Physicochemical approach to evaluating the swelling pressure of expansive soils

Swelling pressure is an important parameter for understanding the expansive nature of the bentonites and natural clayey soils. During the wetting process, the intergranular stresses evolve due to the capillarity and adsorption of clay minerals, and swelling pressure is developed if soil deformation is constrained by the boundary conditions. Strong physicochemical interactions between clay particles and water molecules prevail in the development of intergranular stresses in expansive soils. In this paper, an empirical approach considering Donnan's osmotic effect was proposed to calculate the change in swelling pressure of expansive soils infiltrated by salt solutions with different concentrations. In the model, the intergranular stress addresses the physicochemical interactions between the minerals of clay and saturating fluids with ion concentration gradients induced by fixed negative surface charges. The differential volumetric strain assuming nonlinear elasticity identifies two mechanisms for the variation of the swelling pressure with the concentration of salt solution. The Donnan osmotic pressure decreases as the concentration increases, and the bulk modulus decreases as the concentration increases. The experimental results show that the variation of the Donnan osmotic pressure dominants the evolution of the swelling pressure with the salt solution concentration for a given soil. The comparison between the simulations and the available experimental data in the literature illustrates well the prediction of the swelling pressure of expansive soils by using the proposed model.