Electro-osmotic Consolidation Research Articles

Elastoplastic numerical approach for predicting the electro-osmotic consolidation behaviour of soft clays

A finite element (FE) formulation for electro-osmotic consolidation (EOC) analysis is developed in this paper which was implemented in a FE analysis (FEA) program. This formulation is integrated with the elastoplastic Modified Cam Clay (MCC) model to capture the nonlinear stress–strain behaviour of the soil during EOC using the fully coupled analysis approach. The numerical model is verified against triaxial EOC test data and close form solutions of typical one-dimensional EOC problems. Details of the formulation and the results of the validation analyses are reported in this paper.

Analytical solution for axisymmetric electro-osmotic consolidation

Electro-osmotic consolidation is a potential method for soil improvement. An axisymmetric electro-osmotic consolidation model with coupled horizontal and vertical seepage is proposed and the analytical solution is derived without the equal strain hypothesis, which was used in previous models. The proposed analytical model can predict the distribution of pore-water pressure in the radial direction, and comparison with previous analytical models indicates that the degree of consolidation is overestimated if the model is simplified from an axisymmetric model to a planar model. A design chart is developed to facilitate the evaluation of applying electro-osmotic consolidation for soil improvement.

Finite-difference model for one-dimensional electro-osmotic consolidation

Small strain consolidation theories treat soil properties as being constant and uniform in the course of consolidation, which is not true in the case of electro-osmosis-induced consolidation practices. Electro-osmotic consolidation leads to large strain, which physically and electro-chemically affects to a non-negligible extent the nonlinear changes of the soil properties. For the nonlinear changes, iterative computations provide a mathematical approximation of the soil consolidation when the time steps and spatial geometry are intensively meshed. In this context, this paper presents a finite-difference model, EC1, for one-dimensional electro-osmotic consolidation, and this model is developed based on a fixed Eulerian co-ordinate system and uses a piecewise linear approximation. The model is able to account for the large-strain-induced nonlinear changes of the physical and electro-chemical properties in a compressible mass subjected to electro-osmotic consolidation and to predict the consolidation characteristics of the compressible mass. EC1 is verified against exact analytical solutions and test results obtained from an experimental program. Example problems are illustrated with respect to the numerical solutions of large-strain electro-osmotic consolidation.

Non-mechanical behaviors of soft clay in two-dimensional electro-osmotic consolidation

To investigate the soil behaviors in a direct current field on both spatial and temporal scales, a 1: 5 scale model test was conducted in laboratory to simulate the two-dimensional (2D) electro-osmotic consolidation of soft clay foundation. Volume of drainage, intensity, voltage, water content and pH value of water collected in the cathodes were monitored. The pH values of soil and the mass of anodes were measured before and after the test. The test results indicate that the unsaturated state, resultant from fissures induced by the differences in water contents, is favorable to dynamic compaction of soil during electro-osmotic drainage. The results also demonstrate that water content, degree of saturation and electric potential distributions can be used to deduce the electro-osmotic drainage process. Water content of soil decreases first near electrodes, while keeps nearly constant in the center of the model. The area with constant water content is larger than half of the sample surface. Moving anodes towards cathodes by about one third of the electrode spacing is effective to improve the treatment effect after electro-osmosis stops due to the large resistance. Moreover, it is observed that during electro-osmosis, the corrosion rate of anodes becomes smaller, while the variation in pH values of soil near anodes becomes larger.

Numerical model of electro-osmotic consolidation in clay

Electro-osmotic consolidation is a unique soil improvement technique. A physics-based model for electro-osmotic consolidation is proposed, in which Biot's consolidation equation is coupled with the electro-osmotic flow and control equation for an electrical field, and the displacement of soil mass and pore-water pressure can be obtained. Furthermore, the non-linear variation of the mechanical and electrical properties of the soil mass during consolidation is incorporated into the model. The numerical solution agrees well with the analytical solution. The developed model can predict the ground displacement and provide useful data for the design of an electro-osmosis treatment system.

