On coupled THMC modelling of engineered barriers incorporating microstructural changes

Compacted swelling clays are often envisaged as the main component of engineered barriers for radioactive waste disposal. These barriers are subjected to thermal loading due to the heat emitted by the waste and to hydration from water coming from the adjacent rock. As a consequence of these thermohydraulic phenomena, mechanical and chemical changes arise that, in turn, may affect all other aspects of behaviour. A correct understanding and prediction of these barriers require, therefore, the performance of fully coupled thermo-hydro-mechanical and chemical (THMC) numerical analyses. A common feature of the formulations commonly used in coupled analyses is the assumption of the clay barrier as a single porosity material. However, the pore structure of expansive clays presents more than one type of voids. In the last few years, a number of tests have been performed revealing a strong influence of the pore structure on the behaviour of expansive clays. Therefore, a proper THMC modelling of a clay-based engineered barrier should incorporate the following components: a fully coupled THMC formulation that incorporates the main phenomena occurring during heating and hydration of the clay; a double structure model that takes into account the evolving microstructure of the swelling clay; chemo-mechanical model that allows for the effect of chemical variables on the mechanical behaviour of the clay. The contribution describes each of these components and their incorporation into a computer code, CODE BRIGHT. (authors)