Studies on the mechanical evolution of compacted bentonite subjected to environmental actions

Deep geological disposal is currently the most feasible option for the long-term isolation of radioactive waste, consisting in emplacing the waste into tunnels or drifts excavated at great depths in suitable geological formations. The use of bentonite, a highly expansive clay, is considered in many repository designs as backfill and sealing material because of its favorable properties. Throughout the lifetime of a repository, the bentonite will be subjected to a series of environmental actions, mainly heating, as a result of the decaying radioactivity of the waste, and hydration from the groundwater flow of the surrounding rock. This thesis aims at advancing the understanding of the mechanical behaviour of bentonites subjected to environmental actions by means of a coupled hydro-mechanical modelling framework. The stress path dependent response of bentonite is studied on the basis of new experimental data. It appears that the virgin compression behaviour at saturated states is independent on the stress path followed to saturate the material and on the initial state of the bentonite. The water retention mechanisms of bentonite are analysed with a focus on the water adsorption as interpreted from X-ray measurements. In line with these analyses a water retention model is formulated using an explicit distinction between adsorbed water and free water. All these features lead to the use of a two-way coupled hydro-mechanical constitutive model for interpreting the mechanical response of bentonite subjected to hydration. In order to analyse multiphysical processes in geological media, the model has been extendend to non-isothermal conditions and implemented in the computer code LAGAMINE. The suitability of the proposed formulation has been validated simulating a full scale in situ test that operated for 18 years. Good predictions of total stresses, dry density and water content are achieved with the model, providing also an insight on the causes of the final heterogeneous state of the bentonite barrier. Finally, a numerical analysis of a bentonite buffer based on the Swiss concept is presented, where the nuclear waste is emplaced on pedestals of compacted bentonite blocks and the remaining space between the tunnel and the canister is backfilled with grains of high compacted bentonite. The model predictions indicate that the bentonite blocks and grains tend to homogenise as the buffer reaches full saturation, even if significant segregation of the granular bentonite occurs during emplacement. Overall, this thesis offers new insights on the role of hydro-mechanical couplings in the mechanical behaviour of bentonite buffers.