Evaluation of hydro-mechano-chemical behaviour of bentonite-sand mixtures

Bentonite-sand mixtures are widely used in engineering barrier of deep geological disposal of high-level radioactive nuclear waste and anti-seepage barrier of civil geotechnical engineering.Under the action of groundwater solution infiltration and external stress,the hydro-mechanical(HM)behaviour of bentonitesand mixtures,i.e.the swelling characteristics and permeability,will change.Once the anti-seepage and filtration effect is weakened or lost,the pollutants will spread to the biosphere.Therefore,it is necessary to study the swelling characteristics and permeability of bentonite-sand mixtures under coupled mechanochemical(MC)effect and to establish corresponding prediction model.For this reason,swelling tests under salt solution with different concentrations are conducted on pure bentonite and its mixtures with 30%,70%and 90%sand contents,the compression tests are carried out on saturated samples,and the saturated permeability coefficient k of the sample under each load is calculated by Terzaghi’s one-dimensional consolidation theory.The concepts of true effective stress pe,montmorillonite void ratio em and critical sand content as are introduced to determine the em-pe relationship and finally the k-em relationship of bentonite-sand mixtures.It is found that when the sand content aas,the em-pe relationship of the mixture is linear and independent of the salt solution concentration,and when a>as,the em-pe relationship of bentonite-sand mixture is bi-linear with the true effective deviatoric stress pesa as the intersection.In addition,the em-k relationship also shows the linear trend when aas,and the slope of the line increases with the increase of the salt solution concentration.When a>as,the k-em relationship will deviate from the linear relationship.Moreover,the larger the sand content is,the farther the deviation is.On the basis of summing the regularity,a model for predicting the HM behaviour of bentonite-sand mixture under the coupled MC effect is proposed.By comparing the swelling and permeability test results with model prediction results of different types of bentonite and its sand mixtures,the predictive model is verified.The study on the HM behaviour of bentonite-sand mixtures under salt solution infiltration and the model establishment can provide experimental and theoretical basis for the design and construction of anti-seepage engineering by bentonite-sand mixtures.

Swelling ability and behaviour of bentonite-based materials for deep repository engineered barrier systems:Influence of physical,chemical and thermal factors

Compacted bentonite-sand(B/S)mixtures have been used as a barrier material in engineered barrier systems(EBSs)of deep geological repositories(DGR)to store nuclear wastes.This study investigates the individual and combined effects of different chemical compositions of deep groundwaters(chemical factor)at potential repository sites in Canada(the Trenton and Guelph regions in Ontario),heat generated in DGRs(thermal factor),dry densities and mass ratios of bentonite and sand mixtures(physical factors)on the swelling behavior and ability of bentonite-based materials.In this study,swelling tests are conducted on B/S mixtures with different B/S mix ratios(20/80 to 70/30),compacted at different dry densities(ρd=1.6-2 g/cm^(3)),saturated with different types of water(distilled water and simulated deep groundwater of Trenton and Guelph)and exposed to different temperatures(20℃-80℃).Moreover,scanning electron microscopy(SEM)analyses,mercury intrusion porosimetry(MIP)tests and X-ray diffractometry(XRD)analyses are carried out to evaluate the morphological,microstructural and mineralogical characteristics of the B/S mixtures.The test results indicate that the swelling potential of the B/S mixtures is significantly affected by these physical and chemical factors as well as the combined effects of the chemical and thermal factors.A significant decrease in the swelling capacity is observed when the B/S materials are exposed to the aforementioned groundwaters.A large decrease in the swelling capacity is observed for higher bentonite content in the mixtures.Moreover,higher temperatures intensify the chemically-induced reduction of the swelling capacity of the B/S barrier materials.This decrease in the swelling capacity is caused by the chemical and/or microstructural changes of the materials.The results from this research will help engineers to design and build EBSs for DGRs with similar groundwater and thermal conditions.