## 3 Results and Analysis
### Freezing State (\(-5\)\({}^{\circ}\)C) Reinforced Soil Interface Characteristics
When the water content increases from 2% to 4.5%, the peak shear stress corresponding to the normal stress of 40 kPa, 60 kPa and 80 kPa increases by 28.75%, 10.96% and 12.15%, respectively. When the water content increased from 4.5% to 7%, the peak shear stress decreased by 1.15% and 0.7% under normal stresses of 40 kPa and 60 kPa, respectively. Under 80 kPa normal stress, the corresponding peak shear stress increased by 0.7%. When the water content increases from 4.5% to 7%, the peak shear stress changes little and is basically flat. The strength of the reinforced soil interface in the frozen state is jointly contributed by the cohesion between the soil skeleton, bite force, cementation force of the frozen pore ice, strength of the soil particles, interlocking effect of the geogrid on the soil and blocking effect of the transverse ribs [32; 33]. When the water content is 2%, the pore ice content generated in the soil is small, and its cementation effect on the soil is weak. Therefore, the peak shear stress of 2% water content is the lowest under the three normal stresses. When the water content is 4.5% and 7%, the peak shear stress is basically the same. However, the analysis of the composite mechanism of frozen soil and reinforcement [33; 34] shows that the contribution factors of shear strength are not the same when the water content is 4.5% and 7%. When the water content is 4.5%, the density of the soil increases, and the interlocking effect of the geogrid mesh on the soil particles is enhanced. The blocking effect of the transverse ribs plays a leading role in the contribution of shear stress. The cementation force of pore ice in frozen soil plays a secondary role in the contribution of shear strength. When the water content is 7%, although there is much water, unfrozen water has been converted into pore ice in the frozen state. The pore ice in frozen soil further enhances the cementation between soil particles and between reinforcement and soil. Therefore, increasing the water content does not cause the peak shear stress of the reinforced soil interface to change significantly. Figure 12: The relationship curve between the shear strength and normal stress of the reinforced soil interface under the freezing state (\(-5\)\({}^{\circ}\)C). Influence of Water Content and Temperature on the Shear Strength Index of the Reinforced Soil Interface  
To study the influence of the water content and temperature variables on the shear strength between the geogrid and coarse-grained soil interface, the peak shear stress corresponding to normal stresses of 40 kPa, 60 kPa and 80 kPa under each working condition is linearly fitted. The parameters are shown in Table 3. The shear relationship of the reinforced soil interface conforms to the Mohr-Coulomb strength criterion. The normal stress \(\sigma_{n}\) of the reinforced soil interface in the direct shear test can be calculated by the following formula:  
\[\sigma_{n}=\frac{P}{A} \tag{1}\]  
where \(P\) is the normal load of the reinforced soil composite in kN, and \(A\) is the shear box area in m\({}^{2}\). The relationship between the interface shear strength \(\tau_{f}\) and normal stress \(\sigma_{n}\) can be expressed as follows:  
\[\tau_{f}=c_{s}+\sigma_{n}\tan\varphi_{s} \tag{2}\]  
where \(c_{s}\) is the interface cohesion of the reinforced soil composite and \(\varphi_{s}\) is the interface friction angle of the reinforced soil composite. The apparent friction coefficient is a comprehensive strength parameter reflecting the friction effect of the reinforcement-soil interface, which is defined as the ratio of the peak shear stress to the normal stress of the reinforcement-soil composite. To explore the influence of the water content and temperature on the apparent friction coefficient of the reinforced soil interface, the apparent friction coefficient is used as an index to reflect the friction characteristics of the reinforced soil interface.