Modelling the swelling and osmotic properties of clay soils. Part I: The phenomenological approach

Clay soils with a high montmorillonite content in their mineralogical composition are characterized by swelling and osmotic properties like biological tissues and polyelectrolyte gels. These phenomena are caused by the very high specific surface (≈760 m2/g) and the negative electric charge of montmorillonite lamellae, which determine an interaction with the charge of the ions present in the pore solution. The interest in modelling the behavior of such soils is related to the evaluation of their performances as hydraulic and contaminant barriers in landfill and soil remediation applications. The theoretical approach of the thermodynamics of irreversible processes is applied to find suitable phenomenological constitutive equations, under the assumptions of a unidimensional geometry, infinitesimal strains of the solid skeleton and isothermal conditions. The approach is related to a saturated porous medium, whose voids are filled by an electrolyte solution containing an unspecified number of ions. To make the approach purely phenomenological, the parameters introduced into the constitutive equations are expected to be measured by macroscopic experimental tests, without any specification of the physical and chemical phenomena that occur at the pore scale. The constitutive equations allow the coupled transport and consolidation problem to be formulated for a clay barrier.