## 4 Analysis of the Salt-Insulating Effect of the Aggregate Insulating Layer  
Engineering practices have shown that the aggregate insulating layer affects the separation of water and salt migration processes in saline soil, and, as a bearing layer, the aggregate layer can also improve the overall strength of the foundation and reduce the uneven settlement. To separate the water and salt migration processes in coarse-grained saline soil subgrades and prevent the occurrence of subgrade damage, in this experiment, an aggregate insulating layer was laid to analyze the water- and salt-insulating effects. The specific laying position and thickness are shown in Figure 14. According to the analysis of the water and salt migration results for saline soil under freeze--thaw cycles, water and salt mainly accumulated at a distance of H = 28.2 cm from the bottom. Therefore, an aggregate insulating layer was placed at H = 10.0 cm to prevent the upward migration of external water replenishment. To determine a reasonable thickness of the aggregate insulating layer and provide the most economical and feasible reference for actual construction, aggregate insulating layers were laid with thicknesses of 10 cm (Figure 14a), 20 cm (Figure 14b), and 30 cm (Figure 14c). No sensors were buried within the aggregate layer, namely, no sensors were buried at H = 18.8 cm when the aggregate insulating layer thickness was 10 cm. Moreover, no sensors were buried at H = 18.8 and 28.2 cm when the aggregate insulating layer thickness was 20 cm, and no sensors were buried at H = 18.8, 28.2, and 37.6 cm when the aggregate insulating layer thickness was 30 cm. The remaining experimental settings and test conditions remained unchanged. The actual liquid level height after five freeze--thaw cycles is shown in Figure 14. When the thickness of the aggregate insulating layer was 10 cm, the liquid level height exceeded that of the aggregate insulating layer (Figure 15a). When the thickness of the aggregate insulating layer was 20 cm, the liquid level occurred only approximately 5 cm from the top of the aggregate insulating layer (Figure 15b). Figure 13: Variation curves of the liquid water content and temperature with respect to the freeze—thaw time. When the thickness of the aggregate insulating layer was 30 cm, the liquid level occurred approximately 20 cm from the top of the aggregate insulating layer (Figure 15c). The reason for these findings is the continuous filling of the aggregate insulating layer with coarse-grained saline soil, in which the gaps between the aggregate insulating layers are naturally filled, resulting in the presence of soil particles in the insulating layer and the formation of a notable matrix suction force, causing the water and salt in the lower soil to move upwards. The 10 cm aggregate insulating layer is relatively thin, and liquid water directly submerged the insulating layer under the action of the matrix suction force of the soil, continuing to migrate upwards. The 20 cm and 30 cm layers are relatively thick, and there are fewer soil particles in the aggregate gaps. Although liquid water moved upwards under the action of the matrix suction force, it did not submerge the insulating layer. The final moisture and salt contents at the different heights inside the soil column after five freeze--thaw cycles were compared with the initial values, and the experimental results are shown in Figure 16. Under the condition of an aggregate insulating layer thickness of 10 cm, water and salt still yielded two water and salt aggregation zones at H = 28.2 and 65.8 cm, similar to the water and salt aggregation effect without an aggregate insulating layer. When the thicknesses of the aggregate insulating layer were 20 and 30 cm, because the rising height of the liquid water did not exceed the aggregate insulating layer, the soil did not yield a salt accumulation zone at H = 28.2 cm, and only salt accumulation occurred at H = 65.8 cm. Figure 14: Location and thickness of the aggregate insulating layer.