Modelling of the thermomechanical behaviour of saturated clays: application to the radioactive wastes disposal

During the waste disposal of containers, the clay barriers of backfill and the confining medium, which is essentially composed of clay, are submitted to heavy thermal stresses which induce volume change and can result in material failure. The clay, composed of solid skeleton, adsorbed water, and free water, is submitted to physico-chemical interactions which influence its thermomechanical behaviour, itself quits different from granular media such as sand. The principal factor responsible for this response is the effect of temperature on the clays water. Thus, the loss of special structure of adsorbed water and the increase in thickness of the diffused double-layer provoke microstructural rearrangement mechanisms of particles. Those mechanisms are strongly correlated with the mechanical state of material. When it is highly over-consolidated, an irreversible swelling occurs during thermal cycle, accompanied by a breaking up of the particles and a permanent expansion of meso-pores. The greater the OCR, the more important the thermal swelling. When the material is normally consolidated, the particles settle during heating under the external stress, which results in a denser rearrangement of the material. With a slight over-consolidated material, all the intermediate stages between the above mechanisms can be reached. However, cooling produces only a weak reversible compression characterising the thermal contraction of the components. Those microscopic phenomena have been used to elaborate a macroscopic thermomechanical model based on the Cam-Clay and the Hujeux Models. The model formulation includes a thermal softening, on one hand, by the reduction of the mechanical yield surface fc and the translation of the thermal yield surface fT (PTL), and, on the other hand, an irreversible thermal expansive volumetric strain. This approach of the problem was tested along various thermomechanical paths and especially on the laboratory tests, on the expansive and non expansive clays. (author)