Characterizing groundwater flow in the fractured rock at Äspö, Sweden

The underground Hard Rock Laboratory (HRL) at Äspö, Sweden, is located in granitic rock and dedicated to investigations concerning deep geological radioactive waste disposal. Several in-situ experiments have been performed in the HRL with respect to geotechnical barriers which are intended to protect the waste canisters against the prevailing groundwater. Among them are the recent Buffer-Rock Interaction Experiment (BRIE) and, on a much larger scale, the long-term Prototype Repository (PR) experiment. Modelling the surrounding groundwater flow systems has been performed with the code \(\hbox {d}^{3}\hbox {f}\) using an approach where large fractures are represented discretely in a model while the remaining set of smaller fractures—also called “background fractures”—is assumed to act like an additional homogeneous continuum. It has been firstly applied to the BRIE in a cube-like domain of 40 m side length. Calibration of the model resulted in a considerable increase of matrix permeability due to the influence of the background fractures. To check the validity of the approach the calibrated data for the BRIE were applied to a model for the much larger PR which is also located in the HRL but at quite some distance from the BRIE. Only moderate modifications of the initially used permeabilities sufficed to fit the numerous outflow data for the PR-tunnel as well as for the six “deposition boreholes”. The chosen approach for the BRIE can thus be considered to be successfully transferred to the PR building considerable confidence in the conceptual approach.