Strength Development in Clay Soil Stabilized with Fly Ash

The presence of optimum fly ash content in soil to increase the unconfined compressive strength (UCS) of clayey soil has been studied in the present work. Two types of fly ash, collected from coal combustion in an electric power plant, are used; one is Bangladeshi and the other one is Indian. The soil, collected from paddy field of Shahjalal University of Science and Technology (SUST), is classified as a mixture of inorganic silts and organic clays of medium to high plasticity. UCS tests have been conducted on moulds prepared with soil only and with soil containing fly ash with optimum moisture content (OMC) of 19%similar to 24% at a curing time up to 3, 7, 14, 28 and 90 days. Observations showed that maximum dry density (MDD) and OMC of pure soil are 1.615 g/cm(3) and 20.30%, respectively. The addition of fly ash content decreases MDD and increases OMC of the soil. With the increase in fly ash content, two zones of strength development have been observed: the active zone and the deterioration zone. The optimum fly ash content is found as 5% for both types of fly ash. 5% Bangladeshi fly ash content in the soil-fly ash mix produces 430.96 kPa and 474.53 kPa UCS at 28 and 90 days, respectively, while Indian fly ash generates 424.63 kPa and 434.01 kPa UCS, respectively.