| Underground soil and water pollution is a serious problem in China,which threatens the health and ecological environment of urban residents,so there is an urgent need for effective control and governance.The soil-bentonite barrier uses a very low hydraulic conductivity of backfill materials to enclose the contaminant and isolate it from the outside environment to achieve the effect of preventing the spread of pollutants.It has broad application prospects in water and soil pollution prevention and restoration.The hydraulic conductivity of soil-bentonite cutoff wall is the most important factor to determine its contaminant-resistant performance.However,the research on the permeability of soil-bentonite is mainly carried out by laboratory test,and it is lack of systematic study of bentonite engineering characteristics,laboratory test to in situ permeability coefficient evaluation.In view of the above research situation,two kinds of bentonite,such as Wyoming bentonite,Shandong Weifang bentonite and Fujian standard sand and Qiantangjiang silty soil was used as soil-bentonite materials,based on the study of the basic engineering properties of the above materials,the hydraulic conductivity of soil-bentonite cutoff wall was studied systematically by means of laboratory test and in situ detection.(1)The basic engineering characteristics of mineral composition,specific gravity,liquid plastic limit,cation exchange capacity,specific surface area and swell index of two kinds of bentonite in Wyoming and Weifang were measured respectively.The results showed that the two bentonites were sodium bentonite,and the montmorillonite content,swell index and specific surface area of Wyoming Bentonite was significantly higher than Weifang bentonite,It can be concluded that the engineering performance is better than that of Weifang bentonite.(2)Based on the range of the head difference between the slurry and the groundwater head in the soil-bentonite slurry trench,the hydraulic conductivity of bentonite cake was estimated using the API fluid loss test under the air pressure of 20 kPa to 300 kPa,and it is larger than the permeability coefficient that obtained at the air pressure of 690 kPa by foreign scholars.In order to overcome the shortcomings that the stress distribution of bentonite cake is uneven in API fluid loss test,the permeability coefficient of bentonite cake with uniform effective stress distribution was measured by using a flexible wall permemeater.Comparing the results of the two methods,it is found that when the effective stress is small,the hydraulic conductivity of bentonite cake by the improved API filtration test method is smaller than that of the flexible wall permemeater test.(3)The effects of bentonite content and species,base soils and consolidation pressure on soil-bentonite permeability coefficient were studied by flexible wall permemeater system.The research shows that the decrease trend of soil-bentonite permeability coefficient with the increase of consolidation pressure is not obvious when the amount of bentonite is low and the base soil is low compressible sand.When the species and content of bentonite are the same,the hydraulic conductivity of silt bentonite is less than sand bentonite.Therefore,it is suggested to select fine grained soil as the base soil in the cutoff wall project.(4)A large-scale field test was constructed in a landfill in Jiangsu,Jingjiang,and it is the first soil-bentonite cutoff wall that had been designed and constructed in China.The in-situ hydraulic conductivity of soil bentonite was measured by CPTU pore pressure dissipation test,and an accurate and reliable data processing method is proposed,the test results show that the test section can meet the requirements of the seepage resistance of the conventional soil bentonite cutoff wall less than1.0×10-9m/s. |
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