Evaluation And Application Of Characteristics Of Expansive Soil And Polluted Soil Based On Electrical Index

The rapid evaluation of heavy metal contaminated soil and expansive soil is a technical problem in the current environmental assessment and engineering construction in China.Exploring the evaluation technology of special soil,which is fast,efficient and economical,is one of the urgent needs to be solved in the current environmental geotechnical engineering community.The electrical parameter is the basic conductive properties of the soil;furthermore,it can also represent the microstructures and concentration of heavy metal ions in the soil.An evaluation and application method of expansive soil and contaminated soil based on electrical parameters is proposed in this dissertation.The study of electrical and engineering properties of heavy metal contaminated soil and expansive soil is intended to provide a scientific basis for the application of this new technology.Based on this background,under the financial support of the Key Project of Natural Science Foundation of China?Grant No.41330641?,the "Twelfth Five-Year" National Science and Technology Support Plan?Grant No.2012BAJ01B02?,and the Research Innovation Project of Ordinary University Graduate Student of Jiangsu Province?Grant No.KYLX15₀140?,the postgraduate program of high-level university of national construction?201606090143?,the natural or artificial heavy metals contaminated soil and expansive soil were used in this study.Targeting engineering property evaluation and application,the mechanism and electrical properties of heavy metals contaminated soil and expansive soil were studied by laboratory tests,theoretical analysis,and model tests.The main works and conclusions of this dissertation are shown as following:?1?The electrical test methods and mechanisms of soil were systematically summarized,the physical and mechanical properties,electrical resistivity and engineering estimation methods of heavy metals contaminated soil and expansive soil are analyzed.The existing problems are critically reviewed.?2?The variation in the electrical properties of cohesive soil are thoroughly studied.The effects of cohesive soil state parameters on electrical parameters were studied by low frequency four-electrode conductance test technique and high-frequency dielectric reflectance test technique.The effective medium double water?EMDW?model was proposed based on the double water structure and self-similarity principle.In addition,the feasibility and advantages of the effective medium dual-water model were analyzed.The results indicate that the resistivity decreases with the increase of water content,density and saturation of cohesive soil,while the real dielectric constant and resistivity increase with the increase of porosity.Due to the existence of electric double layer structure on the surface of soil,there is a significant influence on the overall conductance characteristics of cohesive soil.The new proposed model can effectively describe the overall conductance characteristics of clay soil.?3?The variation in the physicochemical,mechanical properties and microstructures of expansive soils with different expansion characteristics are systematically studied.An evaluation method of expansion characteristics based on electrical parameters was proposed,furthermore,the indoor resistivity CPT model test was used to analyze the application of electrical indicators in in-situ evaluation.The results indicate that the liquid limit,plasticity index and clay content increase with the increase of soil swelling performance,in addition,the expansion performance of artificially expanded soil increases with the increase of bentonite content.As the proportion of bentonite increases,the soil specific surface area and water-saturation capacity increase,which resulted in an increase in soil swelling performance.It is feasible to evaluate the expansion properties of expansive soil using electrical parameters,due to the sensitivity of electrical parameters in soil microstructure.The applicability of electrical evaluation models has been verified by the indoor resistivity CPT model test.?4?The changes of soil physicochemical,microstructure and chemical composition of heavy metals contaminated soil were analyzed by a series of micro-chemical analysis including scanning electron microscopy?SEM?,energy dispersive spectroscopy?EDS?,X-ray diffraction?XRD?,mercury intrusion analysis?MIP?and related physical and mechanical tests.The results indicate that there is an obvious agglomeration of soil particle in microstructure after the intrusion of heavy metal ions.The shear properties of heavy metal contaminated soils are improved,due to the shrinkage of combined water on the surface of cohesive soil particles,which was caused by the adsorption of heavy metal ions on the surface of soil.Increasing concentration of heavy metal polluted soil caused a decrease in liquid limit,plasticity index and clay content,while the plastic limit remained stable.?5?The physicochemical,mechanics and microscopic characteristics of heavy metal contaminated clay were analyzed based on electrical parameters?conductivity and dielectric constant?.Introducing the particle surface conductivity to analyze the correlation between electrical conductivity and physical properties.Furthermore,an evaluation method of contaminated soil for the engineering properties based on electrical parameters was established.The results indicate that there is a linearly correlation between the logarithmic function of the surface conductivity and the clay particle content C,the plasticity index PI and the pH value.The normalized cohesive force and friction angle parameters linearly increase with the resistivity.The wet density,volumetric water content and the initial uncontaminated soil resistivity were used to establish a prediction equation for the pollution degree of heavy metal contaminated soil.The real and imaginary parts of the dielectric constant of metal-contaminated soil and polymer-modified soil can form a Cole-Cole circles,in addition,a similar research results are obtained in three-dimensional space.?6?A model test apparatus is designed to conduct the electrical resistivity cone penetration test?RCPT?in heavy metal contaminated soils with different pollution concentrations,compactness and water contents.The feasibility and superiority of the resistivity evaluation method of heavy metal contaminated soil were demonstrated by the comparison between the low frequency four-electrode conductance test and the electrical resistivity cone penetration test.The results indicate that the resistivity decreases with the increase of water content of heavy metal contaminated soil at constant concentration.Furthermore,the resistivity decreases with the increase in the compactness of heavy metal polluted soil,which means the decrease in porosity of soil.It is shown that there is a decreasing trend in the resistivity with the increase of heavy metal ion concentration in the case of constant water content.At the same time,the measured resistivity of resistivity CPT test and the indoor low-frequency four-electrode conductance test are basically similar.?7?Using the analytic hierarchy process as the research method,combined with the three-step analysis method of hazard identification,risk characterization and site engineering characteristics,the heavy metal contaminated site evaluation system are established by the three criteria layers of soil characteristic parameters,physicochemical parameters and electrical pollution parameters.The given various index parameters of the established evaluation system can characterize heavy metal contaminated sites according to the correlation.In addition,the importance scale of each index was analyzed and the proportion weight of each index through the judgment matrix was calculated.Finally,the index of site contamination SQ is calculated by weighted calculation,the states of contaminated sites are estimated by the index of site contamination SQ.The results obtained from laboratory test and model investigation indicate that the resulting evaluation as per the proposed evaluation system is consistent with the real condition,which proved that the evaluation system built in this study has the value of application.