Effects of electro-osmotic consolidation of clays and its improvement using ion exchange membranes

Electro-osmosis is an established method of expediting consolidation of soft, saturated clayey soils compared to commonly used methods, such as preloading with wick drains. In electro-osmotic consolidation a direct current (DC) is applied via inserted electrodes. This causes hydrated ions in the interstitial fluid to migrate to oppositely charged electrodes. Because the clay particles have a negative surface charge, the majority of ions in the interstitial fluid are positively charged. Therefore, the net flow will be towards the negatively charged electrode (cathode), where the water can be removed and thus consolidation is achieved. Certain problems, such as pH changes in the soil around the electrodes, make the method inefficient and prevent the widespread use of electro-osmotic consolidation, especially in developed nations. There is limited experimental or theoretical analysis on how exactly an electric field removes water from clay pores. Hence in this research a new theory is developed based on colloidal chemistry. Furthermore, to a new technology using ion-exchange membranes is proposed and investigated to improve the power consumption of electro-osmotic consolidation. The first part of this research develops a new method of estimating the expected flow of water through clayey soils (electro-osmotic conductivity) under the influence of an electric field based on work done in the area of colloid chemistry. This new method shows clear advantages over the currently used Helmholtz-Smoluchowski model. The flowrate due to electro-osmosis and the soil consolidation is measured for different electrical conductivity values and compared with the developed theory. The results confirm the validity of the new model, showing that electrolyte composition in the interstitial fluid is a significant factor in estimating electro-osmotic consolidation. Additional laboratory tests show that electro-osmotic consolidation achieves effects similar to secondary consolidation settlement, but over a hundred times faster. This observation is used to accurately predict electro-osmotic consolidation for specific voltages. The second part of the research described herein evaluates the ability of ion exchange membranes to improve electro-osmotic consolidation of clay soils. By inserting the membrane between the soil and the electrode a barrier is created that prevents hydrogen ions generated at the anode from moving to the cathode through the soil. When using the membrane, the change of soil pH around the anode is reduced by at least 10% and the increase in electrical resistance is slowed down by almost five times. This results in a 75% increased settlement for electro-osmotic consolidation tests with a membrane. This can prove to be a significant improvement to commercialize the use of the technology in developed nations