INFLUENCE FACTORS OF SANDBENTONITE MIXTURES ON HYDRAULIC CONDUCTIVITY

Bentonite is attracting a greater attention as buffer and backfill material of the engineering barrier for geological disposal of highlevel radioactive nuclear waste,because it can offer impermeability and swelling properties. But the pure compacted bentonite strength decreases with increasing hydration,which will reduce the buffer capability in the practice engineering. To solve this problem,sand is often used to form compacted sandbentomite mixtures(SBMs)to provide high thermal conductivity,excellent compaction capacity,longtime stability,and low engineering cost. The factors affecting the hydraulic conductivity of SBMs are studied in this paper. They include bentonite content,grain size distribution,moisture content,dry density,compacting method and energy,and bentonite type. The studies show that the hydraulic conductivity of SBMs is controlled by the hydraulic conductivity of the bentonite. It also decreases as dry density and bentonite content increase. But when the bentonite content reaches a critical point,the influence of increasing bentonite to decrease the hydraulic conductivity is limited. A fine and wellgraded SBM is likely to have a lower hydraulic conductivity than a coarse and poorly graded material. The internal erosion or erodibility based on the grain size distribution of the SBMs has a negative effect on the final hydraulic conductivity. The lowest hydraulic conductivity is gained when the mixtures are compacted close to optimum moisture content. Also,the mixtures compacted at moisture contents slightly above optimum values give lower hydraulic conductivity than when compacted at slightly under the optimum moisture content．