EFFECT OF CEMENT AND BY-PRODUCT MATERIAL INCLUSION ON PLASTICITY OF DEEP MIXING IMPROVED SOILS

Cement deep soil mixing is one of the most widely used ground improvement techniques to enhance the strength of weak soils for construction purpose. One of the engineering parameter that influences the strength performance of the improved soil is plasticity. This paper investigates the effect of cement and inclusion of waste materials on the plasticity of deep mixing improved soils. Two waste materials namely, Pulverised Fuel Ash (PFA) and Ground Granulated Blast Slag (GGBS) were considered. The investigation was carried out on five different soil samples with natural plasticity of 5%, 10%, 15%, 37% and 45%. In the first phase of improvement, samples were mixed with 5%, 10%, 15% and 20% cement (CEM I) content by weight of dry soil. In the second phase, the cement contents were reduced by 50% and replaced with PFA. In the third phase, cement was further reduced by 33.3% and replaced with equal amounts of PFA and GGBS. All improved samples were cured under 100% relative humidity and subjected to liquid and plastic limit test after 3days. Analysis of results showed that for cement improved soils, increase in cement content beyond 15% by weight of dry soil increases plasticity index of improved soils. The inclusion ofPFA and GGBS to cement during deep soil mixing reduces the plasticity index of the improved soil and may enhance the strength gain over time. PFA and GGBS could be used in deep soil mixing with reduced amount of cement and thus reducing cost, CO2 emission and the environmental impact of cement deep soil mixing. The results have shown that 15% cement is the optimum amount of cement required for deep mixing improvement of soils with natural plasticity index of 5-45%. The inclusion of GGBS and PFA in the blended soil reduces the amount of cement required for optimum binder content and resulted to 20% Cement/GGBS/PFA optimum binder content in the ratio of 1:1:1. This study has shown that addition of 15% cement content and 20% Cement/GGBS/PFA resulted to improved soils with plasticity index less than 17% making the investigated soils suitable for use as embankments and pavement for light to medium traffic. It could also be added that soil-binder interaction depends on soil type and the extent of improvement in plasticity index depends on plasticity index of the natural soil. A generalised flow chart based approach as a function of plasticity index of the natural soil have been developed for selection of binder for use in construction where increase in strength is envisaged.