Homogenization of bentonite upon saturation: Density and pressure fields

Abstract

Bentonite structures are a core element of engineered barriers for geological radioactive waste repositories, preventing radionuclide release into the biosphere. These are often assemblies of elements with different densities (bricks, pellets, powder), or may contain regions of different density due to the building process (technological voids, powder segregation). The performance of these barriers is largely dependent on this material reaching a final state of homogeneous hydraulic and mechanical properties upon water intake. In order to understand the coupled processes of swelling and water transport in bentonite at play at the laboratory scale, an experimental approach was designed to characterize in detail the evolution of the state of samples saturated under isochoric conditions. Firstly, three-dimensional fields of water content, solid density and displacement of a heterogeneous powder sample submitted to unidirectional hydration were measured using X-ray microtomography, as a function of time. These fields allowed to gain insight into the phenomenology of non-monotonic water and clay transport resulting from the heterogeneous and coupled evolutions of the permeability, mechanical and friction properties. Then, samples with controlled heterogeneities such as pellets, powder, technological voids or compacted blocks of different densities - but with similar average properties - were saturated in isochoric conditions with a large set of local stress measurements. The different swelling pressure average evolutions and variability, as well as post-mortem measurements provided rich data on the effect of the initial density heterogeneities on the saturation process and the final state as well as insights into the role of wall friction. In particular, hydration kinetics were shown to depend on a complex manner on the initial heterogeneities. Regions first in contact with water were shown to exhibit a non-monotonic hydromecanical response and the development of macroporosity.