Analytical and numerical evaluation of one-dimensional transient stress release at Wellenberg, Switzerland

Abstract

In the exploration of the Wellenberg site, central Alps, where a repository for low- and intermediate-level radioactive waste is proposed, water pressure measurements from boreholes showed significant underpressures. Pore water pressures be· low hydrostatic, in the order of 600 m (6 MPa), occurred within the Valanginian Marl formation. Several studies evaluated different scenarios that would explain this anomalous trend. Because neither regional groundwater flow nor chemical effects could explain this phenomenon, it is attributed to geomechanical factors due to glacial retreat. A one-dimensional vertical model, considering hydromechanical processes and time scales associated with the dissipation of the pressure disturbances, is proposed. The results show that the rebound hypothesis due to glacial retreat is plausible. The abnormal pressure zones are reproduced reasonably well. The time scales associated with the stress release due to the perturbation are in accordance with the one-dimensional diffusivity equation. Today about 75-84% of the effect of the perturbation has dissipated, but it may take another 20,000 to 30,000 years for 95% to 97% of the pressure to adjust to the new steady-state conditions. The implications for the Wellenberg site are that the local groundwater system is under non-equilibrium conditions due to the geological changes (i.e.; glaciations), that are still influencing the behaviour of the system. This work was used as the basis for more advanced, two- and three-dimensional analysis with a local or regional model which includes the transient effects (Rivera and Senger, 1993).