\) to 24.51\({}^{\circ}\). While it shows a slight decreasing trend with minor fluctuations, the change is not significant, and the variation is substantially smaller compared to that of cohesion. Consequently, it can be inferred that the reduction in cohesion is the principal factor contributing to strength degradation.