The homogenisation behaviour of BCV bentonite – A laboratory and numerical study

Bentonite is considered as a suitable sealing material for radioactive waste geological repositories because of its high swelling capacity and low permeability. Regardless of the construction procedure and initial form of the bentonite, i.e. blocks, pellets, powder or combinations and mixtures thereof, the placement of the bentonite barrier results in initial heterogeneities which could potentially form preferential pathways for the migration of radionuclides. This study focused on the laboratory investigation of the homogenisation of Czech BCV bentonite. The analysis comprised the performance of four sets of tests on dual-density samples composed of two layers of bentonite compacted to various dry densities. Each set of tests considered two identical samples saturated from different sides under constant volume conditions. Homogenisation was investigated through the measurement of swelling pressure on both sides of the samples and via the determination of the final density distribution in a vertical profile from the post-mortem analysis. The results confirmed a substantial degree of homogenisation, with only a small density gradient revealed by the post-mortem analysis. The density distribution profiles indicated a gradual change in the vertical direction with no distinct border between the original layers. A higher degree of homogenisation was found in the samples with a higher overall density and, consequently, a higher swelling potential. No clear effect of the direction of saturation on homogenisation was identified from the density profiles. The final swelling pressures corresponded well to swelling pressures obtained on single-density samples with density equal to the global density of the dual-density samples. The numerical modelling represented well the final swelling pressures and the swelling pressure evolution curves for both the single- and dual-density samples (including the distinct experimentally-observed swelling pressure peak when the dual-density sample was saturated from the high-density end). More accurate results were obtained while considering the porosity and the degree of saturation-dependent permeability. However, the model did not fully capture the homogenisation of the dual-density samples in terms of the dry density profiles. While the tendency towards homogenisation was predicted correctly, two distinct density zones were evident in the simulations, while the experiment revealed a gradual density distribution.