Evaluation Of Impermeability And Physical-Mechanical Characteristics Of Soil-Bentonite Cutoff Wall Based On CPTU

With the accelerated process of industrialization and urbanization in China,the environmental problems are becoming more and more prominent,so the environmental geotechnical engineering is developing rapidly.In environmental geotechnical engineering,the problems brought by domestic waste are very harmful.Landfill is the main disposal method of municipal solid waste at present.If the contaminant control ability of the landfill site is poor,the landfill leachate will cause serious damage to the surrounding surface water and soil,so the impermeability of the landfill site is particularly important.The soil bentonite cutoff wall,as an in-situ vertical barrier,is composed by the base soil and bentonite,and it can effectively control the transport of pollutants and improve the risk control ability.The soil bentonite cutoff wall has the advantages of low hydraulic conductivity performance,low cost and short construction period.At present,the research progress of soil bentonite cutoff wall in China are lagging behind those in Europe and America.According to the soil bentonite vertical engineering barrier materials and the pollutants,the research on the evaluation and analysis of the impermeability effect by the soil bentonite cutoff wall is still lacking.The theory and technology of impermeability of soil bentonite cutoff wall should also be improved for landfill site.The study is financial supported by the National Natural Science Foundation of China and National High Technology Research and Development Program of China?led by Zhejiang University?.And the object of this research is the soil bentonite cutoff wall of a landfill site.The research aims to evaluate the impermeability and physical-mechanical characteristics of the soil bentonite cutoff wall in landfill site,by the in-suit CPTU and the laboratory test,including the one dimension consolidation test,direct shear test,numerical simulation based on ABAQUS.The major conclusions are listed as following:?1?The compression characteristics of the soil bentonite cutoff wall materials were studied by one dimensional consolidation compression test,and the relationship of bentonite content and initial water content on the compression properties of the isolated material were quantitatively investigated.The relationship between the remolded yield stress of soil bentonite and the initial water content was studied.The relationship between the compression index C_c of soil bentonite,the swell index C_s and the initial water content were found,and the relationship between the compression index C_c and the remolded yield stress e_yr was studied.At the same time,the results indicate the relationship beteween the void ratio at??=1kPa and of compression index C_c of soil bentonite.The coefficient of consolidation of the soil bentonite cutoff wall was evaluated by the Casagrande method.The horizontal coefficient of consolidation was converted by the vertical consolidation coefficient,and the result was compared with the value measured by the CPTU investigation.?2?The relationship between shear stress and shear strain of soil bentonite cutoff wall materials was obtained by the direct shear test.Through different vertical stresses,the relationship between the vertical stress and the shear strength of the soil bentonite cufoff wall material was studied,and the cohesive force and the internal friction angle at different depths of the soil bentonite wall materials were evaluated.The results of the direct shear test were compared with the results from CPTU.?3?The new concept of the effective consolidation stress ratio was proposed to interpret the relationship of the bentonite content and the coefficient of consolidaton of soil bentonite cutoff wall.The effect of the consolidation characteristics of soil bentonite by different bentonite content were studied.The method of predicting the bentonite content was established.The safety reserve of the soil bentonite cutoff wall can be improved.?4?The physical-mechanical characteristics and stress behavior of the soil bentonite cutoff wall are studied by CPTU technology.The stress behavior was evaluated by the arching model,lateral squeezing model and modified lateral squeezing model.For the in-situ state parameters,the relationship between the pre-consolidation pressure and the net cone tip resistance was obtained,and the parameter k value suitable for the soil bentonite cutoff wall was evaluated.The coefficient of lateral stress of soil bentonite cutoff wall was evaluated by net cone resistance and OCR and effective internal friction angle method.The total cone tip resistance factor N_k,the effective cone tip resistance factor N_ke and the excess pore pressure factor N_?u,which were suit for the soil bentonite cutoff wall,were tested through the CPTU test.Sensitivity of soil bentonite was evaluated by sensitivity parameter N_s and friction ratio.For the deformation characteristics,the values of a_n for soil bentonite are obtained,and the undrained Young modulus E_u of soil bentonite based on different parameters n was studied.?5?For the standard dissipation type,the coefficient of consolidation of soil bentonite was evaluated by t₅₀ method proposed by Torstensson.For non-standard dissipation,the coefficient of consolidation of the soil bentonite cutoff wall was evaluated by the modified t_50m method proposed by Chai et al,the time square root method and the time logarithm method proposed by Sully et al.?6?The hydraulic conductivity of soil bentonite cutoff wall was evaluated by different methods.For the dissipation behavior with dissipation time t less than t₅₀,the dissipated pore pressure of the soil bentonite was predicted and the coefficient of consolidation and permeability coefficient of the soil bentonite cutoff wall was calculated by the hyperbolic fitting method.The hyperbolic fitting method can effectively save the engineering time and improve the application of CPTU technology in the geotechnical engineering.?7?The penetration process of the CPTU and the consolidation process of soil bentonite are studied by the numerical simulation technology.The plastic influence range by the penetratton of CPTU was studied,and the distribution of the stress,strain and pore pressure at different locations of the cone was obtained.The pore pressure spatial distribution and dissipation rule based on the numerical simulation technology was studied.