Experimental Study On Evaluation Of Heavy Metal Contaminated Soil By Thermal Conductivity Method

During the urbanization of China,risk control of contaminated sites is an important issue for sustainable urban development.It's an important subject of environmental geotechnical engineering that researching on the engineering characteristics and related physical and mechanical indicators of contaminated soil.This dissertation is under the financial support of the Key Project of Nature Science Foundation of China(Grant NO.41330641).Typical soils of cohesive soil and silty clay soil in Nanjing were used in this research.And hydroxyapatite(HAP)was used to solidify the contaminated soil.In this dissertation,the thermal conductivity and physical properties of contaminated soil before and after being polluted and solidified were researched in detail.Besides,an evaluation system of heavy metal contamination site is built based on the Analytic Hierarchy Process,so the theoretical research on heavy metal contaminated soil has been improved.The main research contents and results are as follows:(1)The heat transfer characteristics and variation laws of heavy metal contaminated soil were systematically studied.Through laboratory experiments,the effects of pollution concentration,moisture and dry density on the thermal conductivity of heavy metal contaminated soil were researched.The results show that the thermal conductivity of contaminated soils increases linearly with the increase of pollution concentration.Also the thermal conductivity increases in exponential function with the increase of moisture content,and it increases linearly with the increase of dry density.Under the same conditions,the thermal conductivity of zinc-contaminated soil is greater than that of lead-contaminated soil.The order of the impact degree is pollution concentration,dry density and moisture.After the thermal conductivity of the contaminated soil is normalized by the thermal conductivity of the uncontaminated soil in the same state,there is a linear relationship between the normalized thermal conductivity and the pollution concentration.(2)Through laboratory experiments,the effects of pollution concentration and pollutant type on microstructure and physical mechanical index of heavy metal contaminated soil were studied,and the relationship between thermal parameters and physical and mechanical parameters was established.The results show that after pollution,the soil structure appears to be agglomerated,and the number of pores increases.Besides,the effective contact area between the particles decreases,and the skeleton structure is broken.As the increase of contaminated concentration,the unconfined compressive strength of the soil decreases,the clay content decreases,and the total soluble salt increases.The changes in the microstructure and indicators of zinccontaminated soil are more obvious.The fitting relationship between thermal parameters and physical mechanical parameters is good,so the changes of soil engineering properties can be judged by measuring thermal conductivity.(3)The evaluation system for heavy metal contamination site is built based on the Analytic Hierarchy Process.In this system,the guidelines layer includes site pollution parameters,physical mechanical parameters,and soil variation parameters,and the index layer includes contaminant content,type of pollutant,type of site,thermal conductivity,unconfined compressive strength,clay content,and total soluble salt.(4)Through laboratory experiments,the effects of curing agent content and curing age on the microstructure,compressive strength and thermal conductivity of hydroxyapatite solidified heavy metal contaminated soil were systematically researched.The test results show that hydroxyapatite has obvious improvement on the soil structure of heavy metal contaminated soil.As the increase of curing agent content and curing age,the unconfined compressive strength gradually increases and the thermal conductivity decreases.Besides,hydroxyapatite has better repair effect on Pb contaminated soil.