An appropriate understanding of the interaction and effects of thermal, hydraulic, mechanical and chemical (THMC) phenomena occurring in compacted swelling clay is greatly assisted by the development and use of coupled theoretical formulations. In this contribution one example of such a formulation is presented. Special attention is given to phenomena likely to be encountered in clay barriers and seals used as part of containment systems of nuclear waste. The types of processes considered in the chemical formulation include hydrolysis, complex formation, oxidation/reduction reactions, acid/base reactions, precipitation/dissolution of minerals and cation exchange. Both kinetically controlled and equilibrium-controlled reactions have been incorporated. The coupling between geochemical variables and mechanical behaviour is ensured by a chemomechanical constitutive law that is described in this paper. The formulation has been implemented in a numerical code. An application is presented concerning the performance of a small scale test simulating high-level nuclear waste repository conditions. Also numerical simulations of triaxial and oedometer tests are presentedwhere the samples were subjected to a sequence of mechanical and chemical actions.

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