Freezing induced deformation and hydraulic response of saturated rock

Safety assessments of deep geological repositories for high level radioactive waste require a robust quantitative and qualitative characterization of the future repository environment over time horizons up to one million years. For instance, in Germany this time-frame spans approximately ten glacial and interglacial cycles, during which ice sheet advance, permafrost formation, and thawing can repeatedly modify the thermal and hydraulic regime of the overburden from near surface to intermediate depths. For assessing radionuclide-transport during glaciation the necessary parameters for process based numerical predictions are required and have to be obtained from laboratory measurements under controlled conditions.

In this study, an improved rock volumetric deformation testing system is used to measure freezing induced deformation and hydraulic response. System and coolant effects were calibrated using an stainless steel standard. Freeze thaw tests were performed on consolidated and unconsolidated materials to obtain temperature dependent deformation behavior, water migration characteristics during freezing, and the evolution of permeability with freeze thaw cycle number.

These experimentally derived relations were then used to constrain a coupled thermos-hydraulic numerical framework implemented via a FEFLOW plug in. The model resolves liquid water and ice phase change in saturated porous media, incorporates nonlinear temperature dependent hydraulic conductivity and effective thermal properties, and represents lateral groundwater flow driven by hydraulic gradients. Permeability levels and their freeze thaw induced evolution were parameterized using the laboratory results, and the simulated stage wise redistribution of water during cooling was evaluated against the measured inlet and outlet flux signatures. The calibrated framework was applied to quantify how advective heat transport associated with lateral flow controls the timing, persistence, and spatial heterogeneity of the freezing front.