Hydro-mechanical Properties Of Granular Bentonite

Bentonite is considered as a preferred buffer/backfill material for the engineered barrier in high-level radioactive waste repositories.Its high expansibility,low permeability,and strong adsorption characteristics play an effective role of isolation and anti-seepage through the prevention of nuclides migration and the infiltration of groundwater.Improving the compactness of bentonite is an effective way to obtain high expansibility and low permeability,therefore,researchers have focused on enhancing the degree of compaction by controlling water content,and compaction effort,nonetheless,the improvement ranges obtained are limited.Consequently,this work intends to discuss the feasibility of improving the degree of compaction from the perspective of regulating grain size distribution.Firstly,the granular bentonite with different grain sizes was prepared,then,the optimum water content and optimum granular ratio were determined.After that,the swelling and permeability properties of the granular bentonite with the determined optimum water content and granular ratio were studied.The main research contents and conclusions are as follows:1)Granular bentonite was produced through the wet-dry agglomerate method.The hydro-mechanical properties of bentonite before and after agglomeration were compared and analyzed by performing tests such as free swelling ratio,constant volume swelling pressure,saturated hydraulic conductivity,and suction.It was found that the swelling,permeability and water retention capacities were not affected by agglomeration,and the swelling and self-sealing capacities were still good.The results from scanning electron microscope analysis showed that after agglomeration,the bentonite was denser in structure,larger in size,and more distinct at the boundaries.The pore distribution curves indicated that the agglomeration behavior only affected the inter-particles pores,while the influence on mesopores and micropores less than 7 nm were almost negligible.2)The compaction tests for four kinds of bentonite with different grain size distribution were carried out,and it was found that the dry densities of all samples initially increased and then decreased with the increase in water content.Under 45 kN load,the optimum water content was about 20%,and the dry density was improved significantly by regulating grain size distribution,among the samples,the maximum dry density of granular bentonite?GB-3?was up to 1.72 g/cm³.Under the optimum water content state,the dry density of granular bentonite?GB-3?and original bentonite?OB?minimally increased,while the optimum water content decreased with the increase in compaction effort.The results from mercury intrusion tests showed that the water content and compaction effort only affected the macropores,while the grain size distribution affected both the macropores and mesopores.Through regulation of the grain size distribution of bentonite,increasing the dry density achievable.3)The granular bentonite?GB-3?and the original bentonite?OB?with the same dry density?1.69 g/cm³?were infiltrated by deionized water and 2.0 mol/L CaCl₂ solution,successively.The results indicated that the swelling pressure of granular bentonite?GB-3?was slightly lower than the original bentonite?OB?,however,the hydraulic conductivity of the two showed little difference.Moreover,the swelling pressure reduced significantly and the hydraulic conductivity greatly increased under infiltration with CaCl₂ solution as compared to the deionized water infiltration.The reduction range of swelling pressure was about 30%,nonetheless,the values of swelling pressure and hydraulic conductivity recorded still satisfied the design requirements for the disposal repository.