Studies On The Electric Field Mechanism And Multi-field Coupling Theory Of Electro-osmosis In Soft Soils

With the economic development of China and the urgent need of building harmony between human beings and nature,the dewatering problems of soft soil,dredged silt,sludge and tailings are presented to geotechnical engineers.Although the natural soft soils,silt sludge and mine tailings are from different sources,they all exhibit the geotechnical characteristics of high water content,poor permeability and high compressibility.The drainage consolidation methods based on hydraulic osmosis are often limited by the hydraulic permeability coefficient and tend to be failed.The drainage rate of electro-osmosis method is not controlled by the hydraulic permeability coefficient and is highly efficient especially for fine-grained soils;and electro-osmosis is regarded as a promising technology by many engineers because of its ability to remove heavy metal contaminants from the soil to a certain extent.However,the existing problems of high energy consumption,unclear mechanisms,lack of electric field research and chaotic energization schemes limit the further extension of electro-osmosis.Considering the shortcomings of existing electro-osmotic permeability model that cannot take into account the electric field,from the study of the electrical mechanism in electroosmosis,a new model that involves ions migration and current density is developed in this thesis.To solve the shortcomings of existing interfacial resistance model which cannot take into account electrodes reactions,a new model is developed firstly from the electrodes reactions in this thesis.Based on the divergent reports of energization methods in the literature,this thesis explains the mechanism of intermittent current and electrodes reverse through theoretical analysis and experimental verification,and suggests using the index of interfacial resistance to evaluate the effectiveness of reverse and intermittent.Based on the electro-osmotic permeability model and interfacial resistance model established in this thesis,a multi-field coupled consolidation calculation method is proposed that can take the electric field real-time changes into account.The main work and corresponding results of this thesis are as follows:(1)Based on the Nernst-Planck equation and H-S theory,an electro-osmotic permeability model that can consider the electric field was developed from the perspective of ions migrations.By comparing with the results of several tests,it is proved that the calculation results of the proposed model are more accurate and can explain the existence of the optimum salt content more reasonably.(2)Considering that existing interfacial resistance models are based on Holm’s electrical contact theory,which is applicable to electronic contact elements but not to electrode-clay interfaces.In this thesis,the calculation method of interfacial resistance are derived in detail from the electrode reaction,and the general expression of interfacial resistance is established theoretically.The interfacial resistance of metallic Fe,Cu and Zn electrodes as well as nonmetallic EKG electrodes was measured by rigorous chamber tests,which showed that current density is the most important factor affecting interfacial resistance,other influencing factors include electrode material,ions concentrations,etc.The interface resistance general expression developed in this thesis has a high fitting coefficient to the test results and can explains the concept of starting voltage.(3)This thesis explains the mechanism of electrodes reverse from the view of interfacial resistance.And by designing tests,it has been demonstrated firstly that the accumulated water discharge of EKG electrodes reverse can also exceed that of regular continuous current with reasonable reversal parameters.It was further demonstrated by constant current tests that interfacial resistance changes are an important mechanism of electrodes reverse.This thesis demonstrates experimentally that reasonable reverse parameters are helpful for electroosmotic drainage and further demonstrates the practical value of the interfacial resistance model.(4)This thesis analyses the mechanism of intermittent current from theory and validates it by designing experiments.By measuring the current,the interfacial resistance and the drainage of EKG electrodes at different intermittent current parameters,the mechanism of intermittent current was demonstrated to be mainly:1)The product ions on the electrodes surface can migrate and the reactant ions are replenished during intermittent;2)The cations that accumulate near the cathode during the intermittent time continue to attract water molecules towards the cathode and are expelled,resulting in a longer drainage time.(5)Based on the variation rules of interfacial resistance measured in intermittent current and electrodes reverse tests,the interfacial resistance is proposed as an evaluation parameter to evaluate the effectiveness.The interfacial resistance can evaluate the interface between electrodes and soil in real time,with timely feedback,high repeatability and accuracy.(6)A multi-field coupled consolidation theory that takes the electric field into account has been developed,and remedies the deficiencies of existing electro-osmotic consolidation theory that cannot adequately consider the electric field.This multi-field coupled consolidation theory links the electric field and osmosis fields through the electro-osmotic permeability model based on ion migration.The electric field changes caused by the interfacial resistance and soil resistance are incorporated into the coupling calculation process,and a true coupling of the electric field-osmosis field-stress-strain field is achieved.