Electro-Osmotic Consolidation Experiments on an Organic Soil

A series of electro-osmotic consolidation experiments were carried out on an organic soil from the central Sarawak region of Malaysia. Commercially available EVD was used to induce electro-osmosis and drainage of pore water. The influence of the method of drainage and pumping interval of drained water on electro-osmotic consolidation was investigated. The combination of electro-osmotic consolidation and chemical stabilization techniques was investigated by installing lime or cement columns near the anode end of the test bed. The binder factor in the lime and cement columns was varied. The paper presents the experimental set-up, test procedures, experimental program, results and conclusions of the study. Electro-osmosis, using EVDs, was effective in the electrokinetic treatment of the organic soil. The two types of drainage, namely drainage at bottom and drainage tubes, were effective. Electro-osmotic consolidation improved with decrease in pumping interval. The lime and cement columns, used in combination with electro-osmosis, increased the undrained strength further.

Electro-Osmotic Consolidation Studies on Peat and Clayey Silt Using Electric Vertical Drain

Peat and soft soil deposits are abundant in Malaysia. Studies using electrokinetic geosynthetics (EKG) such as prefabricated electric vertical drains (EVD or ePVD) have shown the potential for the application of these materials in electrokinetic strengthening of soft clays and increasing solids content in mining tailings and sludges. There are no studies reported on the effectiveness of electro-osmosis using EVD in peat. Laboratory experiments were carried out to investigate the effectiveness of electro-osmotic stabilization of two peat samples and a slightly organic clayey silt sample, from the North Sarawak region. Commercially available EVD was used to induce electro-osmosis and drainage of pore water. The influence of voltage gradient, organic content, initial water content, anode–cathode configuration and presence of roots on electro-osmotic consolidation was investigated. Electro-osmotic consolidation using EVDs was effective in the improvement of peat, over a large range of initial water contents. An increase in the voltage gradient generally improved consolidation. The maximum voltage gradient for optimum results appeared to be in the region 120 V/m. The 2anodes-1cathode configuration performed better than the 1anode-1cathode configuration. The presence of roots reduced the electro-osmosis effect.

Interface Electric Resistance of Electroosmotic Consolidation

There will be transition zones of electric current near the electrodes, if the electric conductive area of electrodes is smaller than that of soil. Electroosmosis tests show that the electric current in the transition zones follows a complicated two-dimensional path, while the electric current outside these zones is approximately one dimension. The thickness of transition zones is potty compared to the whole thickness of soil between anodes and cathodes. Conception of interface resistance on zero thickness interfaces, which is a simplified expression for finite thickness transition zones, is presented in this paper to simplify the two-dimensional problem within the transition zones into one dimension. Studies show that the interface electric resistance is inversely proportional to the ratio of electric conductive areas between electrodes and soil. A brief formula is deduced to predict the in situ interface electrical resistance, which presents a more accurate estimation of electric current and energy consumption to the design of electroosmotic consolidation engineering.

A case record of electroosmotic consolidation of soft clay with improved soil–electrode contact

Electroosmotic (e.o.) consolidation of clays has been known in geotechnical engineering for many decades but is in fact seldom used. The analysis of published data from case records indicates that the power losses at the soil–electrode contact constitute a major problem for the field application of e.o. consolidation. A laboratory study, where field conditions were reproduced, has demonstrated the efficiency of a chemical treatment at the electrodes to enhance potential transfer to soil. The chemical treatment was evaluated in a large field demonstration test where over 700 m3of clay was consolidated by electroosmosis. The paper describes the site, the field setup, and the results and analysis of the monitoring during the 48 days of the e.o. treatment. The results of the post-treatment geotechnical investigation after 12% of clay compression due to electroosmotic consolidation are also presented and discussed. These results confirm that soft clay deposits can be successfully treated by e.o. consolidation, at a competitive cost compared with other alternatives, when power losses at the soil–electrode contact are controlled.Key words: case record, electroosmosis, soft clays, consolidation, soil improvement.

Coupling of electrochemical and mechanical processes in soils under DC fields

Direct current (DC) electric fields have been applied in several geotechnical and geoenvironmental engineering applications, including electro-osmotic dewatering and consolidation, ion injection and contaminant removal. Such applications cause electrochemical effects in the soil, leading to changes in the soil’s physical, chemical and mechanical properties. Some of these changes have been evaluated extensively (such as those occurring under electro-osmotic consolidation) based on the assumption that electrochemical conditions are uniform between the electrodes. However, recent studies have shown that nonlinearity in these conditions develop between the electrodes leading to changes in the chemical, physical and mechanical properties of the soil. This paper evaluates the nonlinear effects on these properties as measured in some specific applications. In particular, the application of DC fields to soft soils is shown to alter the soil in both its mechanical and physical properties. The soil’s undrained shear strength is increased by interparticle cementation brought about by the electrochemical injection of ions into the soil under the DC field. In addition, soils can consolidate or swell under these fields. In both of these cases, the development of a nonlinear voltage distribution across the soil is clearly measured, and can affect the progression of any electrochemical soil treatment as well as the accuracy with which it is modeled.

