Influence of Hydromechanical Couplings on the Resaturation of Engineered Barriers
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ABSTRACT This paper presents a fully coupled model for unsaturated deformable materials like swelling clays of engineered barriers. A thermodynamic framework is adopted which allows to take into account the influence of the mechanical variables (stress and strain tensors) upon the hydraulic part of the state equations (suction - water content relation). So, a fully coupled behaviour formulation is stated which, when combined with phenomenological relations, makes it possible to study the influence of the mechanical state of the barrier on the kinetics of its resaturation. INTRODUCTION Many projects of underground repositories for high level radioactive waste involve an engineered clay barrier placed between the waste canister and the surrounding rock. Due to its hydro-mechanical coupling properties, this barrier swells when hydrated, which provides a good sealing of all technological gaps and a good watertightness. Beside this hydro-mechanical function, the barrier also plays an important kinetic role during the repository's life, the first of which is towards resaturation: a low water permeability clay is needed to provide a slow resaturation of the barrier. Finally, the barrier material is asked for two different types of «< qualities >>:hydrodynamic (slow resaturation) and mechanical (good swelling and sealing of gaps). But the interaction and eventual incompatibility between them is scarcely studied. The aim of this paper is a first attempt to evaluate a possible influence of the mechanical properties on the resaturation time. HYDRO-MECHANICAL MODELLING OF UNSATURATED POROUS MEDIA Let us consider a porous medium containing three interstitial constituents: a liquid phase (I, namely: water), its vapour (v) and another gas (a), assumed non reactive (occluded air before resaturation). In our description of an unsaturated porous medium, the gas mixture (vapour + inert gas) is assumed ideal and the vapour in equilibrium with its liquid phase - following Kelvin's law. Considering the previous assumptions, the mass conservation of these interstitial fluids reads: - (pi0i)
-div(piOivi ) + mi
for i = 1,v or a (without summation)
(I)
where 0. stands for the fraction of the volume occupied by fluid (i), v, its velocity, pi its density and m,° its mass variation due to phase change or chemical reaction by unit of global volume. As a consequence of its non reactive character, the local mass variation of the inert gas (g) is only due to transport (m,=0). On the contrary, the exchange of matter due to phase change between the liquid phase (I) and its vapour (v) implies: m, 0= - mv0 . The generalisation of Darcy's fluid conduction law to unsaturated porous media is widely accepted, as far as their saturation degree remains high enough (more than 40%, which is the case for engineered barriers). Which means that, neglecting the couplings between different interstitial constituents in the conduction laws, we assume each fluid phase velocity proportional to its partial pressure gradient as follows: 375 Mat. Res. Soc. Symp. Proc.
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