Electrokinetic treatment of desiccated expansive clay

Abstract

Structures built on shallow foundations in expansive clay, where vegetation and weather condition can significantly reduce the moisture content, must overcome the risk associated with excessive desiccation condition such as shrinkage and settlement. For many existing structures, removal of trees may not necessarily be an effective solution as it might cause swelling and heave on the long-term. This study explores a system that could be operated during prolong drought periods to rehydrate and neutralise the negative pore pressures of expansive clay avoiding the development of excessive desiccation. Electrokinetic (EK) is a proven method that has increasingly been adopted for dewatering and consolidation of fine-grained soils. Our hypothesis is to reverse the EK process by extracting water from saturated stratum below groundwater level and force it into the “early-stage” desiccated area, accelerating the rehydration and reducing suction. The theory was examined by a laboratory model using moderately expansive clay. After draining the soil for 23 days reaching moderate desiccation state, water was allowed into the lower part of the soil while applying the EK treatment using nine electrodes, placed in a radial pattern and inserted 110mm into the soil with approximately 30% of their lengths below the water level. As a result of the electro-osmotic process, the average moisture content increased in the model by more than two-fold within 8 hours. The suction measurement (taken in the middle of the desiccated area) showed an initial slow response followed by fast and consistent reduction rate, where suction dropped by 93% within 5 hours and ultimately down to 1kPa at the end of the treatment. The paper offers some insight into the factors controlling the proposed system and provides a good basis for future research to manage the risks associated with desiccation of expansive soils.