Optimized electrically conductive vertical drains for electro-osmosis treatment of soft soils

Abstract

The thesis presents a study of the optimized conductivity and other electrical parameters needed to initiate effective electro-osmosis (EO) treatment in soft soils. EO deals with the movement of pore water and associated ionic components in soft soil when an electrical potential difference is applied across the soil mass. The present study was conducted on EO consolidation of soft soils via newly developed Electrically Conductive Vertical Drains (EVD). It comprises extensive series of laboratory tests and a large-scale field trial using different types of EVDs. Four series of laboratory tests were conducted using three different types of soft soils. It was observed that at each increment of constant applied voltage, there was a sudden jump in measured current, followed by an immediate dissipation. However, at some voltage increment, it will arrive to a stage where the current was found to gradually increase to a peak and then reduce during the application of this particular constant voltage. This signifies the effective EO process. It was found that there is a notable voltage loss at the electrodes and the soil-electrode interfaces resulting in reduction in EO efficiency. The effective dewatering and shear strength gain were also observed in electro-osmotic treated soil. The latter is perhaps due both to reduction in water content and the cementation effect among soil particles. In addition, simultaneous mobilization of a substantial percentage of heavy metallic ions from contaminated soils was observed.The field trial has shown that the soft clay, beneath an 18m of sand fill, can be treated effectively by EVD. Based on the measured field shear strength, it is estimated that EO improvement is about 10 times faster than the improvement caused by the conventional PVD treatment. In the light of the present laboratory investigations, the field trial observations and analysis of other reported field tests, it is proven that there is a great potential in using EVD technology for improvement of soft marine clay, and perhaps it is the only available technology for contaminated site remediation with heavy metallic ions without using electrolytes or chelating agents.