Compressibility of a binary bentonite-based mixture with particular emphasis on pellet orientation

Abstract

Binary mixtures of Wyoming-type bentonite, consisting of 80 % mass high-density pellets and 20 % granular bentonite, are currently considered candidate materials for the French concept of vertical sealing systems for deep and long-term disposal of radioactive wastes. At low emplacement water contents, the hydro-mechanical behaviour is primarily controlled by contact forces between pellets, forming a coarse grain-supported structure over which granular bentonite is poured. These pellets were uniaxially compacted at elevated stresses to reach high dry densities. Subsequent pellet unloading resulted in anisotropic features due to fissuring and delamination, followed by water absorption. This study investigated compressibility changes upon loading under laterally confined conditions of a well-oriented pellet-supported structure, mimicking the setup of the in situ VSEAL 1 experiment at Tournemire (France). Two pellet orientations at the same dry density and coordination number of pellet contacts were examined in a pure pellet skeleton and a mixture to account for potential heterogeneity during pouring. A discrete element method was used to simulate the compression results with pellets represented as a clump with a heterogeneous void ratio distribution due to fissuring. These simulations were instrumental in understanding the important anisotropic deformation properties of pellet-supported structures under two distinct orientations.