Influence of soil structure heterogeneities on the behaviour of backfill materials based on mixtures of bentonite and crushed rock

Abstract In situ compaction of a mixture of bentonite and crushed rock or sand has been proposed for backfill or buffer materials for the concept of nuclear waste disposal in many countries. At present a mixture of 30% bentonite and 70% crushed rock is used for backfilling tunnels in two full-scale tests in the Swedish underground laboratory Aspo HRL. In advance of those tests, mixtures of 0–50% bentonite and different ballast materials have been investigated in the laboratory regarding hydraulic conductivity, swelling pressure and other geotechnical properties. By comparing the results from different tests and also comparing them with expected properties, assuming that the bentonite is evenly and homogeneously distributed with a constant clay density in the ballast structure, a substantial influence of the soil structure has been exposed. Inhomogeneous distribution of the clay matrix means that there are clusters of clay with higher density than the average clay density and also that there are pockets with very low or no clay. These clusters and pockets are believed to be of the same size as the grain size of the ballast material or up to 20 mm for the backfill used in AHRL. Laboratory results show that these inhomogeneities may result in several changes in properties. The differences increase with increasing heterogeneity. The following conclusions regarding the influence of heterogeneities on the soil structure and the backfill properties were drawn: • The influence of the soil structure on the hydro-mechanical properties of backfill materials is very strong • Heterogeneities in grain size and bentonite distribution in centimeter scale in a backfill cannot be avoided with present mixing techniques • In general, the heterogeneities increase with decreasing clay content at identical mixing procedure • The influence is strong on swelling pressure and hydraulic conductivity • Swelling pressure and hydraulic conductivity increase with increasing heterogeneities