Study On The Compressive Strength Of Compacted Bentonite-Sand Mixture As HLW Buffer Material

Deep geological disposal of HLW is based on a unique multi-barrier system consisting of engineering barriers?waste tanks and buffer backfill material?and natural barriers?surrounding rock?.Buffer material is an engineering barrier between waste disposal tanks and natural surrounding rock,which plays an important role in buffering surrounding rock pressure,blocking underground seepage and conducting radiant heat.Compacted bentonite has the advantages of low permeability and self-healing property of expansion,which makes it the preferred buffer material in the world.By adding appropriate amount of quartz sand,the thermal conductivity of the buffer material can be improved while the excellent properties of pure bentonite can be guaranteed.Buffer materials are mainly constructed by compacted block masonry.During the operation of the HLW underground repository,the buffer materials stacked in the form of blocks will produce compression deformation and even damage under the action of large vertical load,which will seriously affect the engineering barrier effect of the buffer backfill materials.In order to systematically characterize the mechanical properties of the buffer material and ensure the safety and stability of the disposal repository during operation,the compressive strength of the buffer material needs to be studied in depth.Highly compacted small samples and large buffer blocks of bentonite-sand mixture with different compacted dry density,water content and sand content were prepared.The compressive strength of different small samples was measured by unconfined compression tests,and the deformation failure and compressive strength of the samples were analyzed with the variation of the compacted dry density,sand content and water content.Combined with microstructure tests and existing research results,the influence mechanism of compressive strength with compacted dry density,sand content and water content of bentonite-sand mixture was analyzed.The compressive strength and dry density of different parts of the blocks were measured by partition sampling,the spatial distribution law of compressive strength and dry density of the block was explored and the homogeneity of the block was analyzed.The compressive test results of the small samples show that all the compacted bentonite-sand mixtures samples were fractured along the compression direction,and the samples were subjected to tensile failure in the range of compacted dry density,sand content and water content controlled by the tests.The stress-strain curves were all strain softening with obvious stress drop.The compressive strength of the samples increases with the increase of the compacted dry density,the decrease of the water content and the decrease of the sand content.When the compacted dry density is greater than 1.8g/cm³,the sand content is less than 30%and the water content is less than 12%,the compressive strength of the compacted bentonite-sand mixture is greater than 3.30MPa.The analysis results of the strength influence mechanism show that the compressive strength of the samples is affected by the particle diameter distribution and cohesion on the micro level,which are respectively controlled by the compacted dry density and water content.With the increase of the compacted dry density,the large pores between the particles decrease and the material becomes denser and the compressive strength increases.With the increase of water content,the cohesion decreases and the compressive strength decreases.Based on the concepts of effective clay density and effective water content,the influence of sand content on the compressive strength of samples was analyzed,and the concept of clay-water effective density(?_bw)was proposed.The compressive strength of the bentonite-sand mixture can be expressed as a single variable by?_bwand the expression of the compressive strength of the samples is a power function.The compressive test results of large buffer blocks show that the compressive strength values of different parts in compacted bentonite-sand mixture blocks are different,but generally close to the compressive strength of small samples and the homogeneity of the blocks is preferable.In the direction of height?Z?,the compressive strength of the blocks corresponds well with the compacted dry density,and the compressive strength of the parts with higher compacted dry density is also higher.Due to the rebound of expansion,in the direction of height?Z?,the compressive strength value of the upper part?Z₁?of the blocks is generally lower than that of the lower part?Z₂?,and the compacted dry density has the same change rule.