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The study of argillaceous rocks is experiencing increased interest due to its potential as host rock for nuclear waste disposal facilities. Low permeability and self-sealing capabilities mitigate the risk of radioactive materials transport to the biosphere. Nevertheless, damage phenomena to the host rock need to be assessed, not only during the excavation, waste deposition, and repository sealing phases but also during the following operation, as thermal and chemical processes may affect the integrity of the repositories.
Assessing the safety and integrity of the geological seal during this thermal phase requires a deep understanding of the evolution of the permeability under thermal and mechanical solicitations. Moreover, the damage and crack propagation must be studied at scales much smaller than the repository scale. At these scales, clay rocks exhibit a complex and heterogeneous microstructure, significantly affecting macroscopic behaviour.
As a result, a multiscale approach is preferred as it considers a micromechanical description of the material with multi-physical couplings at this scale and captures the main features of clay rock macroscopic behaviour. The double-scale framework relies on replacing the material constitutive equations with the results of numerical simulations on a Representative Elementary Volume (REV), considering the microstructure heterogeneities and the constitutive behaviour of the materials at that scale.
The present work proposes a thermo-hydro-mechanical model for argillaceous rocks based on a computational homogenisation FE2 scheme. The implementation in Finite Element code Lagamine [1] is a continuation of the works from Frey [2] and van den Eijnden [3] on hydro-mechanical double-scale models for argillaceous rocks, where the thermal processes and the resulting couplings are introduced. The thermo-mechanical homogenisation is based on the work proposed by Ozdemir [4], although thermally-induced damage was not considered there.
In order to have a microstructure that is representative of porous material behaviour, not only an accurate representation of the solid components (i.e., clay matrix and mineral inclusions) but also a representation of the pore space is needed. Two pore size distributions are observed from the experimental work of Menaceur [5]. Pores in the clay matrix (smaller than 0.01μm), and pores along the mineral inclusions (median of 12μm). After calibration of the microstructure to the behaviour of COx, the model is validated with simulations at the laboratory sample scale.
The model shows that it is capable of modelling the failure process due to thermally induced over-pressurization, as well as the evolution of the microstructure under such solicitations.
| References | [1] Charlier, R. (1989). Approche unifiée de quelques problèmes non linéaires de mécanique des milieux continus par la méthode des élements finis. [1] Frey, J., Chambon, R., & Dascalu, C. (2013). A two-scale poromechanical model for cohesive rocks. Acta Geotechnica, 8, 107-124. [2] Van den Eijnden, A. P., Bésuelle, P., Chambon, R., & Collin, F. (2016). A FE2 modelling approach to hydromechanical coupling in cracking-induced localization problems. International Journal of Solids and Structures, 97, 475-488. [3] Ozdemir, I., Brekelmans, W., and Geers, M. G. (2008). FE2 computational - homogenization for the thermo-mechanical analysis of heterogeneous solids. Computer Methods in Applied Mechanics and Engineering, 198(3-4):602–613. [4] Menaceur, H., Delage, P., Tang, A. M., & Talandier, J. (2016). The status of water in swelling shales: an insight from the water retention properties of the Callovo-Oxfordian claystone. |
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| Country | France |
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