Detection of potential voids in the granulated bentonite mixture and concrete plug in the Full-scale Emplacement experiment using a parallel-wire cable as an extra-long TDR waveguide

For quality control of a concrete plug constructed for the Full-scale Emplacement (FE) experiment at the Mont Terri rock laboratory and to monitor the backfilling of granulated bentonite, a commercial parallel-wire cable designed for audio speakers was applied as an extra-long time-domain reflectometry (TDR) waveguide. A procedure for estimating the length and position of a potential air void was first established based on laboratory test results. Three cables with a length of 8.0 or 8.5 m were installed along the lined perimeter of the tunnel with a diameter of about 2.7 m. The first cable in the granulated bentonite detected the potential presence of two air voids, 0.79 and 1.17 m in length along the cable, on either tunnel shoulder. Their positions suggested that the voids could have resulted from the flow of granules on both sides of the single screw feeder used to backfill the uppermost portion of the tunnel. The second cable in the concrete plug detected a potential air void (0.98 m in length along the cable) at the tunnel crown likely to have formed when fresh concrete was cast. The third cable did not show any sign of an air void in the concrete at the tunnel crown, implying that the void detected by the second cable was likely localized. The cables were also used to monitor resin-based contact grout injection. The data before and after grout injection suggested that the grout did not seep into the granulated bentonite behind the concrete plug and sufficiently filled gaps in the vicinity of the injection lines. Although the outcome above suggests a promising possibility for observation, it is based on our interpretation of the acquired data. There is no direct observation as of yet, thus, our interpretation remains to be verified when dismantling the FE experiment.