## 3 Results and Discussions
### Discussions  
Based on the above analyses, the mechanism of swelling difference of different expansive soil layers can be explained as follows. As shown in Figure 8, during the saturation process of water (de-ionized water) injection from top to bottom, the divalent cations (Ca\({}^{2+}\), Mg\({}^{2+}\)) in the surface soil layer migrate downward along with water molecules and aggregate around clay particles, eventually form the electrical environment. In this process, the cation exchange with expansive soil occurs where the bivalent cations (Ca\({}^{2+}\), Mg\({}^{2+}\)) enter into the DDL of clay particles and interlayer of montmorillonite crystals, resulting in the modification effect to restrain the swelling potential of the lower expansive soil layer. On the other hand, the hydrated exchangeable cations adhere to soil particles and form the adsorbed water film surrounding the aggregates [11], which produces the crystalline swelling and the diffuse double-layer swelling [15; 27; 28]. Followed by the swelling of aggregates, this induces the collapse and rebuilt of soil skeleton at the expense of porosity which decreases in volume under the confined condition [21; 29]. After the cations further migrate downward and aggregate, the aggregates continue to swell and fill the micropores [29]. As high concentration of solution will cause the reduction in the DDL thickness and consequently a decrease of the diffuse double-layer swelling based on the DDL theory [15; 30; 31; 32]. Thus water molecules tend to adsorb in the interlayer space rather than in the micropores in the case of the confined condition [33]. Moreover, with the increase of concentration of the migrating and aggregating cations, the adsorbed water, particularly the inner water molecules layer surrounding the surface of aggregates, may be in the state of solid water without movement and exchange under the influence of Coulumbian electrical charges [8; 34]. Therefore, the water molecules are hard to penetrate the adsorbed water film surrounding the aggregates into the interlayer space, resulting in the inhibition effect on the swelling of the lower expansive soil layer. On the contrary, the swelling potential of surface expansive soil layer is released because of the downward migration of bivalent cations. Consequently, the swelling difference between the upper and lower expansive soil layers can be observed during the saturated process. Such a difference is mainly attributed to the joint effect of two factors that can be treated as an electric charge effect, namely (i) the modification effect, (ii) the electrical environment. In addition, the concentration of the migrating and aggregating cations increases with the thickness of overlying expansive soil layer, and so the swelling potential of the middle expansive soil layer is higher than that of the bottom expansive soil layer. In the literature reported by [PERSON] and [PERSON] [7; 8], the reason why the 1.2 m thick soil layer can completely restrain the swelling of the lower black cotton soil is that the electric charge effect can completely balance its swelling pressure, the water molecules only accumulate in micropores rather than entering into the soil particles and montmorillonite interlayer, and thus it will not lead to swelling deformation. Based on the above analyses, the electric charge effect on expansive soil provides helpful insights into the swelling behavior of expansive soil under seepage. For the subgrade, foundation and cross-drainage structures on expansive soil areas, the damage to structures caused due to volumetric changes occurring through the active zone of expansive soil has always been of concern to civil engineers [35]. Conservative design of a structure on expansive soil must consider the maximum amount of heave that can occur in the lifetime of the structure [36]. In order to predict the maximum heave of expansive soil, it is necessary to define the largest active zone of expansive soil that can be wetted and determine the properties of expansive soil in that zone [36].