Abstract: Theswelling deformation, crack growth and development and reduction of shear strength induced by the mineral-water interactions of expansive soils that including changes of water content and overburden loads are very important to the safety of expansive soil slopes. In addition, because of the electric charge on their mineral surface, the swelling behaviors of the expansive soils in pure water and salt solution are very different and always separated from each other, which and lack a unified theory of swelling deformation. The mineral-water interactions are explained by the isothermal water adsorption. According to the isothermal water adsorption, the increment of water volume (V_w) absorbed by montmorillonite during the swelling of expansive soils is dependent on the vertical overburden load and the surface fractality of mineral aggregates in voids. The water volume absorbed by montmorillonite is related to the surface fractal dimension and the vertical overburden pressure. The maximum swelling deformation is predicted according to the correlation of the absorbed water volume to vertical overburden pressure. The swelling deformation of expansive soils is affected by the osmotic suction in saline solutions. Based on the fractal model for the swelling deformation of expansive soils, the osmotic suction is transferred into the modified effective stress ($p^{\text{e}}$) which is distinguished from the Terzaghi’s effective stress concept. The V_w/V_m-p^e relationship is expressed using a unique curve for expansive soils in saline solutions. The unique curve of the absorbed water volume is validated by the experimental data of swelling deformation of expansive soils.