[29] revealed that the water in the pores of unsaturated frozen soil includes liquid water, water vapor, and ice. The freezing process in unsaturated soil is a liquid water--heat--vapor coupling process, and, in low-saturation soils, vapor migration may be the main mechanism of water transfer. [PERSON] et al. [30] reported that temperature gradients in unsaturated frozen soil leads to the transfer of liquid water and water vapor to the cold end. [PERSON] et al. [31] considered the combined effect of the temperature gradient, moisture gradient, and gravity potential on water heat vapor migration in frozen soil. [PERSON] and [PERSON] [32] experimentally revealed the interactions among water vapor migration, heat transfer, ice-water and water-vapor phase transitions,  
Figure 1: Multiphase compositions of frozen saline soil. as well as the deformation in unsaturated frozen soil. [PERSON] et al. [33] analyzed the water and heat transfer properties of unsaturated frozen soil on the basis of previous research. To block flow channels of upward water and salt migration in saline soil, a geotextile partition layer can be installed during roadbed construction, and geotextile materials are generally selected. However, [PERSON] et al. [34] analyzed the salt content distribution in a roadbed with a geotextile partition layer in the saline soil area of Xinjiang under natural environmental conditions and reported that the impermeability of the geotextile partition layer led to salt accumulation underneath it, resulting in a covering effect. At present, there are also new geotechnical materials, such as wicking geotextiles. [PERSON] et al. [35] validated the effectiveness of using wicking geotextiles in aggregates through indoor model experiments. [PERSON] et al. [36] studied the water migration mechanism of wicking geotextiles in compacted aggregates through indoor soil column tests and analyzed the effective working distance. However, new materials have not yet been developed for practical engineering applications. In saline soil areas, such as Xinjiang and Qinghai in China, engineering practices have revealed that aggregate insulating layers impose a certain effect on separating saline soil. In summary, scholars have investigated the migration of water and salt in saline soil under freeze--thaw cycles and have focused on this migration process by mixing Na2 SO4 or NaCl into plain soil. There is relatively little research on natural coarse-grained sulfate saline soils. There are few analysis studies of the mechanisms of the liquid--vapor--ice phase transition of water and the phase transition of the crystallization solution under freeze--thaw cycles, and it is difficult to distinguish between liquid and water vapor migration. In addition, many freeze--thaw cycle tests have been conducted in closed systems, but, owing to the significant impact of groundwater on water and salt migration in soil, closed systems cannot fully capture the heat and mass transfer processes in saline soil under the action of groundwater. Moreover, the actual construction of saltwater migration-insulating layers has not been investigated in depth, and the use of materials such as geotextiles is mostly ineffective. The selection and design parameters of the location of the aggregate insulating layer are also unclear. Therefore, in this study, a self-designed water-salt migration test device was adopted to conduct freeze--thaw cycle tests of unsaturated sulfate saline soil under pressureless water replenishment conditions. With the use of fluorescein to label liquid water, a CS655 temperature--humidity--conductivity soil three-parameter sensor, Markov bottle, and camera were employed to monitor the changes in temperature, moisture, conductivity, external water replenishment, and fluorescein-labeled liquid water levels in saline soil in real time, revealing the water--vapor--salt migration trend in coarse-grained saline soil subgrades under multiple freeze--thaw cycles.