Compaction-induced variability in the 80/20 sand-bentonite mixture: Spatial variation in dry density at different scales in the laboratory and large-scale gas seal test

Abstract

Sand-bentonite (SB) mixtures are considered as backfill materials for sealing designated sections of nuclear waste repositories. The dry density of the SB mixture is the crucial parameter for the quality control during emplacement because it strongly controls the hydraulic and mechanical properties. Backfilling of an SB mixture is often performed layer by layer by keeping track of the average dry density of each layer. The compaction is typically done in a dynamic manner leading to more compaction near the compaction surface. In this study, an SB mixture with 80% sand and 20% bentonite was compacted in the laboratory by static and dynamic procedures to a height of 10 cm and the dry density profile was characterized by destructive slicing and microfocus X-ray CT imaging. When dynamically compacted, a distinct dry density gradient was identified. The variation in the dry density identified by the slicing was as much as 0.4 g/cm3 which leads to hydraulic conductivity variation by a factor of 4.8. The SB seal emplaced in the large-scale gas seal test (GAST) for demonstrating the effective functioning of the gas permeable seals was also analyzed to identify if any variability was present at the field scale. The samples collected from the top 2 cm of each 10 cm-thick horizontal layers yielded a higher overall average by 0.06 g/cm3 than the average of the mass balance method. In conjunction with the laboratory studies, it was implied that sampling results, if only collected from the very top few centimeters of the layers, could lead to an overestimation in the global dry density. Overall, the results with different methods at different scales indicated that the dynamic compaction-induced variability within a single layer was more pronounced than the variability in the larger scales detected in the mass balance and sampling methods.