Abstract：A certain proportion of quartz sand is added to pure bentonite in deep geological disposal project of high-level radioactive wastes(HLW) to optimize the thermal conductivity and constructability of buffer/backfill materials. Proceeding from the concept of composite soil，the compacted bentonite-sand mixture specimens with the same dry density，water content and different sand ratios are designed to reveal the controlling mechanism of sand ratio to shear strength by shear test. Gaomiaozi bentonite from Inner Mongolia(GMZ001 bentonite)，mixes with quartz sand in a weight ratio of 0%–50%，is selected as test specimens. The shear test results indicate that，with an increase in sand ratio from 0% to 50%，the compacted mixtures yield a shear behavior from strain-softening to strain-hardening；and the cohesion and internal friction angle decrease correspondingly，which means that the shear strength decreases. Based on a pore structure hypothesis，according to analogy analysis，the bounded sand ratio governing the shear behavior of clay-sand mixture are estimated from the published data；and the supporting scanning electron microscope(SEM) pictures of pore structure are provided. By introducing the effective clay density and effective water content，the physical states of clay matrix among quartz sand particles is described，the controlling mechanism of quartz sand content to the shear strength of bentonite-sand mixtures as buffer/backfill material is reasonably explained.
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