Experimental And Theoretical Study On The Mechanism Of Electroosmotic Reinforcement For Soft Clay Foundation

Soft soils which are wildly located through coastal region and dredged mud produced through dredging and land reclamation engineering are usually characterized by high moisture content,high compressibility,low permeability and low shear strength.The pretreatment is often required before engineering application while the conventional foundation treatment methods are time-consuming,even fail to achieve the expected purpose.Electro-osmosis method is considered to be an efficient method to treat soft soil foundation with high water content and low permeability,since its drainage rates are independent of its pore size distribution.However,there are still some shortcomes which restrict its widespread application,for instance,the mechanism is not quite clear and the difference between the theoretical and actual value are non-negligible.On the basis of previous studies,this paper conducts the researches about the electroosmotic reinforcement of soft soil foundation both on the macroscopic and microscopic level.First,the techniques and principles for studying the microstructure of cohesive soils are summarized.Following the principles,three indoor experiments focusing on the microstructure of soil under electro-osmosis are conducted.The first experiment compared the performance of copper and pure aluminum as electrodes materials,mainly from the aspects of electroosmotic drainage,energy consumption,SEM image surface characteristics and pore distributions before and after treatment.After the treatment,the clay structure changed from cellular-frame structure to aggregate flocculation structure and the soil particles are connected from edge-to-edge or edge-to-face into face-to-face.Pores with diameters of 0.1?1 ?m take up the majority.The second experiment deals with the changes of soil's microstructure during electroosmosis.The test results indicate that soil microstructure is greatly affected by the moisture content which is highly related to the treatment period and the distance to the water outlet.Quantitative analysis of SEM images of anode soil shows that electroosmosis has no regular influence on the directional fractal dimension and probability entropy.Therefore,the effect of electroosmotic treatment on the directional distribution of soil pores is not obvious.The third laboratory test explained the mechanism of electroosmosis combined with chemical grouting to strengthen soil foundation from the microscopic perspective qualitatively and quantitatively.The results show that the bearing capacity of soil surface is negatively correlated with the fractal dimension of soil porosity.Then,based on the observation in the experiments above that soil changes from saturated state to unsaturated state,a one-dimensional electroosmotic drainage test is conducted to systematically study the relationship between electroosmotic permeability coefficient and saturation.When soil is saturated,the measured electric permeability coefficient is usually one order of magnitude smaller than the predicted value by H-S theory,the reason could be attributed to the fact that the theory overestimate the driving force applied on the water flow and neglect the force applied on the surface charge by the remaining ions in the solutions.When the soil saturation interval is from 50%to 100%,there is an approximate power function between the electric permeability coefficient and the saturation Sr,Which can be expressed as ke,rel=a(Sr)b,in which a and b are the fitting parameter related to soil properties.The electropermeability coefficient of unsaturated soil is related to the pore distribution of soil.The smaller the pore size,the faster the value of electric permeability coefficient decreases when the soil saturation drops.In order to compare the electric permeability coefficient of different soil,this paper proposes the standard procedure for laboratory determination.An improved Miller Soil Box along with clintheriform stainless steel as electrodes material is adopted,and a voltage gradient near 1 V/cm is applied.The initial moisture content should be liquid limit or slightly higher than the liquid limit.The measurement period should be at the early stage,that is,the period when the drainage rate has not decreased sharply.Then,one-dimensional and two-dimensional electroosmotic consolidation theory for saturated condition are established based on the principle of effective stress and the assumption that the flow caused by water head difference and electric potential difference can be superimposed linearly.The analytical solutions for pore pressure and degree of consolidation are obtained given the boundary conditions which is free drainage at cathode and impervious at anode.In addition,fnite element software,Comsol is used to simulate the two-dimensional process and the improper boundary conditions can be fixed.By comparing the pore pressure distribution under different ratio of electric and hydraulic permeability coefficient,the results show that electro-osmosis method is more suitable for the treatment of clay whose ke/kh is greater than or equal to 10.Finally,considering the fact that the majority of soil in the nature world is highly saturated soil,the one-dimensional electroosmotic consolidation equation for high saturation soil among which the pore fluid is regarded as a mixture compressible fluid is derived through combining the mass continuity equation and effective stress formula of unsaturated soil.The example results indicate that the degree of consolidation of highly saturated soil is only slightly slower than that of saturated soil,but the final pore water pressure is significantly lower than that of saturated soil.On the other hand,the moisture transport equation in unsaturated soil is derived based on darcy's law,ohm's law and mass continuity equation.The calculated results are in good agreement with the actual data in the literature,which proves the practicability of the equation.