A large deformation model for chemoelastic porous media: Bentonite clay in spent nuclear fuel disposal

Expansive clays, such as bentonite, swell when water adsorbs into their mineral structure during wetting. This characteristic property makes them ideal sealing materials for many geotechnical applications, such as geological disposal of spent nuclear fuel. For this specific application, clay is dug from natural deposits, dried, processed to grains, compacted and installed around the heat producing disposal canister where it re-saturates with the local groundwater. To understand the behaviour of the clay components in the system and to assess their performance, computer simulations with continuum level, multiphysical material models are often utilized. Commonly, the general soil models that have been built originally for capillary soils are extended to the water adsorbing swelling clays, which leads to mismatches between model and experimentally observed phenomena as well as to inconsistencies between conceptual model descriptions and their mathematical realizations. Moreover, the mechanical parts of the models often consider only small deformations, which limit their usefulness for real life applications where deformations are often large. To overcome these limitations, a new large deformation model for materials where adsorbing water induces high volume changes and which also include macroscopic scale porosity has been developed in this thesis. The starting point of the model development has been taking experimentally observed basic phenomena in the disposal environment as the basis on which a general guideline for model development is formulated. Following the guideline, a conceptual model is created. It, in turn, serves as the basis for the developed mathematical model. To obtain a physical and mathematical consistency, the principles of mechanics and thermodynamics of continua in a large deformation mathematical setting are followed. Acknowledging the adsorption of water as the primary cause of swelling and the movement of adsorbed water as a significant water transport mechanism in swelling clays are the key factors in obtaining the new conceptual view on bentonite. Besides the adsorbed water, the model describes macroscopic scale porosity which enables straightforward means to include effects of water salinity on swelling and also capillary driven water transport into the model. The strong coupling between the adsorbed water and the material mechanical behaviour formulated in the thesis consistently combines previously separately considered models: a) a swelling pressure model for clay-water mixtures without mechanical component and b) a mechanical model with a swelling component but without mechanical effect on the water movement. The obtained field equations also generalize the above results to large deformations. In overall, the model created in the thesis provides a novel large deformation model framework for chemoelastic porous media.