Study On Macro And Micro Engineering Characteristics Of Contaminated Soft Soil Under Municipal Solid Waste Dump

Limited by the early technology and conditions,household garbage of some cities in China is stacked randomly rather than treated by sanitarty landfill,resulting in some permanent or temporary dumping sites.Dense piles of waste produce leachate and it seeps into the soil,consequently pollutes the foundation soil.In recent years,with the acceleration of urbanization,construction land was increasingly tense,a large number of researches of reusing contaminated soil have been put on the agenda,and related geotechnical engineering issues have also attracted the attention of scholars.Among them,the macro-microscopic characteristics of the soil before and after contamination and the contamination mechanism were the basic research topics.However,at present,researches of physical and mechanical properties home and abroad mainly focus on the changes physically and mechanically,while few studies were made on the change rules and mechanism of soil properties of soft soil before and after it was polluted by combining macroscopical and microcosmic methods.In this paper,NaOH,H₂SO_4,FeCl₃、Zn（NO₃）₂ were selected by analyzing the composition of municipal solid waste leachate,taking the typical soft soil in Ningbo area as the research object.Artificially prepare contaminated soil by soaking in different concentrations of pollutants,and a series of tests of physical and mechanical properties and microstructure of contaminated soil were carried out.Through these studies,it lays the foundation for the investigation and recycled of the contaminated soft soil in the field of municipal solid waste landfill.The specific research contents and experimental conclusions were as follows:（1）The material composition and micro-structure of contaminated soil and natural soil were compared and analyzed by X-ray fluorescence spectrometry（XRF）and scanning electron microscopy（SEM）.The results show that,in contaminated soils,the composite corrosion occurs mainly,and the void ratio and equivalent diameter go up with the increase of the concentration of the contaminant,but the abundance shows opposite.After soil destruction,the original structure becomes more and more irregular,among which the change of material composition and micro-structure of acid-contaminated soil is the largest.（2）Compare and analyze contaminated and natural soil through the physical property test of contaminated soil.The results show that the quality,moisture content,relative density of soil particle,liquid plastic limit and plastic index of contaminated soil all change with the contamination concentration.（3）Compare and analyse the permeability coefficient of contaminated and undisturbed soil through the test of changing head.The results show that all the permeability coefficient is increasing,and that of zinc and alkali contaminated soil increased 3-3.5 times and 4 times than that of natural soil respectively..（4）Compare and analyze the mechanical properties of contaminated and soft soil by direct shear and compression tests.The results show that alkali contaminated soil has the tendency of solidification,zinc and iron contaminated soil has no obvious change,and acid contaminated soil has the tendency of softening.（5）Through the above test results,the corrosion mechanism of contaminated soil was analyzed.The results show that the main reason for the change of physical and mechanical properties of soil was the compound reaction of soil body and polluted liquid,which leads to the change of material composition,micro-structure,combined water film and so on,and finally affects the properties of soil.（6）Through the study of the physical and mechanical properties of the contaminated soil mentioned above,this paper built up the relationship functions between shear strength and liquid property index,compression exponent and plastic index,shear strength and concentration of contaminant,and porosity ratio of soil to concentration of contaminant and consolidation pressure.The internal relation of the change of physical and mechanical properties of contaminated soil was analyzed.Thus,it can predict the change of mechanical properties of soil in engineering.