Chemo-mechanical coupling in saturated porous media: elastic–plastic behaviour of homoionic expansive clays

Chemically active saturated homoionic clays are considered in a two-phase framework. The solid phase contains the clay particles, absorbed water and salt. The fluid phase, or pore water, contains free water and salt. Water, and possibly salt, can transfer between the two phases. In addition free water may diffuse through the porous medium. A global understanding of the phenomena involved, namely deformation, transfer and diffusion, is proposed. Emphasis is laid on the chemo-mechanical constitutive equations in an elastic–plastic setting. Elastic chemo-mechanical coupling is introduced through a potential, in such a way that the tangent poro-elasticity matrix remains symmetric. Material parameters needed to quantify the coupling are calibrated from specific experiments available in the recent literature. The elasto-plastic behaviour aims at reproducing qualitatively and quantitatively the typical experimental responses observed on almost pure Na-Montmorillonite clays during chemical and mixed chemo-mechanical loadings. Increase of the salinity of pore water at a constant confinement stress leads to a volume decrease, so-called chemical consolidation. Subsequent exposure to a distilled water solution produces swelling: however, the latter is smaller than the chemical consolidation so that the chemical loading cycle results in a net contractancy, the amount of which increases with the confinement. In fact, plastic yielding takes place at low salinities of pore water, and when it stops, chemical preconsolidation is generated. Natural clays which contain cations of different species are considered in a companion paper, Gajo et al. [Int. J. Solids Struct., this issue], as they require to account for electro-chemo-mechanical couplings