A hydro-mechanical model for unsaturated bentonite incorporating the effect of microstructure

Document available in extended abstract form only. Highly compacted swelling clays, such as bentonite, have been considered by nuclear waste agencies as buffer and backfill materials for high-level radioactive waste disposals. The important role of these materials, as an engineered barrier, is to avoid preferential pathways along the galleries and shafts of the repository and to limit the transfer of radionuclides to the environment. Despite its crucial function, the behaviour of bentonite in in-situ conditions still remains not completely comprehended. Because of the numerous uncertainties concerning the in-situ conditions, laboratory tests have been carried out in laboratories in order to help understanding the complex behaviour of bentonite. In this context, numerical modelling of the experiments should help interpreting the fundamental aspects of the observed response. In a further stage, it should also provide information on large time periods which are not accessible to experimentalists. Existing mechanical models for unsaturated materials are generally based on the Barcelona Basic Model. However, the BBM has shown some limitations in reproducing the behaviour of highly swelling clayey materials. Indeed the behaviour of these materials is relatively complex owing to the strong interactions between both the mechanical and hydraulic aspects and between the different levels of structure which characterize such materials. Two main levels of structure are generally recognized in swelling materials: a microstructural level at which swelling of clay particles occur and a macro-structural level responsible for the rearrangement of the granular-like skeleton formed by the aggregates. Accounting for this particular behaviour of highly compacted swelling clays, the new hydro-mechanical model distinguishes microstructural volumetric strains from macro-structural ones. The model hypothesizes that variations in microstructural volumetric strains are solely linked to variations in suction. A permeability dependence on the macro-porosity has then been introduced to account for the decrease in permeability as the macro-pores close. Kozeny-Carman's law was used as first approximation although other relations may be introduced. The model has been used to simulate the hydro-mechanical behaviour of bentonite during the hydration stage of the BENTOGAZ2 experiment performed at the CEA laboratory. The experiment consists in its first stage in a hydration test at constant volume. The tested material is a mixture of WH2 bentonite (70%) and sand (30%) with an initial water content of 12,4%. The sample has a diameter of 120 mm and a height of 100 mm. It is subjected to hydration from its basis (constant water pressure of 0,1 MPa). Different laboratory tests performed on the same material have allowed calibrating parameters such as the retention curve, water permeability and the elastic slope relative to suction change. Figure 1 shows the evolution of the swelling pressure and the injected water volume during the experiment. Good agreement between experimental and numerical is obtained and should encourage investigating the structure of the material. The influence of the model parameters has also been analysed and put into evidence their role in the model predictions