# Research on Mechanical Behavior of Geogrid-Soil Interface Under Rainfall Infiltration
## 3 Results and Discussion
### Effect of Shear Rates on Geogrid-Soil Interface Characteristics  
Figure 7 shows the shear stress-shear displacement curves for different working conditions at shear rates of 1.0 mm/min, 1.5 mm/min, and 2.0 mm/min, respectively. As can be seen from Figure: 1. Comparing Figure 7a-c, it can be found that the shear rate basically does not affect the shear strength of unreinforced sand. 2. Comparing Figure 7d-f, it can be found that the shear strength of the geogrid-reinforced soil decreases with the increase in the shear rate, which is due to the fact that the geogrid acts through a certain amount of deformation, and the larger the shear rate, the faster the deformation of soil, so that the geogrid is too late to give full play to its role, which leads to the reduction in the interface shear strength.  
Figure 7: Shear stress–shear displacement curves at different shear rates. (**a**) S-T0-A; (**b**) S-T0-B; (**c**) S-T0-C; (**d**) J-T10-A; (**e**) J-T10-B; (**f**) J-T10-C.  
Figure 8 shows the comparison of the shear stress-shear displacement curves for different working conditions at a normal stress of 100 kPa and shear rates of 1.0 mm/min, 1.2 mm/min, and 1.5 mm/min. As can be seen from the figure, the peak shear stresses of the geogrid-sand are greater than those of the unreinforced sand, and the peak shear stresses of the dry sand are all greater than those of the sand infiltrated by rainfall for 10 min. Taking the shear rate as 1.0 mm/min, the peak shear stresses under three working conditions are 131.5 kPa, 48.4 kPa, and 44.1 kPa, which satisfied the above conjecture, i.e., under the conditions of the test, the effect of reinforcement is greater than that of rainfall infiltration on the shear strength. It can be seen that the reinforcement has a significant effect on the strength of the soil, and under certain working conditions, the reinforced soil is still able to maintain high strength under rainfall infiltration.  
Figure 9 shows the peak shear stress-shear displacement curves of geogrid reinforced specimens at normal stresses of 50 kPa, 100 kPa, and 150 kPa, shear rates of 1.0 mm/min, 1.5 mm/min, and 2.0 mm/min, and a rainfall infiltration time of 10 min. The data from the shear test results are fitted and analyzed, and the results are shown in Table 4.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Shear**} & \multicolumn{3}{c}{**Strength Index**} & \multirow{2}{*}{**Fitting**} \\ \cline{2-2} \cline{4-4}
**Rate/(mm-min\({}^{-1}\))** & & & **Friction** \\ \cline{2-2} \cline{4-4}
1.0 & 22.7 & 43.8 & 0.995 & y = 0.959\(\times\) + 22.7 \\
1.5 & 16.7 & 42.0 & 0.999 & y = 0.899\(\times\) 16.7 \\
2.0 & 9.6 & 40.0 & 0.994 & y = 0.839\(\times\) + 9.6 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Strength index at different shear rates.  
Figure 8: Comparison of shear stress under different working conditions and different shear rates.  
Figure 9: Peak shear stress–shear displacement curves at different shear rates.  
As shown in Table 4, the direct shear friction characteristics of the geogrid-soil interface varied with the shear rate. The friction angles at three different shear rates are 43.8\({}^{\circ}\), 42.0\({}^{\circ}\), and 40.0\({}^{\circ}\), and the cohesions are 22.7 kPa, 16.7 kPa, and 9.6 kPa, respectively. It can be found that the 1 friction angles and cohesions both decreased with the increase in shear rates. This is because the faster the shear rate, the shorter the time of shear failure, and the soil particles do not have enough time to rearrange themselves, resulting in the weakening of friction and occlusion, and the reduced friction angle. At the same time, the faster the shear rate, the more the soil's internal water cannot be discharged in time, which leads to an increase in the pore water pressure and decreases in the effective stress, the shear strength, and the cohesion [26; 27].