Tunnels and underground rock laboratories offer the opportunity to study the properties of the rocks of interest at larger scale compared to the deep boreholes. It is emphasised that the tunnels studied are constructed in areas of much stronger tectonic overprint than in the siting regions for the deep geological repository. This applies also to the Mont Terri rock laboratory, situated in the Folded Jura in Western Switzerland.

Gautschi (2001) and earlier literature cited therein screened the hydro­geological observations in the Opalinus Clay in five railway tunnels and five motorway tunnels. The overburden thickness at the locations in the Folded Jura ranges from 100 to 800 m. The level of mapped details in the available documentation is variable. Despite the complex tectonics, comprising numerous faults in a water-saturated environment, only two damp patches and three measurable water inflows are reported for a total of 6'600 m of tunnel length. All these indications of water flow are from sections with low overburden thickness (less than 200 m) and are either associated with a calcareous – sandy facies not observed in the siting regions in Northern Switzerland or with faults. The detection limit for visual identification of damp patches along faults in ventilated tunnels is estimated to a transmissivity of 10^-9 m²/s.

In the framework of the construction of the new Belchen motorway tunnel in the Folded Jura, pulse tests in two boreholes indicated rock mass transmissivities ranging from 1.3 × 10^-9 to 4.3 × 10^-11 m²/s (Renz et al. 2019, Ziegler et al. 2022). These data refer to intervals with tectonic faults and are probably affected by the excavation damage zone of the large tunnel diameter.

Becker & Vogt (2020) studied the occurrence of so-called 'wet spots' in the Mont Terri rock laboratory, where overburden thickness above the tunnels in the Opalinus Clay ranges between 220 and 320 m. The term wet spot is used for damp patches observed on tunnel walls, dripping points or water inflows (also termed seepage waters). The study of Becker & Vogt (2020) focused on the BBB-3 borehole and provided evidence of localised flow along features with a maximum thickness of a few centimetres and local hydraulic conductivities in the range of 10^-9 – 10^-11 m/s. Hydraulic cross-borehole connections were observed over distances of 0.8 – 1.0 m, suggesting that the wet spots are a spatially limited phenomenon. The authors consider small-scale variations of lithological properties in the transition zone from the carbonate-rich to the sandy facies in conjunction with stress redistributions due to the extensive tunnelling activities in the URL as the probable cause. Whether fracture planes, e.g. originating from the excavation damage zone, in these low clay-mineral content layers also contribute to preferential flow paths could not be clari­fied. Overall, the occurrence of wet spots can be (at least partly) explained in the context of small-scale variation in hydraulic conductivity and the activation of these systems by the tunnel construction (high hydraulic gradients, possibly disadvantageous stress situation). The profiles of natural tracers provide evidence against large flow systems active over long periods of time. Both at Mont Terri and at the neighbouring Mont Russelin, the profiles of the natural tracers can be explained by diffusion as the dominant transport process, despite the stronger tectonic overprint compared to the siting regions in Northern Switzerland (e.g. Wersin et al. 2020, Waber & Rufer 2017, Mazurek et al. 2009).