Hydro-chemo-mechanical characterisation of sand/bentonite mixtures: with a focus on the water and gas transport properties

According to Swiss regulations, low and intermediate level wastes (L/ILW) should be stored in deep geological repositories excavated about 300 to 400 meters below the ground surface, in suitable rock formations. The materials used for the construction of these repositories must guarantee a perfect isolation of the wastes from the surrounding geological formations under strong variations of environmental conditions. Sand Bentonite (S/B) mixtures, at low bentonite content (i.e. 20 to 30 % of bentonite), are considered as appropriate backfilling material for these underground structures. In this framework, understanding and quantifying the hydro-chemo-mechanical (HCM) phenomena governing the water and gas transport through an 80/20 sand/bentonite mixture is crucial for ensuring the safety of L/ILW repositories at both short and long terms. To this purpose, following a systematic approach, this research conducts an experimental characterisation of the mixture at macroscopic and microscopic level. At the macroscopic level, free and confined swelling tests are performed on specimens compacted to different dry densities and wetted with different pore fluids. Controlled suction confined swelling tests are carried out for determining the water retention curve, as well as the suction-swelling pressure relationship for two different densities. The water retention capacity is determined for a wide suction range. The water permeability of the mixture is measured under full saturation conditions, for a wide range of densities and using different pore fluids. The relationship between the unsaturated water permeability and the degree of saturation is determined for two target dry densities. Gas injection tests on the mixture are performed in both saturated and unsaturated conditions. Two different experimental setups are adopted to investigate the influence of the method used for preparing the specimens, as well as how the boundary conditions affect the gas permeability and the breakthrough pressure. A highly advanced triaxial cell, equipped with a laser scanning system for measuring the radial deformations, allows analysing the volumetric response of the mixture during the gas injection. The results reveal that the swelling pressure and the saturated hydraulic conductivity depend on the applied matric suction and on the pore water chemistry. In particular, at a low dry density, the mixture in contact with aqueous solutions loses most of its swelling capacity which causes strong increases of the saturated hydraulic conductivity. For a higher dry density, the swelling capacity of the mixture is better preserved and thus its hydraulic conductivity remains relatively low. Two distinct regions are identified in the water retention curve of the mixture: the first at a high suction, where the curve does not depend on the void ratio, and the second at a low suction, where the curve is significantly affected by it. Finally, an extensive microstructural characterisation is performed using the mercury intrusion porosimetry and a high-resolution scanning electron microscope. This allows relating the observed HCM behaviour at the laboratory scale to the evolution of the different pore networks during a wetting-drying cycle. The transition from double to single structure is clearly detected upon wetting. This transition justifies the generation of the swelling pressure, as well as the significant reduction of the gas permeability upon wetting. A specific relation


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