| The bentonite/additive mixture is an optional buffer backfill material in the geological disposal of high level nuclear waste.From the perspective of time,the buffer backfill material will undergo a series of volume change processes such as compression.swelling,recompression and rebound;for the spatial position,the buffer backfill material at different locations in the repository is under different boundary conditions;from the perspective of material science,there is a variety of additives including quartz sand,graphite,granite powder and other additives can be selected for buffer backfill materials.Therefore,understanding the hydro-mechanical coupled characteristics of bentonite/additive mixture under multiple processes,multi-class materials and various boundary conditions is highly significant for the safety evaluation of the repository and the performance optimization of the buffer backfill material.Specific work contents and main conclusions of this thesis are as follows:(1)Six groups of calcium bentonite/sand mixtures were prepared with different contents of quartz sand,i.e.0%,10%,20%,30%,40%and 50%.Then a series of experiments including uniaxial confined compression,saturation under 0.2 MPa vertical pressure and reloading tests were carried out in sequence.The hydro-mechanical coupled characteristics of bentonite/sand mixtures under multiple processes condition was understood.It is found that:The compression behaviour of mixture significantly depends on sand content,saturation state and density.As the mixture is unsaturated and at relative low dry density(pd<1.7g/cm3),the compression index decreases linearly with increasing sand content.However,as the mixture issaturated and at relative high dry density(pd<1.7g/cm3),the compression index of mixture is independent of sand content,and the corresponding value is much less than that at unsaturated and loose state.The periodical characteristic was observed in swelling process.Both the swelling strain and swelling pressure decrease with the increase of the sand content,but they increase exponentially with the increasing bentonite effective dry density.As the sand content in the mixture is higher than 40%,volume collapse can occur during saturation,and collapse extent is enhanced by increasing sand content.The observed differences in volume change characteristics are mainly related to the distribution of bentonite and sand in the mixture,and their dominant effect on soil skeleton.Generally,the higher sand content and higher density,the volume change behavior the mixture is more conditioned by sand.(2)Two types of mixtures were prepared by mixing variable amounts of crushed granite powder or quartz sand(a range of 10%to 50%by dry mass of aggregate addition)with pure GMZ sodium bentonite.To investigate the volumetric deformation characteristics of the bentonite/additive mixtures under coupled hydro-mechanical conditions,sequential oedometer tests comprising four phases were carried out.including preliminary compression,swelling under constant vertical stress,re-compression,and unloading-rebounding.Experimental results indicate that the compression behavior of the mixture is not only affected by the saturation state,density and additive content,but also by the type of additive;bentonite/granite mixtures have higher compressibility than bentonite/sand mixtures.The quantitative relationship between the swelling strain or swelling pressure and the bentonite effective dry density depends not only on the type of bentonite but also on the type of additive;for the same bentonite effective dry density,the final swelling pressure of bentonite/granite mixture is higher.Mechanical effect and physical-chemical effect are the dominant mechanism of rebound behavior,and it is significantly affected by the initial state,additive content and type:the swell index increases linearly with the increase of the additive content,but the swell index of the bentonite/granite mixture is higher than that of bentonite/quartz sand.The bentonite type has a significant effect on hydro-mechanical coupled characteristics of the mixture;the final swelling pressure of sodium bentonite/sand mixture is higher than that of calcium bentonite/sand mixture,while the calcium bentonite/sand mixture is easier to compress than the sodium bentonite/sand mixture.(3)Three groups of GMZ sodium bentonite were prepared with different dry density,i.e.1.5 g/cm3?1.6 g/cm3 and 1.7 g/cm3.Then a series of swelling tests under different boundary conditions(constant mean stress,constant volume and constant stiffness),different initial load(25 kPa?100 kPa and 400 kPa)and different test path(CMS-CV and CV-CMS)were carried out.The swelling equilibrium limit curves(SEL)of sodium bentonite were determined and the effects of initial state and test path on SEL were studied.The results show that:the boundary condition is an important factor affecting the swelling indices:the final swelling pressures corresponding to each boundary condition are as follow:constant volume>flexible boundary>constant stress,while the final swelling strains corresponding to each boundary condition are as follow:constant stress>flexible boundary>constant volume.The initial conditions have an important effect on SEL:changing the initial dry density or load will cause the position of the curve to move horizontally orvertically,and the effect of dry density on the shape of the curve is more pronounced.The test path has slight influence on the determination of swelling indices:the swelling index measured under variable boundary condition is slightly higher than the initial SEL.moreover the final swelling index depends mainly on the boundary condition applied in the early stages.(4)Aiming at the drawbacks of existing geotechnical test equipment,a multi-boundary conditional dilatometer was designed and developed.The instrument can not only fully simulate the different boundary conditions in the repository,but also enable achieve the free switch between different boundary conditions during the test process.On this basis,typical swelling test methods under multi-boundary condition and multi-test path were proposed. |
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