Hydro-mechanical and homogenization behaviour of GMZ bentonite pellets/block assemblies upon hydration

Bentonite pellets and block are jointly used to construct engineered barrier systems in geological repository. The hydro-mechanical behavior and homogenizations of bentonite pellets/block assemblies are of significant concern to the long-term operational safety of the repository. In this study, Gaomiaozi (GMZ) bentonite pellets were combined with block of different initial dry densities in three types of assemblies (I, II and III) and subjected to hydration under isochoric conditions. Evolutions of axial and lateral swelling pressures as well as local water contents and dry densities were measured. The pore structures of specimens after different durations of hydration were detected and analyzed with resort to X-ray μCT and mercury intrusion porosimetry (MIP) techniques. Results show that, the development modes and final values of axial and lateral swelling pressures were highly dependent on the assembly type and the initial dry density of the block. Affected by wall friction and fabric anisotropy, the final lateral swelling pressure on the pellets side was higher than that on the block side and the final axial swelling pressure was in between. No significant water infiltration rate difference was observed among the three assembly types, due to water availability was limited by a thin layer of bentonite gels initially formed near to the specimen bottom and the inter-pellet pores were closed gradually by the swelling pellets. For all the three assembly types, after an initial hydration stage (> 72 h), the pellets zone was compressed by the swelling block zone and the pellets/block interface tended to bend/move towards the pellets side, leading to a rearrangement of pellets, closing of the inter-pellet pores, healing of the interface and thus homogenization of the specimen. The degree of homogenization was evaluated quantitatively to decrease with increasing hydration time using a relatively porosity homogenization index (RPHI). However, the residual heterogeneity still remained even after full saturation, indicating the homogenization will persist for a long term.