Electro-Osmotic Consolidation of Soft Bangkok Clay Using Copper and Carbon Electrodes with PVD

Abstract Electro-osmotic consolidation of Bangkok clay using copper and carbon electrodes with prefabricated vertical drain was studied. A laboratory testing program was conducted on undisturbed and reconstituted samples in a small cylinder cell and a large consolidometer in order to assess the probable effectiveness of electro-osmotic treatment. The tests were performed under the voltage gradients of 60 and 120 V/m with a polarity reversal of every 24 h. The time to achieve 90% degree of consolidation induced by electro-osmosis ranges from 1.4 to 2.1 and 1.2 to 2.2 times faster than the normal consolidation using PVD only for undisturbed and reconstituted samples, respectively. The faster rate of consolidation and higher magnitude of settlement were achieved at a higher voltage gradient. Higher reduction of water content up to 9% and increase in shear strength up to 144% were obtained using electro-osmotic consolidation with PVD compared to using PVD only, especially when using the carbon electrode. The liquid limit, plastic limit, and plasticity index were increased due to increased salinity during electro-osmotic consolidation. With its total dissolved salts of 4050 ppm well below the 6000 ppm limit, the soft Bangkok clay is considered to be suitable for electro-osmotic consolidation.

Feasibility assessment of electro-osmotic consolidation on marine sediment

A procedure is developed for the feasibility assessment of electro-osmotic (EO) consolidation of geomaterials, including fine-grained soils, sediments and mine tailings containing clay minerals. The assessment is based on a series of parametric studies to examine the effectiveness and efficiency of the electro-osmotic process in a site-specific geomaterial. The feasibility study on a natural marine sediment recovered from the seabed of the Korean coast is presented to illustrate steps involved in the assessment. In particular, the effects of soil pore fluid salinity and void ratio on electro-osmotic consolidation using intermittent current are examined. The results show that the effectiveness of electro-osmotic consolidation, assessed by the EO permeability ratio, increases when the soil void ratio decreases, whereas it is not significantly affected by the pore fluid salinity. The study also illustrates that the power consumption in an electro-osmotic consolidation process is predominantly controlled by the electrical conductivity of the soil, which is a function of the soil pore fluid salinity and void ratio. Une procédure est développée pour l'évaluation de la faisabilité de la consolidation électro-osmotique (EO) des géomatériaux incluant les sols granulaires fins, les sédiments et les résidus miniers. L'évaluation est basée sur une série d'études paramétriques pour examiner l'efficacité et l'efficience du processus électro-osmotique dans un géomatériau spécifique. On présente l'étude de l'efficacité sur sédiment marin naturel provenant d'un fond marin de la cote coréenne pour illustrer les é tapes comprises dans l'évaluation. On examine, en particulier, les effets de la salinité de l'eau interstitielle et de l'indice des vides sur la consolidation électro-osmotique en utilisant le courant intermittent. Les résultats montrent que l'efficacité consolidation électro-osmotique, évaluée par le rapport de permeéabilité EO, augmente quand l'indice des vides du sol décroît tandis qu'elle n'est pas affectée de façon significative par la salinité de l'eau interstitielle. L'étude illustre aussi que la consommation de puissance dans un processus de consolidation électro-osmotique est principalement contrôlée par la conductivité é lectrique du sol qui est une fonction salinité de l'eau interstitielle et de l'indice des vides.

