Tensile Strength Of Compacted Gaomiaozi Bentonite And Its Prediction

Safe disposal of high-level radioactive waste is a worldwide problem.Many researches around the world have shown that the deep geological disposal of high-level radioactive waste is only reliable permanent engineering solution.The barriers for the deep geological disposal of nuclear waste consist of the waste canister,buffer layer and natural surrounding rocks.Bentonite has high swelling,low permeability and excellent nuclide adsorption properties,so it is very suitable to be a buffer material between the waste tank and natural surrounding rock.After long-term screening,Gaomiaozi bentonite in Xinghe county,Inner Mongolia was selected as the first choice of the buffer material in China。The buffer material in the deep geological disposal of high-level radioactive waste is made of compacted bentonite blocks.Humidity changes during the storage,handling,and installation due to seasonal and weather conditions.Moreover,the compacted bentonite blocks are prefabricated in the factory and need to be tied and lifted when transported to the site or installed on the site,during which some parts of the bentonite blocks may be subjected to tension,therefore,it is necessary to know the tensile strength of compacted bentonite with different water contents and dry densities.In future disposal works,the bentonite blocks may be placed for a long time after they are prefabricated in the factory for reasons such as construction procedures.Will their tensile strength decrease significantly?This is one of the concerns in engineering design.Therefore,this paper mainly studies the tensile strength of compacted Gaomiaozi bentonite under different initial conditions and different ageing time conditions,and puts forward a formula to predict the tensile strength of compacted Gaomiaozi bentonite,so as to provide references for future nuclear waste disposal projects.Specific research contents and conclusions are as follows:1.The direct tensile test and Brazilian test were conducted on specimens that have been newly completed with different dry densities and water contents,and it is found that tensile strength is related to water content and dry density.The tensile strength increases gradually with the increase of dry density.The higher the dry density,the higher the tensile strength is.The relationship with water content is that the tensile strength increases at first and then decreases with the increase of water content.The data obtained by the two test methods are different,which may be related to the specimen preparation method and the specimen anisotropy.2.The effect of ageing time on the tensile strength of compacted Gaomiaozi was studied under different dry densities and water contents.Under the condition of keeping the dry density and water content unchanged,the specimens were allowed to cure for 0,7,15,30 and 90 days,respectively,and then Brazilian test was conducted.The results show that the tensile strength of Gaomiaozi bentonite decreases with the increase of ageing time.At the early stage,the strength decreases rapidly.The tensile strength tends to be stable after the curing time exceeds a certain number of days.Combined with the measured pore size distributions of compacted bentonite specimens,the reason that the tensile strength decreases with the curing time is:with increasing the curing time,the soil water molecules gradually move towards the montmorillonite interlayer and montmorillonite crystal layer suction unfolding,hydration,which make particles segmentation,then make weak connection between particles,resulting in a decrease in the tensile strength.3.VG and Fredlund-Xing models are used to fit the characteristic curves of compacted bentonite samples.The tensile strength of compacted bentonite with different water contents is predicted by using the existing prediction formula of tensile strength.The results show that the tensile strength predicted by Lu-Likos’formula is far different from the measured tensile strength.Based on Varsei’s formula for predicting tensile strength and using Fredlund-Xing’s model,it is suggested to predict the tensile strength of unsaturated compacted Gaomiaozi bentonite.According to the measured results,the parametersΨ_r,a andξare determined to match predicted values with the measured values well.The parametersΨ_r,a andξare linear relationships with the dry density.Therefore,the tensile strength of compacted Gaomiaozi bentonite with different dry densities and degree of saturation can be predicted,which can be used in practice.