(a) Location of the Mont Terri URL in Switzerland. (b) Cross-section of the motorway tunnel of Mont Terri, whose security gallery was used for excavating the URL. (c) Location of the boreholes sampled in this study. The circles indicate the position of the boreholes opening and the angles indicate the boreholes orientation compared to the ground (a negative angle means a descending borehole). The borehole used in the experiment is BRC-3. Modified from Swisstopo26.