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Investigation of the hydro-mechanical behaviour of compacted bentonite/claystone mixture

Abstract : In the French concept of deep geological disposal, the underground repository is planned to be constructed in Callovo-Oxfordian (COx) claystone formation. To reduce the excavation wastes, a mixture of MX80 bentonite and excavated COx claystone in compacted blocks has been proposed as a candidate sealing/backfill material by the French National Agency for Nuclear Waste Management (ANDRA). When the repository is closed, the compacted blocks are expected to swell, filling up the technological voids, resisting the propagation of excavation-damaged zone and preventing the release of radionuclides into the biosphere. To assess the suitability of the proposed material, a laboratory programme was set up in this PhD study to characterize the hydro-mechanical behaviour of compacted bentonite/claystone mixture under different environmental conditions.Compression, swelling pressure, hydraulic conductivity and mercury intrusion porosimetry (MIP) tests were performed on the compacted MX80 bentonite/COx claystone mixtures with various bentonite fractions, dry densities and water contents for the preliminary performance assessment of the material. By considering the interaction between bentonite and claystone during hydration, the claystone void ratio and volumetric fraction in the mixture were deduced and the swelling pressure of claystone grains was indirectly determined. Moreover, two analytical methods were proposed to predict the swelling pressure and hydraulic conductivity of bentonite/claystone mixtures. Additionally, based on the pore size distribution after hydration, the swelling pressure and hydraulic conductivity of bentonite/claystone mixtures were further evaluated.Afterwards, the aeolotropic swelling behaviour of compacted MX80 bentonite/COx claystone mixture with axial/radial technological voids was experimentally determined. Results show that axial technological voids could decrease the grain orientation and reduce the aeolotropy. By contrast, radial technological voids would increase the heterogeneity and enhance the aeolotropy.In addition, the homogenization process of compacted blocks with technological voids was experimentally and theoretically evaluated. The evolutions of axial and radial swelling pressures and hydraulic conductivity over time were investigated, together with the determination of the variations of water content, dry density, suction and microstructure distribution. Particular attention was paid to the effect of boundary friction between the samples and porous stones and an analytical method was proposed to estimate the final dry density distribution of samples with technological voids.To account for the influences of the pore water in COx claystone and the cementitious solution due to concrete degradation, compacted bentonite/claystone mixtures with various bentonite natures, dry densities and technological voids were hydrated with deionised water, synthetic site water and cementitious solution and the swelling pressure, hydraulic conductivity and microstructure were determined. Results show that the synthetic site water and cementitious solution slightly decreased the swelling pressure and increased the hydraulic conductivity due to the cation exchange and montmorillonite dissolution.Finally, water flow in the unsaturated MX80 bentonite/COx claystone mixture was investigated by carrying out infiltration and water retention tests under constant-volume conditions alongside microstructure observation. The hydraulic conductivity was experimentally determined. Based on the evolving pore structure upon hydration, two-phase hydraulic conductivities were predicted and a numerical model was developed to investigate the contribution of vapour and liquid water flux
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Submitted on : Tuesday, November 16, 2021 - 5:55:13 PM
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  • HAL Id : tel-03431598, version 1

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Zhixiong Zeng. Investigation of the hydro-mechanical behaviour of compacted bentonite/claystone mixture. Mechanics of materials [physics.class-ph]. École des Ponts ParisTech, 2021. English. ⟨NNT : 2021ENPC0010⟩. ⟨tel-03431598⟩

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