Compacted highly expansive clays are envisaged to be employed as buffers for the long-term storage of nuclear waste in deep geological repositories. Buffers are tasked with preventing any form of contamination of the environment by the radioactive matter. Their strong swelling behaviour upon contact with water, which is originated in the structure of the fabric and in the mineralogical composition, is meant to ensure the absence of leakage into the biosphere. Overall, compacted highly expansive clays present different characteristics compared to moderately expansive clays. The aim of the present thesis is to develop a suitable constitutive model for these materials and to conduct a numerical investigation of their hydro-mechanical behaviour. A new constitutive framework is formulated and implemented into the Imperial College Finite Element Program (ICFEP). It accounts for the double-porosity structure that is observed in the fabric of compacted clays. Two levels of structure are considered, the macro-structure and the micro-structure. The overall behaviour of the material is characterised as an interaction of these two levels of structure, which are assumed to be elastoplastic and not independent. The validation of the new model is discussed by means of the analysis of laboratory experiments that demonstrates, among other things, the improved simulation of soil behaviour with respect to the single-structure constitutive model available in ICFEP. The new double structure model is employed to perform a numerical investigation of the behaviour upon wetting of compacted clays observed at the small and large scale. In the former case, a series of swelling pressure tests are studied in order to verify that the model correctly reproduces the fundamental features of the swelling behaviour. In the latter case, a large scale deep geological repository is reproduced in order to assess the predictive capabilities of the model for the long-term state of the buffer.