Advances in studies of multi-scale swelling pressure of highly compacted bentonite

The swelling pressure of highly compacted bentonite, as the engineering barrier in the deep geological repository, is affected by its multi-scale microstructure as well as the near-field coupled thermo-hydro-chemical environment during its service period. Based on the analyses of multi-scale structure of the highly compacted bentonite, a thorough review of the multi-scale swelling pressure of compacted bentonite is introduced. It is indicated that the highly compacted bentonite consists of montmorillonite tactoid, aggregate and the unit specimen at different scales. At the microscale level, the molecular dynamics simulation methods are used to simulate the hydration expansion of montmorillonite interlayers and to elucidate the micro-mechanism of the development of swelling pressure of montmorillonite. At the mesoscale level, the elastic modulus of the montmorillonite aggregates is calculated by particle coarsening, and the structural model for montmorillonite aggregates at different dry densities is established. At the macroscale level, the constant volume swelling pressure tests are conducted to reveal the development of the swelling pressure of the high-density compacted bentonite under confined conditions in the near-field environment. Currently, the researches on the swelling pressure of the highly compacted bentonite at different scales are relatively independent, and there is significant size effect on the swelling pressure among different scales. The multi-scale coevolution mechanism of swelling pressure is not yet clear. Therefore, the systematic studies of the swelling pressure characteristics of montmorillonite tactoid, aggregate and highly compacted bentonite and the establishment of a multi-scale swelling pressure model will be the focus of future researches.