Abstract

The nuclear disposal concept is based on compacted bentonite barrier placed into a borehole in the host rock. In crystalline rock, one of the issues is a non-uniform saturation of bentonite caused by inhomogeneity of the rock, typically the flow being concentrated to fractures. We solve a hydro-mechanical model problem in bentonite in the general engineering multiphysics simulation software ANSYS. The elasto-plastic transition as result of saturation is represented by a special non-linear stress- strain curve of elastic model (saturation-dependent weakening). The model is further extended to a contact-problem formulation, where the swelling is possible either as free swelling until filling the gap or as confined state after. It is a synthetic case of one horizontal fracture crossing a vertical borehole. We demonstrated that for large time period, the saturation is limited to a part of the volume, producing a non-uniform stress field in the whole volume. The stress is also controlled by the gap width.

1. Introduction

Numerical modelling is one of the tools for analysis of the processes in barriers of the spent nuclear fuel disposal concept. Our work is motivated by a potential damage of disposal barriers due to non-uniform swelling of bentonite caused by inhomogeneous hydration. In the repository, the interaction of the bentonite and the host rock (fractured granite considered) is equally important, requiring the joining of the two disciplines of soil and rock mechanics.

Recent studies deal with modelling of thermo-hydro-mechanical processes and also compare results from full scale experiments, e.g. [8, 11]. Additionally, we developed a non-linear hydro-mechanical (HM) method for solution of bentonite hydration with relatively simple implementation which can be easily-defined in a number of commonly used simulation tools. It offers an alternative to complex models of hydraulic or mechanic behaviour of bentonite, e.g. [1, 9]. It extends the previous work [10] on hydraulic-only models of non-uniform saturation process based on data from field experiments.

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