Bentonite Saturation from Discrete Fractures – Numerical Model of Non-Uniform Swelling Over a Gap

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.