## 3 Results and Discussions  
### Swelling Deformation of Expansive Soil Layer at Different Depths  
In this study, the approach of unidirectional water injection from top to bottom was adopted to make the expansive soil saturated. The water height was controlled at 1 cm in the total testing process. When the swelling deformation was stable for 48 h, the model test was considered to be completed. After completion of model test, four cylindrical samples with 61.8 mm diameter and 20 mm height were respectively taken from the three soil layers at depths of 0-30 mm, 90-120 mm and 180-210 mm by stainless steel rings. Then, these soil samples were put into separate containers for moisture content determination by oven drying method. At the same time, the dry density could also be obtained. After that, the void ratio was determined according to the specific gravity of 2.7, then based on this to calculate the saturation. The test results obtained are presented in Figure 2.  
The moisture content, dry density, void ratio and saturation are the average of the measured values of the four samples taken from each soil layer. As shown in Figure 2, the values of saturation of the soil samples show that all soil layers achieved a saturated state during the unidirectional seepage. Unfortunately, because there is no humidity sensor embedded in the soil, the moisture content of each soil layer in the saturated process is unknown and uncontrollable. Additionally, it can be observed that the dry density of surface expansive soil layer is the minimal while the moisture content and void ratio are the maximal. As the depth of soil layer increases, the dry density increases while the moisture content and void ratio are dropping down. Furthermore, these three parameters of the lower soil layer change slightly, while those of the upper soil layer vary greatly. This phenomenon indicates that the swelling deformation of expansive soil is nonlinear with the depth of soil layer and has the apparent difference between the upper and lower soil layers. of lower soil layer, which is consistent with the conclusion of Figure 2. It is illustrated that there is the inhibition effect on the swelling of lower soil layer under saturated process. Figure 4 shows the relationship of the measured swelling deformation of different soil layers and those calculated via a loaded expansion ratio. In this section, the saturated density of each soil layer could be calculated by its average void ratio (Figure 2) and specific gravity (Table 1), so that its saturated self-weight was determined. The applied load of loaded expansion ratio is considered equivalent to the saturated self-weight of overlying soil layer. It is indicated that the loaded expansion ratio of different soil layers is different and controlled by the overburden load. Therefore, if the error of the estimation method is neglected, the calculated swelling deformation of expansive soil at different depths can be determined by the product of the corresponding loaded expansion ratio and the thickness of soil layer. Based on this, the inhibition effect of self-weight of overlying soil layer on expansive soil can also be evaluated. According to the Figs. 2 and 3, there is the inhibition effect on the swelling of expansive soil due to the self-weight of overlying soil layer. However, from Figure 4, the calculated swelling deformation of expansive soil at different depths just decreases slowly with the self-weight of overlying soil layer. Even though the thickness of overlying soil layer reaches 180 mm, the swelling deformation is reduced only 0.62 mm. This means that the self-weight of overlying soil layer only has a weak effect on the swelling of expansive soil. However, it can be observed that the measured swelling deformation of expansive soil is sharply reduced with the increase of soil depth. The swelling deformation of the surface expansive soil layer is 7 mm, and that of the bottom one is only 2 mm.