## 5 Discussion  
The objective of this study was to clarify the water, heat, vapor and salt migration patterns in coarse-grained saline soil, as well as the salt-insulating effect of the aggregate insulating layer. The results of the final moisture and salt contents without the aggregate insulating layer reveal that, 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. This conclusion is consistent with the conclusion of [PERSON] et al. [22] and [PERSON] et al. [23,24] 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. [PERSON] et al. [29] and [PERSON] et al. [30] have come to similar conclusions. 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. A comparison of the final moisture and salt contents of the aggregate insulating layer with the test results without the aggregate insulating layer revealed that, when the thickness of the insulating layer reached 10 cm, the final moisture content at H = 28.2 cm was 19.0%, an increase of 13.0% over the initial moisture content. The final salt content reached 6.8%, an increase of 5.3% over the initial salt content (without an aggregate insulating layer, the moisture content increased by 14.1% and the salt content increased by 5.9%). The increases in the moisture and salt contents were smaller than those when an aggregate insulating layer was not replaced, indicating that the 10 cm aggregate insulating layer imposed a certain inhibitory effect on water and salt migration. However, owing to its small thickness, the insulating effect was not significant. When an insulating aggregate layer was laid at H = 65.8 cm, water and salt accumulated in the 10 cm, 20 cm, and 30 cm aggregate insulating layers. The final moisture and salt contents in the 10 cm thick aggregate insulating layer increased by 5.7% and 2.5%, respectively, at H = 65.8 cm. The final moisture and salt contents in the 20 cm thick aggregate insulating layer increased by 6.4% and 3.0%, respectively, at H = 65.8 cm. The final moisture and salt contents in the 30 cm thick aggregate insulating layer increased by 7.6% and 3.3%, respectively, at H = 65.8 cm. When an aggregate insulating layer was not used, the moisture content increased by 4.7% and the salt content increased by 1.6%. The increases in the moisture and salt contents were greater than those when an aggregate insulating layer was employed, and with the increasing thickness of the aggregate insulating layer, the increase in the moisture and salt contents accordingly increased. The reason for this phenomenon is that, after replacing a portion of the original coarse-grained saline soil with an aggregate insulating layer, the  
Figure 16: Variation curves of the water and salt contents along the height of the saline soil column after the installation of an aggregate insulating layer. aggregate gaps are much larger than the soil particle pores are, and a large amount of water vapor migrates upwards through the aggregate insulating layer. At the cold end, water vapor condenses and exhibits a phase change [41], resulting in higher final moisture and salt contents in the frozen zone of H = 65.8 cm in the experiment with the aggregate insulating layer. Moreover, the thicker the aggregate insulating layer is, the larger the number of aggregate gaps and the greater the migration of water vapor [40], which leads to an increase in the moisture and salt contents at H = 65.8 cm with the increasing thickness of the aggregate insulating layer.