The Barcelona Basic Model (BBM) is an elasto-plastic model used to describe the coupled thermo-hydro-mechanical behaviors of unsaturated soil. BBM is frequently adopted to model the unique swelling behavior of bentonite, which is generally considered as the buffer material between the host rock and the canister containing high-level radioactive waste in deep geological repositories, under the changing thermal, hydraulic, mechanical and chemical conditions during the lifetime of repository. Therefore, a variety of the continuum-based numerical methods tried to add the BBM for modelling the multi barrier systems of geological repository and succeeded to describe the elasto-plastic deformation of bentonite. However, to demonstrate the entire barrier systems the host rock should be modelled simultaneously with the buffer materials, and the continuum-based methods may be limited in their ability to reflect the fracture networks in the host rock which could be the major flow channels of groundwater. This research applies BBM in 3DEC, a three-dimensional block-based discrete element method, and validates the model by comparing the change of specific volume and mean effective stress during three numerical test cases. Discontinuum-based numerical methods with BBM can be extended to describe the coupled thermo-hydro-mechanical processes of multi-barrier systems in geological repositories, with a focus on the interaction between the host rock and bentonite.

For safety assessment of a high-level radioactive waste disposal system, it is important to predict and analyze the coupled thermo-hydro-mechanical (THM) behaviors of bentonite, which is a buffer candidate material in the engineered barrier system. The Barcelona Basic Model (BBM) is a constitutive model to describe the geomechanical behaviors of partially saturated soils. Complicated tests are required to directly measure BBM parameters of bentonite. In this study, we demonstrate that probable BBM parameters can be sought by calibrating the BBM parameters to match simulation results to observed ones for two kinds of simple tests (swelling pressure test and free swelling test) instead of the complicated direct tests. In the swelling pressure test and free swelling test that were conducted by Japan Atomic Energy Agency (JAEA), water was injected into constrained and unconstrained bentonite core samples, and then swelling pressure and displacements were measured, respectively. We find optimal BBM parameters using a quasi-Newton optimization method that reproduce the observed swelling pressures and displacements in hydro-mechanical simulations. The optimal BBM parameters that are sought in the inversion process can be used to predict the THM behaviors of bentonite barriers in a high-level radioactive waste disposal system.