Investigation of the emplacement dry density of granulated bentonite mixtures using dielectric, mass-balance and actively heated fiber-optic distributed temperature sensing methods

Abstract

Granulated bentonite mixture (GBM) is considered to be one of the key elements of the engineered barrier system in the Swiss concept for disposal of spent fuel/vitrified high-level waste. To demonstrate the full-scale emplacement of GBM using a prototype backfilling machine consisting of five screw feeders, an 8-m-long mock-up tunnel in a warehouse and a 50-m-long tunnel in the Mont Terri rock laboratory were backfilled. To ensure the successful emplacement of the GBM in these tests, attempts were made to quantify the dry density of the GBM using conventional mass-balance averaging as well as tools based on dielectric and heated fiber-optic methods. In the mock-up test, the dry density was measured using a set of dielectric tools installed through the tunnel wall. Heterogeneity over the tunnel cross-section was captured, with a general trend towards higher dry density near the screw feeders and lower values near the tunnel wall and crown areas. The in-situ tunnel backfilling in the Mont Terri rock laboratory was performed in twelve steps and the section-wise average dry densities along the tunnel showed a longitudinal variation. Nonetheless, both mock-up and in-situ backfilling tests showed that the overall average dry density exceeded the target value of 1,450 kg/m3. The fiber-optic method involved actively heating the fiber-optic cable installed near the GBM-shotcrete interface at the crown of the tunnel. The GBM dry density profile along the cable was estimated based on the temperature responses. The estimated dry density near the tunnel crown indicated longitudinal variations and was generally lower, which was consistent with the results of the mock-up test. The three methods provided the GBM dry density at different locations with different spatial resolutions. However, the combined results demonstrated that GBM backfilling met engineering and safety-relevant requirements.