Electro-osmosis Reinforcement Technical Research Of Coastal Soft Soil Foundation

A mass of soft soil with high moisture content,high pore ratio,high compressibility and low shear strength is distributed over coastal area in China.For coastal soft soil foundation such as beach silt with high moisture content,usually needs drainage consolidation treatment to strengthen its shear strength for direct application or secondary treatment.Electro-osmosis method has many advantages,for instance,short duration,convenient installation and unlikely instability of the foundation,which provides a better solution for the foundation reinforcement treatment for soft clay with high moisture content and low permeability.But it still exists some engineering problems such as electrode corrosion,high energy consumption and so on.This paper summarizes the existing literature and discusses some common concepts of electro-osmosis in soft soil,including dimensional analysis for the model test.From the aspects of efficiency lifting of electro-osmosis,electro-osmotic treatment effect improving and etc.,the research on the electro-osmosis of coastal soft soil is conducted.First,the composition and particle size analyses of beach silt from Ningbo are conducted.Aiming at the common electrode surface corrosion for beach silt in electro-osmotic process,three typical electrode materials are adopted to conduct the electro-osmosis reinforcement model tests after the surface corrosion pretreatment of electrodes,namely pure aluminum,stainless steel and brass.Several aspects such as electro-osmotic drainage,moisture of soil,current,effective potential,electro-osmotic migration coefficient are analyzed in the treatment.Under the constant current condition,the electrochemical corrosion rate of pure aluminum electrodes and stainless steel electrodes kept essentially constant,which are conformed to Faraday's law.The initial electrochemical corrosion rate of the brass electrodes is generally constant,while the latter surface passivation severely reduces the electrochemical corrosion rate.Under the potential gradient of 1 V/cm,the order of the drainage efficiency of different electrode materials from highest to lowest is pure aluminum,stainless steel,and brass.The anode surface corrosion of the pure aluminum and stainless steel electrodes has little adverse effect on the electro-osmosis,which is not the main negative factor of the electro-osmotic efficiency.Brass electrodes have poor electro-osmotic effects,the surface corrosion during electro-osmosis deteriorates the drainage effect.It is suggested that those mental electrode materials whose surface corrosion deteriorates the electroosmotic effect seriously should be avoided.The interface resistance of the electrode-soil mainly comprises of film resistance caused by electrode surface corrosion and shrinkage resistance caused by the change of electrode area.Under the premise that the surface corrosion of pure aluminum has little effect on the electroosmotic effect.By changing the area ratio of different electrodes,Holm electrical contact theory is introduced to study the relationship between the electrode area and the current for pure aluminum electrodes from the viewpoint of shrinkage resistance.It is suggested fitting method can be used to determine the optimal value of the electrode area.Then,potential gradient and electrode spacing,which are usually used in electro-osmotic treatments,are adopted to conduct electro-osmotic model tests under two kinds of working conditions,that is,the variant potential gradient under equal electrode spacing and the variant electrode spacing under equal potential gradient.The test results of drainage,current and energy consumption coefficient in the process of electro-osmosis are achieved,further,the relationship between energy consumption coefficient,potential gradient and electrode spacing is studied.Under the condition of equal potential gradient with variant electrode spacing,the energy consumption coefficient increases with the enlarging of electrode spacing which subjected to a linear relationship.The energy consumption coefficient,which is deduced by the electro-osmotic flow equation considering the initial potential gradient,is in good agreement with the energy consumption coefficient in tests,Through the fitting curves,the initial potential gradient and the theoretical economic potential gradient of beach silt from Ningbo can be calculated.In practical projects where the direction of the electrodes is vertical and the flow direction is horizontal,the cross-section area of the soil and its interface resistance has no effect on the energy consumption coefficient in electro-osmotic process.It is suggested that the economic potential gradient with the lowest energy consumption coefficient of electro-osmosis should be adopted in electroosmotic treatment,while the reduction electrode spacing for energy consumption saving.Meanwhile,new inexpensive electrical geosynthetics are recommended to reduce the cost of electrode materials.Finally,the one-dimensional and two-dimensional consolidation equations of saturated soft soil are established based on some basic assumptions such as superposable hypothesis of hydraulic flow and electroosmotic flow.Further,the pore pressure and the consolidation degree for different layout of electrodes and common boundary conditions are solved.Meanwhile,the numerical simulation of different ratios of electrical permeability coefficient to hydraulic permeability coefficient is adopted to compare the different distribution of the negative excess pore water pressure in the soil.The results show that electro-osmosis method is more suitable for treating low permeability clay.Considering the pore volume change of soil in the electro-osmotic process,introducing the concept of effective drainage channel to analyze the electroosmotic drainage.For the two extreme cases,one is the pore space occupied by the initial pore water completely incompressible,another is completely compressed,the varying curves of electro-osmotic drainage are calculated such as the electro-osmotic drainage,rate,the electro-osmotic drainage rate,and the porosity of soil when considering the interface resistance.Proposing that under the condition of exists of initial potential gradient,the gradual reduction of the effective potential gradient will cause soil water unable to exhaust completely in the electro-osmotic process.Based on the results of the electro-osmotic experiments for different anode and cathode arrangements,the soil potential distribution is calculated.Combining with the experimental data of the average effective potential difference and the calculation results of soil potential distribution,it can show that expanding the anode area can significantly improve the average effective potential difference when the cathode area is constant and further promote the drainage rate.Expanding the cathode area can enlarge the drainage channel when the average effective potential difference cannot be improved significantly,so electro-osmotic drainage rate cannot be improved significantly.From the aspect of electrode material saving,it is suggested that the layout of the anode should be majorly considered in practical engineering,while the reduction of the cathode use.