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

To assess the present and future conditions of potential deep geological repository sites, understanding their evolution in the past is mandatory. Here, glaciation cycles strongly affected the long-term thermo-hydro-mechanical (THM) evolution of the geosystem.

The AREHS project studies the effects of time-dependent boundary conditions (BCs) 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 complex time-dependent THM BCs. A generic geological model for a clay host rock formation including predominantly sedimentary rock layers is applied. Assuming the plastic flow behavior of the sedimentary rocks to share qualitative features, the same generic material model is used for all layers: The elasto-plastic modified Cam clay model can describe qualitatively a range of relevant effects (dilatancy, contractancy, consolidation etc.) with few material parameters. Special emphasis is put on the specification of a suitable initial state: To this end, an initial simulation is carried out, where a reasonable plastic pre-consolidation is adjusted. Then, the thermodynamic state is transferred in full to the subsequent simulation of two glacial cycles. As a main result, the glacial cycles lead to persistent deviations in the subsurface, e. g. long-term pressure anomalies due to THM coupling. However, under the chosen assumptions, only the first glacial cycle leads to pronounced (contractant) plastic flow whereas the second cycle merely shows elastic rock behavior.

Funding: BASE Grant No. 4719F10402 (AREHS project)