# Research on Mechanical Behavior of Geogrid-Soil Interface Under Rainfall Infiltration
## 3 Results and Discussion  
### Effect of Normal Stresses on Geogrid-Soil Interface Characteristics  
Taking the test results at the shear rate of 1.0 mm/min as an example, the shear stress-shear displacement relationship curves under three distinct normal stresses are presented in Figure 5.  
From Figure 5, the following observations can be made: 1. regardless of the working conditions, a higher normal stress leads to a greater peak shear stress at the geogrid-soil interface. 2. Separate shear tests were conducted for unreinforced sand and geogrid-reinforced soil. The shear stress of the unreinforced sand (Figure 5a) exhibits a continuous increase as the shear process progresses. When the shear displacement reaches approximately 10 mm, the shear stress tends to stabilize, and the specimen enters the plastic deformation stage without a prominent peak. In contrast, the relationship between shear stress and shear displacement for the geogrid-reinforced soil (Figure 5b-e) demonstrates a nonlinear rapid growth. When the shear displacement exceeds 10 mm, the shear stress-shear displacement curves under the three different normal stresses display more conspicuous peaks. Moreover, after reaching the peak, there is a downward segment followed by a tendency to level off, indicating varying degrees of shear softening. This phenomenon occurs because, during the shearing process, the sliding of soil particles induces the deformation of the geogrid, and when the shear displacement exceeds the yield point, the twisting deformation of the geogrid reaches its limit, thereby reducing the shear capacity of the tendon-soil interface. 3. Comparing Figures 5a and 5b reveals that the shear strength of geogrid-reinforced sand is notably enhanced compared to that of unreinforced sand. Additionally, comparing Figure 5b-e indicates that the peak shear stress at the geogrid-soil interface diminishes with the increase in infiltration time during rainfall infiltration, highlighting the detrimental effect of the rainfall on the strength of the soil. Overall, even when affected by rainfall infiltration, at a shear rate of 1.0 mm/min, the peak shear stress at the geogrid-soil interface remains greater than that of the unreinforced soil. Hence, it can be hypothesized that under certain working conditions, the reinforcing effect of the geogrid reinforcement will outweigh the weakening effect of rainfall infiltration on the shear strength. Figure 6 depicts the dynamic moisture content-shear displacement curves for different working conditions at normal stresses of 50 kPa, 100 kPa, and 150 kPa, respectively. From this figure, that the following is evident: 1. The dynamic moisture content of the soil, as measured by the moisture content sensors placed at two different depths, increases with the increase in the shear displacement and exhibits a gradual stabilization trend. 2. When the normal stresses are 50 kPa, 100 kPa, and 150 kPa, respectively, the final moisture content measured by moisture content sensor 1 (located 30 mm above the interface) in Figure 6a is 15.0%, 15.3%, and 15.4%, respectively, and the final moisture content measured by moisture content sensor 2 (located 10 mm above the interface) is 17.3%, 17.7%, and 18.7%, respectively. By comparing the two different depth conditions, it can be observed that the closer the moisture content sensor is to the shear surface (i.e., the deeper the rainfall infiltration depth), the higher the final moisture content of the soil. The curves obtained from other conditions are consistent, which is attributable to the limitations of the test conditions that prevent the water in the soil samples from being discharged, resulting in water accumulation within the shear box, and a slightly higher final moisture content  
Figure 5: Shear stress–shear displacement curves under different normal stresses. (**a**) S-T0-1.0; (**b**) J-T0-1.0; (**c**) J-T10-1.0; (**d**) J-T20-1.0; (**e**) J-T30-1.0. in the lower soil samples compared to the upper ones. 3.