## 3 Results and Analysis  
### Nonfreezing State (0 \({}^{\circ}\)C)-Reinforced Soil Interface Characteristics  
Figure 8 shows the shear displacement-shear stress curve of the interface between the geogrid and coarse-grained soil with different water contents under a nonfreezing state (0 \({}^{\circ}\)C). In the initial stage of shearing, the shear stress increases with increasing shear displacement, and it is in an elastic stage. As the shear displacement increases, the slope of the curve begins to change, and the curve transitions to an elastic-plastic stage. However, after the turning point, the changes in each curve tend to be different. Figure 8 shows that the curves after obvious turning can be roughly divided into two types under different water contents and different normal stresses. In the case of low water content or small normal stress, the curve has no obvious peak and is closer to the ideal elastic-plastic double linear model. The interface shear displacement-shear stress curves between the geogrid and coarse-grained soil with normal stresses of 40 kPa, 80 kPa under 2% water content and 40 kPa under 7% water content are typical. In the case of high normal stress or water content, the curve has a more obvious peak. After the peak, as the shear displacement continues to increase, the shear stress decreases slightly. The curve type is closer to the elastic-strain softening type. In addition, during the test, it is found that when the shear displacement-shear stress curve passes through the stress peak, the friction sound caused by the interaction between soil particles or the interaction between soil particles and geogrids continues to appear on the shear surface of reinforced soil. At this time, the soil presents an overall sliding failure, and the corresponding curve also shows a slight decrease in shear stress or maintains a horizontal stable state. Figure 8 also shows that the peak shear stress of the reinforced soil interface increases with increasing normal stress under different water contents. This is because the increase in the normal stress makes the soil denser, and the mutual occlusion between different graded soil particles is further increased. Therefore, the friction between the soil and soil and between the soil and reinforcement is also greater during direct shear. When the normal stress is 40 kPa, 60 kPa and 80 kPa, the peak shear stress corresponding to 2% water content is 26.408 kPa, 34.238 kPa and 42.734 kPa, respectively. The peak shear stress corresponding to a water content of 4.5% is 32.68 kPa, 39.694 kPa and 47.632 kPa. The peak shear stresses corresponding to a water content of 7% are 26.708 kPa, 38.734 kPa and 45.85 kPa. The relationship between the shear strength and the normal stress of the reinforced soil interface under the nonfreezing state (0 \({}^{\circ}\)C) is shown in Figure 9. When the water content is 2%, the peak shear stress under different normal stresses is the lowest among the three water contents. At 4.5% (optimal) water content, the shear stress under different normal stresses is the highest among the three water contents. The stress curve of 7% water  
Figure 7: The actual temperature distribution of each point at the reinforced soil interface at different temperatures (based on the sheared end of the soil). (**a**) 0 \({}^{\circ}\)C; (**b**) \(-\)5 \({}^{\circ}\)C.  
content is between the two. At the same time, according to Figure 10, the effect of water content on the peak shear stress of the reinforced soil interface is more significant in the nonfrozen state (0 \({}^{\circ}\)C). Under normal stresses of 40 kPa, 60 kPa and 80 kPa, the peak shear stress of the reinforced soil interface corresponding to 4.5% water content is the highest. The water content has a great correlation with the interface characteristics of the reinforced soil in the nonfreezing state.