## 3 Results and Analysis
### Analysis of the Water and Salt Migration Results for Saline Soil under Freeze Thaw Cycles
#### 3.2.2 Analysis of Water--Salt Changes  
After five freeze--thaw cycles, the final moisture and salt contents inside the soil column at different heights were compared with the initial values. The experimental results are shown in Figure 11, and the height of the fluorescein-labeled liquid water is shown in Figure 12. There is a significant accumulation of water (see Figure 11a) and salt (see Figure 11b) in the saline soil column at distances of approximately 28.2 and 65.8 cm from the bottom. For H = 28.2 cm, the final water content reached 20.1%, an increase of 14.1% over the initial water content of 6% (see Figure 11a), and the salt content reached 7.5%, an increase of 5.9% over the initial salt content (see Figure 11b). For H = 65.8 cm, the final moisture content reached 10.7%, an increase of 4.7% compared with the initial moisture content of 6% (see Figure 11a), and the salt content reached 3.2%, an increase of 1.6% compared with the initial salt content (see Figure 11b). However, as shown in Figures 10 and 12, the maximum migration height of fluorescein labeled liquid water was approximately 52 cm, which was greater than the height of water-salt aggregation zone 1 (H = 28.2 cm) but lower than the height of water-salt aggregation zone 2 (H = 65.8 cm). The reason for this finding is that the natural coarse-grained saline soil itself is unsaturated. Although the H = 28.2 cm area is a nonfrozen zone, the content of external replenishment water mixed with fluorescein rapidly increases under capillary action and soil matrix suction, and water mainly accumulates at 28.2 cm. During the upward migration of water, the bottom salt is also moved to a depth of 28.2 cm and more salt in the surrounding soil is dissolved, forming water-salt aggregation zone 1. After 480 h, the liquid water in the aggregation zone migrates to the cold end under the action of upper soil matrix suction and a temperature gradient, resulting in the migration height of fluorescein-labeled liquid water reaching 52 cm. H = 65.8 cm is located in the frozen zone, and its height is much greater than the migration height of fluorescein-labeled liquid water because only liquid water can be labeled, not water vapor. There is no fluorescence color at  
Figure 10: Fluorescence liquid level height change. H = 65.8 cm in Figure 12, so the changes in the water and salt contents can only be caused by the migration of water vapor. Although the larger pores of coarse-grained saline soil are not conducive to the migration of liquid water, they provide favorable conditions for the migration of water vapor. During multiple freeze--thaw cycles, water vapor migrates from bottom to top under the action of the temperature gradient. Owing to the sealing effect of the top film, water vapor accumulates at the top of the saline soil sample, and a series of freezing--condensation--crystallization processes occur as the temperature is reduced, resulting in an increase in the top moisture and salt contents. Water vapor condenses and forms liquid water on the upper part of the heat transfer plate. The condensed liquid water moves downwards under the action of gravity and dissolves more salt in the soil, forming water--salt accumulation zone 2. The variations in the liquid water content and temperature in the frozen zone (H = 65.8 cm) and nonfrozen zone (H = 28.2 cm) of the saline soil column with freeze--thaw time were analyzed under freeze--thaw cycles, as shown in Figure 13. Under freeze--thaw cycles, the liquid water content in the frozen and unfrozen zones of the same part within the saline soil column exhibited a periodic variation pattern with temperature, and there was essentially no hysteresis phenomenon. In the freezing zone at H = 65.8 cm (Figure 13a), when the temperature dropped below the freezing temperature, the water began to freeze and salt crystallized. In the crystallization process, a large amount of liquid water was consumed, resulting in a corresponding decrease in the liquid water content.