Improvements of electroosmotic consolidation of soft clays by minimizing power loss at electrodes

Although it appears promising in many laboratory studies, electroosmotic consolidation of soft clays is seldom used in practical applications. A study of well-documented case records of field applications illustrates the difficulty of transmitting the electrical potential to the soil deposit through the use of electrodes. An important amount of the electrical potential is indeed lost at the soil–electrode contact. The loss of potential at this point has been studied in a laboratory setup designed to reproduce the conditions existing in the field during electroosmotic consolidation. The laboratory results confirm that only about a third of the electrical potential applied at the electrodes is effectively transmitted to the soil and available for electroosmotic consolidation. Moreover, the laboratory studies have shown that a chemical treatment to increase the soil conductivity at the soil–electrode contact can double the electrical potential effectively transmitted to the soil and greatly improve the performance of electroosmotic consolidation.Key words: electroosmosis, soft clays, consolidation, potential loss, electrode treatment, soil improvement.

Feasibility assessment of electro-osmotic consolidation on marine sediment

A procedure is developed for the feasibility assessment of electro-osmotic (EO) consolidation of geomaterials, including fine-grained soils, sediments and mine tailings containing clay minerals. The assessment is based on a series of parametric studies to examine the effectiveness and efficiency of the electro-osmotic process in a site-specific geomaterial. The feasibility study on a natural marine sediment recovered from the seabed of the Korean coast is presented to illustrate steps involved in the assessment. In particular, the effects of soil pore fluid salinity and void ratio on electro-osmotic consolidation using intermittent current are examined. The results show that the effectiveness of electro-osmotic consolidation, assessed by the EO permeability ratio, increases when the soil void ratio decreases, whereas it is not significantly affected by the pore fluid salinity. The study also illustrates that the power consumption in an electro-osmotic consolidation process is predominantly controlled by the electrical conductivity of the soil, which is a function of the soil pore fluid salinity and void ratio. Une procédure est développée pour l'évaluation de la faisabilité de la consolidation électro-osmotique (EO) des géomatériaux incluant les sols granulaires fins, les sédiments et les résidus miniers. L'évaluation est basée sur une série d'études paramétriques pour examiner l'efficacité et l'efficience du processus électro-osmotique dans un géomatériau spécifique. On présente l'étude de l'efficacité sur sédiment marin naturel provenant d'un fond marin de la cote coréenne pour illustrer les é tapes comprises dans l'évaluation. On examine, en particulier, les effets de la salinité de l'eau interstitielle et de l'indice des vides sur la consolidation électro-osmotique en utilisant le courant intermittent. Les résultats montrent que l'efficacité consolidation électro-osmotique, évaluée par le rapport de permeéabilité EO, augmente quand l'indice des vides du sol décroît tandis qu'elle n'est pas affectée de façon significative par la salinité de l'eau interstitielle. L'étude illustre aussi que la consommation de puissance dans un processus de consolidation électro-osmotique est principalement contrôlée par la conductivité é lectrique du sol qui est une fonction salinité de l'eau interstitielle et de l'indice des vides.

Electrically conductive geosynthetics for consolidation and reinforced soil

The concept of electrically conductive geosynthetics (EKG) materials has recently been introduced. These materials extend the traditional functions of geosynthetic materials by incorporating electro-kinetic phenomena. Electro-kinetic geosynthetics offer technical benefits over conventional electrodes in that they can be formed as strips, sheets, blankets or three-dimensional structures. They are light and easy to install and can be structured so as not to be susceptible to electro-chemical corrosion, whilst continuing to provide conventional functions of filtration, drainage, separation, reinforcement or to act as impervious membranes. This paper describes initial laboratory tests on different types of EKG materials which can be used as combined electrodes/drains in electro-osmotic consolidation and as conductive geosynthetic reinforcement used to improve and reinforced weak cohesive soil. Results of the consolidation tests showed that the EKG electrodes were as efficient as a copper electrode and that the filtration and drainage characteristics did not deteriorate under electro-osmotic conditions. Results of the reinforced soil tests showed that EKG reinforcement can be used to increase the undrained shear strength of cohesive fill and that reinforcement/soil bond increases in proportion to the increase in shear strength.

