| China has widely distributed natural soft soils, and lately the booming dredging and reclamation projects have generated large volumes of soft soils. These soils are high in water content and compressibility and low in permeability, and need proper disposals before any constructions. Conventional techniques, which are based on gravity consolidation, have limited effects in strengthening soft soils for the sake of low permeability. In this respect, electro-osmosis is claimed as a promising technique to reinforce soft foundations of high water content and low permeability because its performance is independent of the particle size. Therefore, electro-osmosis has received extensive attentions and researches. However, large energy consumption and lack of reasonable design and calculation theory have hindered the widespread application of electro-osmosis.Disagreements exist among preceding documents as to the electro-osmosis effects of common electrode materials (ferrum, copper, graphite and aluminum). In light of this and also aiming at improving electro-osmotic effect and efficiency, the four electrode materials were thoroughly compared from the performance and ion migration, which revealed the essential impact of electrode material on electro-osmotic process and also ion migration discipline during electro-osmosis. Considering the deficiency of design and calculation theory for electro-osmosis in available research, the relationship between electro-osmotic drainage rate and current, soil electrical conductivity and water content were study comprehensively, and a novel method to calculate electro-osmotic drainage was proposed. Main research work and corresponding results are listed in what follows.(1) Laboratory experiments using ferrum and copper electrodes were conducted to investigate the influence of anode and cathode materials on electro-osmotic effects. It is found that anode materials play dominant rules in electro-osmosis while the cathode materials have small effects.(2) Laboratory experiments using ferrum, copper, graphite and aluminum electrodes were performed under three voltage gradients. In combination with preceding literature the performance of common electrode materials and function mechanism of electrode materials were analyzed and summarized. Such results were achieved:graphite electrode occupies a better performance over ferrum and copper electrode under higher voltage gradient, while worse performance than the other two electrodes with lower voltage gradient; Copper electrode subjected to passivation and graphite electrode can be easily resolved; Most of Fe2+/Fe3+and Cu2+generated by anode reactions accumulated in anodic areas and very small part migrated to the cathodes and precipitated under the alkaline environment in cathodic areas, which demonstrated the poor migration capacity of Fe2+/Fe3+and Cu2+; Al3+has high migration ability. Most of Al3+generated by anode reactions were drained together with water and very small part precipitated in cathodic areas under the alkaline environment. It is revealed that considerable variations of electro-osmotic effect among different electrode materials mainly come from voltage loss, which is a combined result of the erosion, passivation and oxidation of the electrode and the separation from soil mass, rather than migration ability of the ions from electrode reactions.(3) Laboratory experiments using ferrum, copper, graphite and aluminum electrodes were performed and the contents of Na、K、Mg、Ca、Fe、Cu and Al monitored in electro-osmotic drainage, gravity drainage and the treated soils. It was revealed that electro-osmotic dewatering mainly relies on cations of relatively high migration ability as Na+、K+、Mg2+、Ca2+and Al3+. Electro-osmosis effect has a dependency on the transferring of water by counters of low order and small atomic mass. Based on experimental results the developments of water transported per unit charge with time and soil electrical conductivity with water content were analyzed. Water transported per unit charge was found to maintain steady during main dewatering stages, meanwhile electrical conductivity has approximate linear relationship with water content.(4) Systematic summary was conducted with regard to the influence of soil type, water content, voltage gradient and electrode lay-out on water transported per unit charge. The linearly and proportionally development of drainage rate with current was highlighted despite of the soil type. Water transported per unit charge differed for different soils. For the same soil, Water transported per unit charge is mainly subjected to impacts of water content and salinity rather than electrode lay-out and voltage gradient.(5) Systematic summaries were performed as to the theoretical models and empirical formulas of electrical conductivity and water content in the existing researches. Three relationships were put forward, namely linear, exponential and quadratic. Considering soil property (electrical conductivity and water content) changes during electro-osmosis, a novel method to calculate one-dimensional electro-osmotic drainages was proposed based on the relationship between electro-osmotic drainage rate and current, soil electrical conductivity and water content. Results from the new method and the conventional methods upon Esrig theory were compared together with the laboratory experiments. The new method was demonstrated more accurate in estimating electro-osmotic drainage than the conventional method.(6) The influence of loading on water transported per unit charge and electrical conductivity were studied. The relationships between electro-osmotic drainage rate and current, soil electrical conductivity and water content were obtained. Furthermore, one-dimensional electro-osmotic drainage under loading was deduced based on electrical conductivity. Based on soil conductivity formula for axisymmetric-arranged electrodes, axisymmetric electro-osmotic drainage under loading was derived, which was verified by experimental data. Computing and experimental results showed good consistencies. Two-dimensional electro-osmotic drainage under loading was also deduced based on electrical conductivity.(7) Engineering application of the novel method to calculate electro-osmotic drainage by electrical conductivity was studied. Electro-osmotic drainage and soil shrinkage were analyzed. Magnitude of the former is larger than the latter. Difference between these two can be reduced by loading. The conservatism of estimating soil shrinkage by electro-osmotic drainage was thus emphasized. Reasonable advices about the design of involved parameters were also given as well as application route and condition of the method. The objective was to provide guidance for engineering application. |
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