Thermo-hydro-mechanical analysis of the complete lifetime of the bentonite barrier in the FEBEX in-situ test

The FEBEX test was a large-scale demonstration project for the deep geological disposal concept of nuclear waste involving bentonite seals that lasted 18 years. One of the objectives of the test was to evaluate the capabilities of numerical methods to provide reliable predictions of the physical processes in a geological repository. Although previous studies have demonstrated the performance of current models of water, vapour and heat flow to capture the evolution of temperature and relative humidity, some uncertainties remain in the capabilities of constitutive models to predict and interpret the stress–strain behaviour of the bentonite. In this paper a recently developed thermo-hydro-mechanical (THM) elastoplastic constitutive model is used to analyse the bentonite barrier of the FEBEX test by means of the Finite Element method. The model features a two-way hydro-mechanical coupling and includes thermo-plasticity. The associated water retention formulation distinguishes the behaviour of adsorbed water and free water. The predictive capabilities of the model are tested by calibrating the material parameters on the sole basis of laboratory tests. Good predictions of total stress, dry density and water content are obtained and the analysis of the computed THM stress paths provides new insights on the causes of the final heterogeneous state of the bentonite barrier.