Numerical modeling of damage and permeability interaction in rocks. Application for the radioactive waste storage in deep galleries

The relation between damage and permeability in rocks is a very important subject in industrial applications. It is for example the case of cracks around the radioactive waste storage galleries (EDZ) which can rise considerably the permeability and so make a serious problem for the sealing and the safety of these structures. The same phenomena can occur in the surrounding concrete wall of the nuclear power stations and also in the cracking of the oil-bearing rocks reservoirs. The experimental research on this subject, specially in laboratory G.3S-LMS in Ecole Polytechnique made it possible to determine the orders of damage effect on the permeability change. But a numerical modeling of these phenomena for setting a constitutive behavior law was to be done. This is the principal objective of this thesis. In this project, at first the notion of crack damage is defined. This type of damage is modeled by the disc shaped crack distribution in the 3D space. The disc's geometrical characteristics (radius, direction and opening) obey the statistical distribution laws, depending on the type of loading (compression or extension). The upper and lower limits of the characteristics are fixed according to actual observations, carried out on clay-stone (host rock selected for the realization of an underground research laboratory in Bure). In order to modeling the damage in the porous media, the double porosity concept is considered. A method of homogenization is used to simulate the flow through the network of cracks and porosity. It allows to derive the equivalent permeability of the fractured porous media. The study of the correlations between permeability and damage, obtained by this method for various values of fracture density, made it possible to obtain a relation between permeability and crack damage, for classical types of loading as simple extension and simple compression. To generalize this relation in the case of any specified triaxial loading, the crack damage is put in relation with the notion of mechanical damage, resulting from a mechanical model connecting the damage to stress and strain. Finally, the poro-mechanical behavior law with damage is implemented to the finite elements computer program ESAR-LCPC The modeling of an underground gallery proved that damage calculation by this method is feasible and allows to compute the evolution of the permeability and the variation of the flow entering the gallery as a function of this damage. (author)