| A large number of abandoned industrial sites exist in cities in China,leading to severe groundwater and soil pollution,which restrict the reuse of sites.As one of the in-situ vertical engineered barrier,cement-soil vertical barrier is suitable for large-scale,high-polluting industrial site remeditation projects.However,at this stage,the application of the cement-soil vertical barrier in the isolation project of contaminated sites is not enough and the following problems mainly exist:（1）Lack of a mixing ratio of vertical barrier material with feasible economic effects and environmental effects;（2）Chemical compatibility in terms of hydraulic conductivity and contamination control performance of cement-based vertical barrier under heavy metal pollution have been unclear.In this paper,the cement-soil vertical barrier material is taken as the research object.Throughout laboratory test,the influence of the stablizing agent on the engineering performance of the cement-soil vertical barrier material is studied,and the chemical compatibility and contamination control of the cement-soil vertical barrier material under typical heavy metal pollution are clarified.Main conclusions of the research work are given as follows:（1）Based on the laboratory test results of field samples of an in-situ vertical engineered barrier in Yancheng Dafeng,three sets of preferred tests are designed.Through these preferred tests,physical properties,strength properties and impermeabilities of different cement-soil vertical barrier samples based on different mixing ratios of GGBS,mixing ratios of bentonite,and mixing ratios of stablizing agent are studied.The mechanisms of GGBS and bentonite to improve the strength and impermeability of samples are clarified.Taking quality of vertical barrier and economic effects into account,an optimal mixing ratio of stablizing agent（PC:GGBS:B=8%:8%:4%）is selected for chemical compatibility research.（2）The chemical compatibility of cement-based vertical barrier materials under the effects of heavy metal zinc and lead pollution,in terms of hydraulic conductivity,is studied by flexible wall penetration test.The hydraulic conductivities of the samples permeated with different heavy metal solutions are between 1.49 and 10.10 times of the hydraulic conductivity of that sample permeated with tap water.The adverse effect of the type of solution on the permeability of the sample:zinc nitrate-lead nitrate solution>zinc nitrate solution>lead nitrate solution.When the concentration of zinc nitrate solution reaches 100mmol/L and the concentration of zinc nitrate-lead nitrate composite solution reaches 50mmol/L,the hydraulic conductivity of the samples exceed hydraulic conductivity limit regulatory(10-9m/s).The decrease in the impermeability of the cement-based vertical barrier under the action of heavy metal solutions is determined by three factors:heavy metal species,concentration of heavy metal,and pH of pore solution.（3）The hydrodynamic dispersion coefficient and retardation factor of heavy metal zinc and lead transported through the vertical barrier are estimated by soil column tests,and the relationships between them and heavy metal concentration are clarified.The order of the mobility of zinc is faster than that of lead.Van Genuchten analytic solution is used to analyze the influence of seepage velocity,hydrodynamic dispersion coefficient,retardation factor,and concentration criterion for breakthrough time on barrier breakdown time.The service life of vertical barriers with different barrier thicknesses and water heads is also given,as well as the barrier thickness meeting temporary and long-term service conditions. |
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