Authors: Nadia Mokni* Agustín Molinero Guerra†‡ Yu-Jun Cui† Pierre Delage† Patrick Aimedieu† Michel Bornert† Anh Minh Tang†

 

Abstract

The aim of this paper is to investigate the long-term hydro-mechanical behaviour of a highly heterogeneous MX80 bentonite pellet/powder mixture (80/20 in dry mass), which is one of the candidate sealing materials in deep geological repositories.

In spite of the operational advantages related to the use of the mixture, structural heterogeneities resulting from the installation process constitute a matter of concern and require special approaches to describe adequately the material behaviour during hydration.

In this study, a double structure formulation taking into account the initial structural heterogeneity of the material as well as damage to pellets upon wetting is proposed. The formulation is applied to the modelling of a 1/10 mock-up of Sealex large-scale tests.

Hydraulic and mechanical parameters are determined from an extensive laboratory programme carried out on a single pellet of bentonite and on the pellet/powder mixture. To ensure an adequate analysis of the test, the initial heterogeneous structural distribution of the material is determined by image analysis of microfocus X-ray computed tomography observations.

The model allows the anisotropic swelling behaviour of the mixture to be satisfactorily reproduced when accounting for the spatial variation of the material’s initial porosity. In particular, the long-term (decade) hydro-mechanical behaviour of the mixture can be well described.

Detailed analysis of the modelling results demonstrates the existence of dry density gradients at the long term and their influence on swelling pressure anisotropy.

Keywords: expansive soils fabric/structure of soils numerical modelling radioactive waste disposal

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