Automated thermo-hydro-mechanical simulations capturing glacial cycle effects on nuclear waste repositories in clay rock

Abstract

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 studied the effects of changing boundary conditions (BCs) on the long-term evolution of large-scale hydrogeological systems. The focus is on numerical modeling the far-field using the open-source multi-field finite element code OpenGeoSys with full THM coupling. The impact of the glacial THM loading is taken into account using complex time-dependent THM BCs. In the present study, a generic geological model for a clay host rock formation including predominantly sedimentary rock layers is applied. The elasto-plastic behavior of all the layers is described with the Modified Cam clay model. Thus, a range of relevant effects (dilatancy, contractancy, consolidation etc.) can be considered 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 2D 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. Large glacial over- and underpressure cells disturb the in-situ hydraulic gradients and alter the flow fields around the DGR. As the host rock horizons represent aquitard layers they prevent periglacial circulation flow. No dilatant deformation was observed in this study. Contractant plastic deformation in combination with the HM coupling plays an important role as it significantly increases pore pressure peaks during glacial transit.