Electro-osmotic consolidation of soft Bangkok clay with prefabricated vertical drains

The response of reconstituted soft Bangkok clay to electroosmotic consolidation was investigated using the small cylinder electro-osmotic cell and the large consolidometer. Test results indicated that employing electro-osmosis to induce consolidation produced a 3 to 4·4 times faster rate of consolidation. An 11·6–58% increase in shear strength with the reduction of water content was achieved for treated samples. In contrast, only 9·8–19·5% increase in shear strength was achieved without treatment. It was found that the 120 V/m voltage gradient and the 24 h duration of polarity reversal interval had maximum effect on the shear strength distributions and settlements between the electrodes. Nous avons étudié la réponse d'une argile tendre reconstituée de Bangkok à la consolidation électro-osmotique en utilisant la petite cellule électro-osmotique cylindrique et le grand consolidomètre. Les résultats des essais montrent que l'emploi de l'électro-osmose comme moyen de consolidation produit une consolidation de 3 à 4,4 fois plus rapide. Pour les échantillons traités, nous avons obtenu une augmentation de 11,6 à 58% de la résistance au cisaillement avec la réduction du contenu en eau. A l'inverse, sans traitement, nous n'avons obtenu que 9,8 à 19,5% d'augmentation de la résistance au cisaillement. Nous avons trouvé que le gradient de tension de 120 V/m et l'intervalle de 24 heures pour l'inversion de polarité avaient un effet maximum sur les distributions de résistance au cisaillement et les tassements entre les électrodes.

Electroosmosis-enhanced preloading consolidation via vertical drains

An analytical model of electroosmosis-enhanced preloading consolidation via vertical drains is developed for the design of the combined preloading and electroosmotic consolidation of clay soils. The excess pore pressure and average degree of consolidation are computed and examined for the effects of surcharge load, electric field intensity, treatment time, spatial configuration of electrodes, drainage path, and soil properties. The model is evaluated using the results of a well-known electroosmotic consolidation field test reported in the literature.Key words: consolidation, electroosmosis, surcharge preloading, vertical drains, soil improvement, soft clays.

Two-dimensional electro-osmotic consolidation

An analytical model of two-dimensional electro-osmotic consolidation is presented. The negative excess pore pressure and degree of consolidation due to electroosmosis are investigated in comparison with preloading consolidation. The model is evaluated using the results of a well-known electro-osmotic consolidation field test reported in the literature, and is shown to be consistent with the results of the shear strength measurement.

Electrokinetic Remediation. II: Theoretical Model

A mathematical model is presented for multicomponent species transport under coupled hydraulic, electric, and chemical potential differences. Mass balance of species and pore fluid together with charge balance across the medium result in a set of differential equations. Sorption, aqueous phase, and precipitation reactions are accounted by a set of algebraic equations. Instantaneous chemical equilibrium conditions are assumed. Transport of H + , OH − , Pb 2+ , NO-, the associated chemical reactions, electric potential, and pore pressure distribution across the electrodes in electrokinetic remediation are modeled. Model predictions of acid transport, lead transport, and pore pressure distribution display very good agreement with the pilot-scale test results validating the formalisms offered for multicomponent transport of reactive species under an electric field. The model also bridges the gap between the electrochemistry and mechanics in electroosmotic consolidation of soils.

Electroosmotic strengthening of soft sensitive clays

A comprehensive experimental investigation on the electroosmotic strengthening of soft sensitive clay was performed to assess the effectiveness of the treatment and to study the mechanism of the process. A specially designed electroosmotic cell was developed to prevent gas accumulation near the electrodes, to allow better electrode-soil contact, and to improve the treatment efficiency. This apparatus also enables the monitoring of the generated negative pore-water pressure along the sample length, settlement, voltage distribution, and current variation during treatment. The investigation covered two different types of soil trimmed at different orientations: the vertically and horizontally trimmed overconsolidated Wallaceburg clay and the vertically trimmed slightly overconsolidated soft sensitive Gloucester (Leda) clay. Results of this study showed that the voltage distribution and induced negative pore pressure at equilibrium along the sample are linear with steady current flow across the sample, indicating that the electrode design in the electroosmosis test apparatus is efficient. The electroosmotic consolidation curve is similar to that of the conventional consolidation curve, and the preconsolidation pressure was increased by 51–88% with an applied voltage up to 6 V. The undrained shear strength increased to a maximum of 172%, and the moisture content decreased by 30%. The technique of electrode reversal is employed, and a relatively uniform strength increase between the electrodes is observed. Key words: electroosmosis, electroosmotic cell, soft sensitive clay, negative pore-water pressure, preconsolidation pressure, stress–strain behaviour.