Role Of Sand Content On The Shear Strength Of Compacted Bentonite-Sand Mixtures As Buffer/Backfill Material For HLW Disposal

High-Level Radioactive Waste(HLW) is a waste of that very difficult to be disposed of because which contains strong radioactive, heat, toxicity, and long half-life nuclides. The deep geological disposal is currently believed to be the best feasible disposition. A multiple barrier system was accepted as the repository conceptual, is which the buffer/backfill material is the most important barrier to prevent the migration of radionuclides by groundwater.Bentonite is considered to be reliable buffer/backfill material in HLW repositories because of its high absorption and high swelling properties and low permeability. However, pure bentonite been used as buffer/backfill material has two limitations. One is the low thermal conductivity properties and it is not helpful to conducting the heat of radiation into the surrounding rock. The other is the workability, difficult to be prepared and to be compacted to the maximum dry density.Following the recent development trend in deep geological disposal of HLW, compacted bentonite-sand mixtures is recommended by Lanzhou University,China as an innovative buffer/backfill material for its enhanced thermal conductivity and workability.The final research goal to the compacted bentonite-sand mixtures is to investigate the optimum sand addition, under which the mixtures will provide a best adsorption to radionuclides, impermeability, swelling sealing at same time. As a specific research objectives of this study is to reveal the role of sand content on the shear strength of the compacted mixtures so to establish a mechanical basis for final optimum ratio of sand content of the mixtures.Direct shear tests conduced on bentonite-sand mixtures and the role of sand content on the shear strength of compacted mixtures were analyzed and reported. GMZ001 bentonite was mixed with quartz sand in a ratio of 0% to 50%(w/w) and compacted to a series of specimens in same dry density and water content. Test results indicated that, with an increase in sand content from 0% to 50%, the compacted mixtures yield a shear behavior from strain-softening to strain-hardening, and the cohesion and friction angle were decreased correspondingly. From the viewpoint of porosity structure developed by sand and clay in a granular material-clay mixture, sand content limits governing the shear behavior of the mixtures were estimated from published data. The concept of effective density and water content were introduced to describe the physical state of the clay matrix filling between sand particles. By combination of porosity structure and clay matrix state, the effluence of sand content on the shear strength of compacted bentonite-sand mixtures was reasonably explained.