A numerical approach for the hydro-mechanical behaviour of bentonite seals in the context underground radioactive waste disposals

Deep geological disposals have been considered as a reasonable solution for the final management of high
and medium-activity-level nuclear waste.
The waste confinement is ensured by the host rock and/or engineered barrier. Bentonite is the main
component of this barrier due to its very low permeability in saturated conditions and its swelling potential,
namely swelling strain and swelling pressure development upon water saturation (respectively in
unconstrained and constant volume conditions).
In order to assess the long-term evolution of a geological repository, a good prediction of the hydromechanical
response of bentonite subjected to various boundary conditions is needed.
This is the main aim of BEACON project, which, among many other relevant objectives, promotes the
development of an efficient European network between the most important agencies and universities
involved in the nuclear waste disposals research. The objective of this work is to propose a new constitutive
model able to reproduce the bentonite hydro-mechanical response.
Based on advanced numerical methods, this model is implemented in the finite element code LAGAMINE.
In this work, the Barcelona Basic Model is considered for the bentonite mechanical behaviour. Pressure
dependence is taken into account for some mechanical parameters, as well as the dry density dependence
that has also to be considered given the experimental evidences which link the final swelling strain (or
stress) to the initial compaction state of the material.
In parallel, the permeability evolution is analysed. Permeability is linked to pore structure of bentonite, in
which it can be distinguished the volume inside the grains (microporosity), between the grains of clay
(macroporosity) and, in the case of pellet-mixtures, between the pellets (megaporosity). The evolution
during saturation of these porosities leads to the variation of the permeability.
The developed model has been used for the numerical modelling of experimental tests proposed in the
context of the BEACON project. The numerical results obtained are in good agreement with experimental
measurements. Especially, the non-monotonic evolution of the swelling pressure during the hydration
phase is well captured by this model, which is always a challenge for this type of problem.