Effectiveness of Microbial Induced Calcite Precipitation on Sand-Clay Mixtures

Biocementation using microbial induced calcite precipitation (MICP) is a newly introduced and promising soil stabilization technique. In this process, ureolytic bacteria hydrolyze urea to give carbonate ions, which when combined with calcium-rich solution produces calcium carbonate that binds the soil particles together. The amount of calcite precipitate formed depends upon the pore size and hydraulic conductivity of the soil (keeping other factors the same). As the particle size decreases, the pore size and hydraulic conductivity both decrease, and hence the amount of induced calcite precipitate reduces. But the efficiency of this process not only depends on the amount of calcite precipitated but also depends upon how these precipitates bind the soil particles. In this study, the effectiveness of MICP on sand-clay mixtures of different proportions is investigated to find out the optimum gradation of sand-clay mixture for maximum efficiency of MICP treatment. A series of experiments were conducted to determine the unconfined compressive strength and induced calcium carbonate content of both treated and untreated clay-sand mixtures at various proportions. Test results indicated that as the amount of clay content increased in sandy soil, the size of the pore and hydraulic conductivity both reduced, leading to a reduction in the amount of induced calcium carbonate precipitation. The UCS of the treated samples showed better strength than the untreated samples. A post-treated sample with 20% clay soil shows 200 kPa more than an untreated sample of the same clay percentage. The improvement in strength between treated and untreated samples was reduced significantly after increasing the percentage of clay in sandy soil. At a higher amount of clay content, the MICP is restricted due to the improper growth of ureolytic bacteria and a lack of pores. The test results from this study provide an opinion about the applicability of the MICP on soil containing a significant amount of clay.