Analysis of hydro-mechanical behaviour of compacted bentonite/sand mixture using a double structure formulation

In this paper, a double structure formulation is used to describe the behaviour of compacted bentonite/sand mixtures intended as sealing material in underground repositories for nuclear waste. The formulation accounts for the presence of two distinct porosity levels in the material as revealed in the pore size distributions determined by mercury intrusion porosimetry technique. Non-equilibrium hydraulic conditions are assumed between micro and macroporosities. A water exchange term between the two porosities is determined. Experimental data obtained in the laboratory were analysed allowing determination of the hydro-mechanical (HM) parameters used in the modelling. These parameters were then used in modelling a 1:10 scale mock-up representative of the large-scale SEALEX performance experiments. It was observed that the model reproduced well the main features related to the swelling pressure development during hydration under confined conditions. Moreover, it allowed analysing interesting information not available from direct laboratory measurements (micro and macroporosities and suctions). The analysis is then up-scaled to the description of large-scale SEAling EXperiment (SEALEX). A challenging task consisted in predictions of field observations using the same hydraulic and mechanical parameters employed in the modelling of the small-scale laboratory tests. The model was able to predict correctly the global HM behaviour of the bentonite-based seal considering the uncertainties and heterogeneities associated with the fabrication and installation processes of the bentonite-based blocks.