| As an important engineering barrier for high-level radioactive waste(HLW)repository,buffer/backfill materials can effectively guarantee the long-term safe and stable operation of the repository.In deep geologic bodies 500-1000m from the surface,the buffer/backfill materials are highly susceptible to groundwater erosion and swells due to water absorption.At the same time,HLW will generate a large amount of radiant heat during the decay process,which evaporates the water in the buffer/backfill materials,causing the materials to shrink and deform and resulting in potential safety hazard.In addition,the water retention characteristics of the materials may also change during the above-described water field changes.Therefore,it is of great significance for evaluating the long-term safety of HLW repository to master the swelling and shrinkingbehavior of buffer/backfill materials in the process of water absorption and water loss.The main research work and achievements of this paper are as follows:(1)The water retention capacity of buffer/backfill materials based GMZ bentonite compacted samples under different initial dry densities,initial water content and sand mixing ratio during dehumidification was investigated by WP4.The results show that the initial dry density and the initial water content have similar characteristics to the water retention capacity of the sample,that is,in the low suction stage(s<7MPa),the water retention capacity of the sample increases with the increase of initial dry density or initial water content.However,in the high suction stage(s>7MPa),the above factors have little effects on water retention capacity,indicating that the initial pore structure only affects the water retention characteristic at low suction stage,but not at high suction stage.The impact on the high suction stage is negligible;and the water retention capacity of samples decreases with the increase of sand mixing ratio in both low and high suction stages.(2)Swelling behavior of buffer/backfill materials based GMZ bentonite under three boundary conditions(constant mean stress(CMS),constant stiffness(CS),and constant stiffness(CS))and sand particle sizes were investigated through swelling tests.The results show that boundary conditions have an important influence on swelling indexes of the sample.Under different boundary conditions,the final swelling indexes measured after the sample is saturated show a large difference.For the final swelling strain,the relationship among the influences of boundary conditions on it is CMS>CS>CV;For the final swelling pressure,the relationship among the influences of these conditions on it is CV>CS>CMS.The swelling equilibrium limit(SEL)curve of the sample can be determined based on the final void ratio and swelling pressure of the sample after expansion equilibrium.The curve reflects the final stress-strain state when the soil swells to equilibrium,and has importantreference significance for predicting the swelling indexes of soil samples during hydration under complex boundary conditions.The sand particle size has a significant influence on the expansion indexes of samples.Specifically,the larger the sand particle size,the stronger the inhibition effect on the expansion of samples.The three-dimensional structure of the samples with different sand particle sizes was analyzed by micro-CT test.It was found that the larger the sand particle size,the more uneven the distribution of bentonite in the mixture.(3)The effects of initial dry density,initial water content and sand mixing ratio on the evaporating and drying process of the sample were investigated.The results show that the larger the initial dry density,the smaller the evaporation rate of the sample,the longer the evaporation time,and the closerthe final residual water content during the deceleration rate stage.The higher the initial water content,the larger the evaporation rate,the longer the evaporation time.And the residual water content has no obvious change,either.The higher the sand mixing ratio,the larger the evaporation rate and the shorter the evaporation time.And the residual water content of the sample decreases with the increase of the sand mixing ratio.(4)The shrinking behavior of the samples under different initial dry densities,initial water content and sand mixing ratio were investigated.The results show that the final shrinkage strain of the sample decreases with the increase of initial dry density,which is due to the larger the dry density,the smaller the void ratio,the tighter the arrangement or soil particles,and the greater the friction resistance to be overcome in the rearrangement of particles;the final shrinkage strainincreases significantly with the increase of initial water content,which is due to that the higher the initial water content,the higher the saturation of the sample,and the smaller the friction force and connection stiffness among soil particles;and the final shrinkage strain decreases with the increase of soil mixing ratio,but when the sand mixing ratio is greater than 30%,the final shrinkage strain decreases less obviously with the change of the sand mixing ratio.Micro-CT test was used to analyze the three-dimensional structure of samples with different sand mixing ratio..It was found that when the sand mixing ratio reaches 30%,the bonding density of sand particles would increase sharply,forming a skeleton and restricting the overall shrinkage deformation of the test.(5)The anisotropy of buffer/backfill materials based GMZ bentonite during shrinkage was quantitatively analyzed by shrinkage geometry factor.The results show that the water content has a significant effect on the anisotropy during shrinkage.The higher the water content,the greater the proportion of the radial shrinkage of the sample in the total volume shrinkage process.The excessive radial shrinkage can easily cause cracking in the shrinkage process.With the decrease of water content,the sample shrinks with an isotropic tendency,and the contribution of radial shrinkage and axial shrinkage to total volume shrinkage is substantially the same. |
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