On the role of plastic flow in THM simulations capturing glacial cycle effects on German nuclear waste repositories in clay rock

To properly assess the present and future conditions of potential nuclear waste repository sites, understanding their evolution in the past is mandatory. Here, glaciation cycles strongly affect the long-term thermo-hydro-mechanical (THM) evolution of the geosystem. The AREHS project studies the effects of time-dependent boundary conditions on the long-term evolution of large-scale hydrogeological systems. The focus is on numerical modeling using the open-source multi-field finite element code OpenGeoSys with THM couplings. The impact of the glacial THM loading is taken into account using appropriate time-dependent THM boundary conditions. The generic geological model for a clay host rock formation includes almost only sedimentary rock layers. Within the scales of the physical problem, it can be assumed that the plastic flow behavior of the different sedimentary rocks shares qualitative features. Therefore, the same generic material model is used for all layers: The elasto-plastic modified Cam clay (MCC) model can describe qualitatively a range of relevant effects (dilatancy, contractancy, consolidation, normal and overconsolidation effects, etc.) with a manageable number of material parameters. The chosen approach is considered sufficient to demonstrate the effect of inelastic deformations. Comparing the results to the elastic reference case, the role of plastic flow in the context of such a long-term simulation is elaborated. Special emphasis is put on the specification of a suitable initial state: To this end, an initial simulation is carried out, where plastic flow can occur under the gravitational load. The fields for temperature, pore water pressure and effective stress are then transferred as initial values for the glacial cycle. Additional internal variables of the material model are automatically adopted. This means that the thermodynamic state is transferred in full. The simulation results are analyzed with respect to potential safety-critical parameters, such as maximum temperature, hydraulic pressure, subsidence, equivalent effective stress and strain.