## 6 Conclusions  
The water and salt migration patterns in coarse-grained saline soil under freeze--thaw cycles and the insulating effect of the aggregate layers under different design parameters were investigated through periodic soil column model tests with no-pressure water replenishment, i.e., fluorescein-labeled liquid water migration and aggregate partition layer tests were performed under freeze--thaw cycles. The research results provide the following conclusions:  
1. In the freeze--thaw cycle process, the temperature inside the saline soil column periodically changes, and there are depth and hysteresis effects in terms of temperature transfer. The farther away from the cold end, the smaller the temperature change amplitude is. Moreover, the temperature change near the cold end occurs 8 h earlier than the far away change. With an increasing number of freeze--thaw cycles, the freezing depth gradually decreases at the early stage and stabilizes at the later stage, with a freezing depth of 40.5 cm at the stable stage.
2. Under freeze--thaw cycles, the amount of external water replenishment increases with decreasing temperature and decreases with increasing temperature, and the external water replenishment variations lag behind the temperature changes. The liquid water content inside the soil increases with temperature and decreases with temperature.
3. After multiple freeze--thaw cycles, two water and salt accumulation zones form inside the coarse-grained saline soil subgrade. The water and salt accumulation zone in the nonfrozen zone is due mainly to external replenishment, namely, the rapid rise of liquid water under capillary action and soil matrix suction. The water-salt accumulation zone in the frozen zone is due to the vapor--liquid phase transition of water vapor at the cold end, which condenses to form liquid water. The water and salt contents in the saline soil increase with time in the frozen zone, whereas they remain basically unchanged in the unfrozen zone. (4) During construction in seasonally frozen soil areas, the laying of a 20 cm thick aggregate insulating layer at a distance of H = 10 cm from the roadbed base should be considered to prevent the formation of large water-salt accumulation areas due to liquid water migration in saline soil roadbeds. Moreover, a drainage layer should be installed 19.2 cm below the roadbed cover layer (as shown in Figure 17) to prevent the formation of water-salt accumulation areas due to water vapor migration during condensation and liquid water generation at the cold end under multiple freeze--thaw cycles.  
Conceptualization, H.Y. and J.L.; methodology, H.Y.; investigation, B.L.; resources, J.L.; data curation, J.L.; writing--original draft preparation, H.Y.; writing--review and editing, J.L.; supervision, L.X.; project administration, C.H.; funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Xinjiang Transportation Technology Project (KY2022080901), the Xinjiang Transportation Planning Survey and Design Institute Co., Ltd., and the Science and Technology Research and Development Project (KY2022042503).  
Not applicable.  
The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding authors.  
Author [PERSON]; [PERSON]; [PERSON]; [PERSON] was employed by the Xinjiang Transportation Planning Survey and Design Institute Co., Ltd. Author [PERSON] was employed by the Xinjiang Transportation Planning Survey and Design Institute Co., Ltd. and China Gezhouba Group Municipal Engineering Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.