Engineering properties of GMZ bentonite-sand as buffer/backfilling material for high-level waste disposal

This paper reports on the engineering properties of various mixtures of GMZ bentonite and sand, as regards their effectiveness as buffer/backfilling material. The sand ratio in the various test specimens was 0, 20, 30, 40 and 50%. Compaction tests were conducted to determine the optimum water content and maximum dry density of the bentonite-sand mixtures. A series of hydraulic and mechanical tests was conducted to assess Atterberg limits, the soil water characteristic curve, hydraulic conductivity, shear strength, swelling pressure and swelling strain of the bentonite-sand mixtures. The hydraulic conductivities of the mixtures, that had sand ratio varying from 0 to 30%, remained consistent and were low enough to prevent the flow of groundwater through the vitrified waste. The water holding capacity, shear strength and swelling capacity decreased slightly, but that could be partly offset by increasing the effective dry density and average dry density of the buffer/backfilling material. The Atterberg limits decreased by mixing in additional sand, which could offer improved the workability of buffer/backfilling material. It was suggested that the most effective sand ratio of GMZ bentonite-sand mixtures, which are available in China, should not be more than 30%.