Active clays, particularly sodium smectite-rich bentonites, play a central role in a range of environmental and geotechnical engineering applications, most notably as sealing materials in landfills and deep geological repositories. Their suitability stems from their unique hydro-mechanical characteristics: exceptionally high swelling capacity, very low hydraulic conductivity, and a strong ability to retain water through both capillary and adsorption mechanisms. These properties arise from the mineralogical features of smectite, contributing to swelling, permeability reduction, and mechanical stability. Water in clays exists primarily in two forms: capillary water, retained in pore spaces, and adsorbed water, bound to mineral surfaces. Despite their distinct roles in controlling both hydraulic and mechanical responses, the current understanding and quantification of these mechanisms remain incomplete. This thesis advances the understanding, quantification, and modelling of hydro-chemo-mechanical behaviour in active clays, with a focus on sodium bentonite. The work addresses several interlinked gaps: (i) the lack of a robust macroscopic method to separate and quantify capillary and adsorbed water in compacted specimens without disturbing fabric; (ii) the absence of a composition-aware framework for comparing and predicting swelling pressure under varying salinity; (iii) the poor characterisation of water retention at lower dry densities and under saline wettingâ drying conditions, including the definition of â trueâ saturation when interlayer water density differs from bulk water; (iv) experimental uncertainty around saturation itself â saturation protocols for bentonite are difficult to validate using conventional total pressure and pore water pressure criteria because swelling pressure affects stress measurements, leaving full saturation and its confirmation insufficiently understood; and (v) the inconsistent use of â adsorbed waterâ across disciplines and its unclear geomechanical relevance. A central contribution is a thermogravimetric analysis (TGA)-based experimental protocol designed to distinguish and quantify adsorbed water in undisturbed specimens across different hydro-mechanical states, enabling physically meaningful partitioning between adsorbed and capillary water and supporting improved retention curves. Building on this, the thesis develops a semi-empirical predictive framework of swelling pressure that links mineralogical variations and salinity effects â supported by interlayer-spacing evidence â enabling more consistent cross-material interpretation. An experimental campaign is then further developed to investigate how salinity and compaction influence retention, hysteresis, saturation evolution, and adsorption mechanisms under controlled boundary conditions. Further, this thesis consolidates the experimental practice needed to obtain reliable hydro-mechanical data in highly expansive clays, detailing saturation and constant-volume testing challenges, typical sources of error, and methodological recommendations that improve reproducibility and interpretation. Finally, the thesis synthesises adsorption-scale physico-chemical mechanisms into an integrated conceptual definition tailored to geomechanics, establishing a basis for refining water retention formulations and for future developments in effective stress and constitutive modelling of active clays.