Large Strain Analysis of Electro-Osmosis Consolidation for Clays

Abstract

Consolidation of soft clay creates a lot of problems in foundation engineering, because of the very low clay permeability and high compressibility. Primary consolidation takes a long time to complete if the material is left consolidating under atmospheric evaporation, and traditional dewatering techniques, such as surcharge preloading, vacuum preloading or vertical drains, have been used for decades to shorten the consolidation time. Among new soft ground improvement technologies, electro-osmosis consolidation is receiving much attention as a possible time efficient solution. Electro-osmosis is a novel technique to consolidate soft clays, and involves the flow of pore fluid in a soil mass in response to an applied electrical field. The electrodes (positive and negative) are installed in pairs in the soil mass; the direct current then forces ions in the mobile part of the electric double layer (EDL) to move from the anode towards the cathode, causing water flow. Electro-osmosis is found to be more effective in clayey soils because the electro-osmosis permeability is independent of the grain size. This means that electroosmosis can generate flow rates that are 100 to 1000 times greater than hydraulic flows in fine grained soft clays. The purpose of this thesis is to develop a numerical model for simulating multidimensional and fully coupled multi-physics electro-osmosis consolidation, including the elasto-plastic behaviour of soil and time dependent transport parameters at large strain. Special attention is paid to the simulation of complicated geometries and boundary conditions, and to the inclusion of more advanced elasto-plastic constitutive models. The overall goal is to develop a more realistic numerical tool which addresses the main features of electro-osmosis consolidation, and that has potential use in the design and optimization of field applications. In this thesis, numerical models for the electro-osmosis consolidation of soft clays in multi-dimensional domains at large strain are presented, which consider the full coupling of the soil mechanical behaviour, pore water transport, pore gas transport and electric flow. In particular, elasto-plastic constitutive models (i.e. the Modified Cam Clay model and Barcelona Basic Model) are employed to describe the mechanical behaviour of the clay, and some empirical expressions are employed to describe the nonlinear transport parameters. The proposed models have been verified against analytical/numerical solutions and also evaluated with results obtained formlaboratory experiments. Overall, excellent agreement has been found, which demonstrates the accuracy and efficiency of the proposed models. Updated Lagrangian formulations are employed to account for the geometric nonlinearity. The importance of considering large strains in a consistent and proper way is demonstrated, and differences with models based on small strain theory are highlighted. As deformation is the key concern during consolidation behaviour, various numerical examples are investigated to study the deformation characteristics. The ratio of electro-osmosis permeability to hydraulic permeability keo/kw is a key factor in electro-osmosis consolidation. Generally, electro-osmosis permeability and hydraulic permeability decrease with a decrease in the void ratio and degree of water saturation, but the decrease in hydraulic conductivity is much faster than the decrease of electro-osmosis permeability, so the ratio keo/kw increases during the consolidation process. A field test of electro-osmosis consolidation has been analysed, showing excellent agreement between the computed and measured settlements. Various electrode configurations, as well as current intermittence and current reversal approaches for electro-osmosis consolidation have also been investigated using the proposed model. The particular contribution of this thesis is that it introduces a realistic numerical tool for the simulation of electro-osmosis consolidation. It is able to simulate field applications with complicated boundary and geometry conditions, as well as practical applications such as current intermittence and polarity reversal, which are often employed in the field to achieve efficient and economical consolidation. Feasibility studies and a proper design are important for the field application of electro-osmosis consolidation. Hence this numerical tool has potential use in the design and analysis of electro-osmosis consolidation, including the assessment of factors for achieving optimal dewatering effects and estimating the cost.