A triple porosity hydro-mechanical model for MX-80 bentonite pellet mixtures

This paper describes the application of a triple porosity model to characterise the hydro-mechanical behaviour of MX-80 bentonite pellet mixtures. The macroscopic behaviour of the system was assumed to be fundamentally controlled by three functional levels, the microstructure (associated with the pores existing inside the clay particle aggregates), the Macrostructure 1 (which integrates the space between the aggregates inside the pellets) and the Macrostructure 2 (related to the void space between the pellets). The role played by each functional level in the overall behaviour of the system was modelled by means of an overlapping continua approach. The usual framework in the simulation of the behaviour of compacted bentonite blocks has been adapted to describe the behaviour of the microstructure and the Macrostructure 1. To model the behaviour of the Macrostructure 2, a simplified formulation of granular materials was adopted. To illustrate the capacity of the proposed approach, two hydration tests were simulated in which significantly different pellet mixtures were used: an open mixture with a dry density of 953 kg/m3, and a closed mixture with 1520 kg/m3 of dry density. Despite the difference between the two mixtures, the model allowed very satisfactory fits to be obtained using in both cases the same material parameters. This shows the consistency of the proposed conceptual framework. Since the response of the two Macrostructural levels is differentiated, the model provides a richer description of the mixture behaviour than that obtained using a double porosity model. However, the interest of the model lies not only in its descriptive richness. The differentiation of the effect of both Macrostructural levels improves the description of the time evolution of the system compared with that obtained when using a double porosity model. However, if the Macrostructure 2 is small (as occurs in the second of the two tests analysed), or there is sufficient confinement for the inter-pellet void space to be considerably reduced when the microstructure swells, double porosity models can satisfactorily characterise the performance of the bentonite pellet mixture.