Electro-osmosis Consolidation of Soils

Abstract

Consolidation of problematic, soft-grained soils is a major concern in geotechnical engineering. Traditional hydraulic ground improvement methods, such as pre-loading and vacuum pressure, usually take a long time. In addition, using these methods is not applicable in all cases, particularly in slopes. These conventional method limitations draw engineers’ attentions to the electro-osmosis (EO) consolidation technique. In fact, applying electrical potential gradient to the soil leads to fundamental changes in its electro-hydro-mechanical (EHM) behaviour. However, some uncontrolled EO-induced chemical processes could significantly affect the efficiency of the EO consolidation technique. Although a large number of experimental and theoretical studies have been conducted in this topic, the practical application of the EO consolidation in geotechnical engineering is limited due to lack of knowledge in the field of EMH behaviour of soils, and unavailability of an accurate method to predict the expected efficiency and cost of the EO system. In this study, a laboratory-scale experimental programme along with an extensive numerical investigation is conducted to study the soil behaviour during EO consolidation and the efficiency of the process. As the electrical resistivity of soil controls the efficiency of EO process and distribution of voltage in the soil body, the experimental programme is initiated with electrical resistivity tests. The laboratory experiments are carried out in two main phases: (1) Electrical resistivity experiments; and (2) EO tests. In each phase, the required numerical investigations are carried out using FlexPDETM software, which is based on the finite element technique (FEM). In Phase (1), a new apparatus is developed and fully calibrated to consider the effect of boundary conditions and void ratio on the laboratory-measured electrical resistivity of the soil. To be able to extend the results to various specimen sizes and boundary conditions, a numerical model is developed and successfully verified by experimental data from this study. In addition, to be able to capture the spectrum of real soil behaviour, the effects of pore water salinity and level of soil sensitivity (clay content) on the electrical resistivity of soil are investigated. To achieve this, various soil mixtures at different levels of salinity and clay contents are tested in the calibrated apparatus. Then the obtained data are used to verify a proposed physical-numerical model. In Phase (2) of experiments, a new apparatus and a uniform testing framework is developed to carry out the load application and required EO parameter measurements simultaneously in a ii single cell with the same dimension as that developed in Phase (1). Then the data obtained from this phase of experiment is analysed along with the soil electrical resistivity to investigate the EHM and chemical effects as well as EO efficiency and power consumption. Finally, considering those factors, a new technique is proposed to reduce the level of chemical effects at the soil-electrode vicinity, enhance the soil-electrode contact and increase the EO efficiency. The results show that detailed investigation of electrical resistivity is necessary to design an EO consolidation scheme efficiently. It is concluded that the electrical resistivity depends on soil void ratio and clay surface conductivity. In a constant level of salinity, up to 30% increase in the electrical resistivity of the soil and consequently in the efficiency of EO consolidation is observed in New Zealand kaolin clay depending on the initial condition and volumetric strain of soil during the EO process. However, the electrical resistivity drops drastically by increasing the salinity level of the pore solution indicating very low EO efficiency at higher salinity. Furthermore, implementation of uniformly measured EO parameters, such as EO permeability, varying electrical resistivity (up to 30% variation) and time-dependent potential loss due to electrolysis at soil-electrode vicinity (up to 50% variation) into the existing EO consolidation governing equation, improves the estimation of soil behaviour undergoing EO consolidation considerably. In addition, it is found that the coefficient of EO consolidation (Cve) should be utilised rather than the coefficient of consolidation (Cv) to estimate the rate of consolidation. In addition, the proposed model is extended to the field successfully and is verified using a welldocumented field experiment. Finally, to evaluate the effectiveness of the proposed technique to decrease the level of potential loss at soil-electrode interface, a relatively large EO cell is designed, fabricated and tested. It is shown that, as the generated gas is removed from the proposed cathode vicinity, the efficiency of EO system increases noticeably. This study is the first to establish a robust, calibrated and uniform framework to measure the effective parameters affecting EO consolidation namely permeability coefficients, timedependent potential loss and variable electrical resistivity in the laboratory and incorporate them in a single model to estimate the behaviour of the soil undergoing EO consolidation in the field and laboratory, the efficiency of the process and the level of power consumption. Therefore, geotechnical engineers, researchers and scientists are able to adopt the developed framework from this study to design a more efficient EO scheme.