Water content evolution in the Opalinus Clay: pre- and post-backfilling observations from the lined and unlined cold sections in the Full-scale Emplacement (FE) experiment

Commercial moisture probes were installed in the unlined and lined cold sections in the Full-scale Emplacement (FE) experiment. A grout-based installation method was developed to ensure a good coupling between the probe casing and the rock and shotcrete. For converting the raw output to volumetric water content (VWC) in the Opalinus Clay and shotcrete lining, calibrations were established. In the unlined section, distinct desaturation was observed to a depth of ∼0.55 m likely due to the high air mobility in the excavation-damaged zone (EDZ) and air intake/exhaust cycles. Before backfilling, the measured VWC followed the seasonal variation in the ambient air. The amplitude of the variation decreased with increasing distance from the tunnel wall. After the unlined section was backfilled with granulated bentonite, seasonal fluctuation became insignificant and the VWC in the desaturated zone started to increase monotonically. In the lined section, the lining functioned as a ‘hydraulic barrier’ for sufficiently isolating the EDZ in the rock from the ambient air. Desaturation thus only occurred within the lining, and the Opalinus Clay behind remained saturated. Nearly all probes have been functional for ∼13 years, proving that they are a reliable cost-efficient tool for clay rock.