Numerical modelling of mass losses from compacted sodium montmorillonite due to extrusion and erosion within thin fractures

Compacted bentonite swells upon hydration and, under confined conditions, develops a swelling pressure that depends on its dry density. In the presence of thin fractures in the confining host rock, bentonite will tend to expand within these spaces. If furthermore, low salinity water flows along these spaces, bentonite may disaggregate due to colloid formation, and mass can be lost to the seeping water. A significant mass loss can be associated with an increase in permeability and loss of performance of compacted bentonite as an engineered barrier for radioactive waste disposal. In this context, a numerical model was developed to simulate the interaction between compacted montmorillonite, the main constituent of bentonite, and thin fractures with low salinity water flow. Extrusion and erosion were analysed accounting for wall friction. Wall friction, which was associated with shear stress exerted by colloids accumulating at the bentonite-water interface, from now on referred to as rim, was crucial to properly assessing the pseudo-equilibrium between extrusion and erosion. The model was satisfactorily validated with relevant experimental data from 20 small-scale tests including different bentonites, pellet volumes, fracture apertures, initial dry densities, slopes, and flow rates. With the proposed approach, the extrusion of montmorillonite was limited even for low fracture flow rates in accordance with experimental results. Furthermore, the model also reproduced the low erosion rates observed in narrow sloping fractures.