Description of the thesis topic

A large proportion of the earth's surface formations are composed of clay materials, and there are many environmental and/or geo-engineering issues based on their particular properties (low permeability, high specific surface area, high retention). Among their possible uses, clay materials are used as barriers for waste storage; this requires their intrinsic transport properties (ionic, hydraulic) to be estimated. However, these properties are generally the result of multiphysical couplings, which makes it difficult to determine them without very long-term experiments (e.g. through-diffusion experiments) or complex numerical simulations. However, all these experimental and numerical difficulties are even greater in the case of clay materials that are partially saturated with water, conditions that are expected, for example, in the storage of long-lived high and medium-level radioactive waste for a hundred thousand years.
Following an initial series of experiments, the geophysical measurement approach emerged as a new experimental perspective for understanding the singular behaviour of water and solute diffusion in partially water-saturated conditions in the porous media investigated. The preliminary results obtained in 2024 showed that it would be possible to use electrical conductivity measurements as a proxy for estimating diffusion coefficient values in clay media. As these measurements are almost instantaneous (a few minutes), they could be applied to many samples in parallel to better characterise the diversity of clay media. However, before being deployed on a larger scale, this approach needs to be validated experimentally on several types of clay materials and under different conditions. Finally, this experimental approach would benefit from being coupled with the adaptation of an existing numerical code (e.g., CRUNCH Clay) to partial saturation conditions, in order to reproduce and explain in a more mechanistic way the intrinsic couplings that occur in clay media that are partially saturated with water.
The aim of this thesis project is therefore to implement a research programme that combines an experimental and numerical approach to studying transport properties in clay media. This project will be made possible by the co-supervision of the thesis work by three experts in the field: Sébastien Savoye for the ion diffusion experiments (CEA), Christophe Tournassat for the numerical modelling of coupled processes (ISTO) and Damien Jougnot for the development of geophysical tools and models under conditions of partial saturation (METIS).

Work Context

Thesis funded by the CNRS MITI (Mission pour les Initiatives Transverses et Interdisciplinaires) - GeoDiff project: 'Use of GEOphysics to determine DIFFusion properties in unsaturated clay media: experimental and numerical approach'.