Soft Soil Improvement By Electro-osmosis Techniques

Electro-osmosis is an efficient technique for the dewatering and consolidation of soft soil with high water content and low permeability. Based on the state of the art literature review,this study conducted model experiments to further investigateboth the macroscopic and microscopic phenomena of electro-osmosis in soils, and to understand the fundamental mechanisms for electro-osmotic improvement of soils. A theoretical model for electro-osmotic consolidation and analytical solutions were developed to describe the electro-osmotic consolidation process, and numerical simulations were carried out to analyze practical engineering situations and provide suggestions for the application of this technique.A testing system for electro-osmotic consolidation was established, which incorporate model testing facilities for one-dimensional and axisymmetric scenario, real-time monitoring system for electro-osmosis process, and laboratory testing procedure for macroscopic and microscopic properties of soils before and after electro-osmosis. The water content, Atterberg limits, free swelling ratio, zeta potential and cation exchange capacity were measured to analyze the impact on the physico-chemical properties. Moreover, the microscopic characteristics, including microfabric, chemical and mineralogical composition, wereinvestigated to understand the microscopic mechanisms ofelectro-osmosis.The results of the model tests indicated that the initial water content and electrode material presented significant influence on the electro-osmosis process, and the physical and chemical properties of tested soil changed after the treatment. The tests on a kaolinite illustrated that the final discharged water portion increasedwith the increase of the initial water content,and the intermittent current technique had better effect with higher initial water content. The tests on a sodium bentonite demonstrated that reactive electrodes induced better drainage effect than inert electrodes, and the plasticity index and free swelling ratio of soil samples in the vicinityof the anode decreased remarkably after electro-osmosis, indicating that electro-osmosis could reduce the swelling and shrinkage potential of bentonite.The microfabric, chemical and mineralogical composition encountered significant change after electro-osmosis, whichfurther altered the physical and chemical properties of the soil tested. Upon electro-osmosis treatment, the microfabric of the bentonite samples in the vicinity of the anode changed from flocculated fabric to aggregate fabric, resulting in a smaller void ratio and a denser state. The sodium ions in the electrical double layer and between the clay lattices were substituted by multivalenceions, causing the compression of the electrical double layer and the increase of the attractive forcebetween clay lattices. The change in microfabric,electrical double layer and clay lattice characteristics then resulted in a decrease in the water-absorption capacity, and therefore the decrease in the plasticity index and free swelling ratio. The test results also indicated that thesodium ion was the majorcation that carried the pore water to the cathodeunder electric field by electromigration.The multi-field coupling theoretical model for electro-osmotic consolidation was developedwith non-linear variation of the mechanical, electrical and hydraulic parametersof soils.Based on the simplified axial symmetric model, the analytical solutions for pore water pressure and degree of consolidation were derived, and a simplifiedformula for settlement was proposed.The relationship curves among electrode space, settlement and electro-osmosis parameters were providedto facilitateengineering design. Based on the theoretical model, a numerical simulation program was developed to analyze electro-osmosis problems with complex electrical field and boundary conditions. The effectiveness of the numerical model wasverified by comparingwith the analytical solutions and experiment results.Engineering cases of electro-osmotic consolidation were simulated, and the results indicated thatthe tempo-spatial distribution of pore water pressure, soil displacements,as well as stresses and strains during electro-osmotic consolidation could be predictedreasonablywith the developed numerical model.