## 5 Conclusion  
To explore the interface characteristics of coarse-grained soil reinforced by geogrids at low temperature, this paper selects the typical coarse-grained soil in Xinjiang, China, and HDPE unidirectional geogrids as materials to carry out large-scale direct shear tests at 0 \({}^{\circ}\)C and \(-\) 5 \({}^{\circ}\)C with water contents of 2%, 4.5%, and 7% and normal stresses of 40 kPa, 60 kPa, and 80 kPa. The variation law of the interface curve characteristics and shear strength under different working conditions was obtained. The apparent friction coefficient and dilatancy mechanism of reinforced soil composites under different working conditions are analyzed and explained by the strength index and calculation formula of the reinforced soil interface. The above research results can provide scientific basis for the construction design and safety evaluation of reinforced soil engineering in frozen soil area, and also promote the popularization and application of reinforced soil technology in frozen soil area. The main conclusions are as follows:  
* The trends of the shear displacement-shear stress curve of the geogrid-coarse-grained soil interface under the nonfreezing state (0 \({}^{\circ}\)C) and freezing state (\(-\)5 \({}^{\circ}\)C) are basically the same. Under the condition of low normal stress, the curve has no obvious peak value, which is closer to the ideal elastic-plastic double linear model. In the state of high normal stress, the curves have more obvious peaks, and the curve type is closer to the elastic-strain softening type. * In the nonfreezing state (0 \({}^{\circ}\)C), there is a strong correlation between the shear strength and water content. When the water content is 4.5%, the peak shear stress under each normal stress is the largest. However, the larger water content will reduce the interface shear strength of the reinforced soil composite. The shear strength of the interface between the frozen state (\(-\)5 \({}^{\circ}\)C) and the nonfrozen state (0 \({}^{\circ}\)C) is different. When the water content is 4.5% and 7%, the peak shear stresses under different normal stresses are very close. The main sources of the difference are the cementation of pore ice in the soil skeleton under the freezing state (\(-\)5 \({}^{\circ}\)C), the improvement in the strength of the soil particles and the further interlocking effect of the geogrid on the soil. * Compared with coarse-grained soil reinforced by geogrids in the nonfreezing state (0 \({}^{\circ}\)C), the shear strength of the frozen state (\(-\)5 \({}^{\circ}\)C) is significantly improved. Under normal stresses of 40 kPa, 60 kPa and 80 kPa, when the water content is 2%, the corresponding peak shear stress increases by 19.39%, 21.71% and 11.34%, respectively. When the water content is 4.5%, the corresponding peak shear stress decreases by 29.98%, 16.17%, and 13.83%. When the water content is 7%, the corresponding peak shear stress decreases by 50.85%, 18.64%, and 21.96%. * The apparent friction coefficient between the geogrid and coarse-grained soil interface in the nonfreezing state (0 \({}^{\circ}\)C) and freezing state (\(-\)5 \({}^{\circ}\)C) decreases with increasing normal stress, and the downward trend is steep to slow. This is mainly because the soil particles are more likely to move under the action of the transverse ribs of the geogrid under the condition of low normal stress. With the decrease in temperature, the dilatancy phenomenon of the reinforced soil composite is more obvious. Data curation, Q.B. ; Writing--Original draft, Q.B. ; Investigation, Q.B., H.D. and B.W. ; Formal analysis, Q.B. ; Writing--Review and Editing, Q.B., J.L. and Y.W. ; Supervision, J.L., Y.W., H.D. and B.W. ; Conceptualization, J.L. ; Funding acquisition, J.L. and Y.W. ; Project administration, H.D. All authors have read and agreed to the published version of the manuscript. This study received funding from Xinjiang Transport Planning Survey and Design Institute Co. Ltd (Grant Nos KY2020112301; KY2021100901). Not applicable.