Improvement of strength behaviour of residual soil via enzymatically induced calcite precipitation

Abstract

Enzymatic induced calcium carbonate precipitation (EICP) is a biomediated soil improvement technique that utilizes free enzyme to produce a biocement material. This research investigated by performing test tube test the effect of varying urea-CaCl2 concentrations from 0.25 to 1.25 M on the mass and efficiency of calcium carbonate precipitation formed via EICP technique. The study also evaluated, by conducting UCS test, the strengths of EICP-treated residual soil prepared at various concentrations of cementation solution and different moulding water contents (– 2% to + 4% OMC). The test tube test results showed that the mass of calcium carbonate precipitation increased with an increment in the urea-CaCl2 concentrations from 0.25 to 1.00 M. Maximum mass of calcium carbonate precipitation of 1.577 g, 1.603 g and 1.603 g were obtained due to 1.00 M urea-CaCl2 at 3, 7 and 14 days, respectively. The effect of curing period beyond 3 days were found to be insignificant on the amount of CaCO3 precipitations. The strength of the EICP-treated soil increased with increasing concentrations of urea-CaCl2 and decreased with an increment in moulding water content. The maximum strengths values at all moulding water contents were obtained at 1.00 M urea-CaCl2. The highest shear strength of 728.5 kPa was determined at -2% OMC when 1.00 M was used as urea-CaCl2 and 4.33% calcium carbonate content was yielded. It was determined that there is a strong linear relationship (R2 = 0.8696) between UCS and calcium carbonate content in the treated soil. The amount of calcium carbonate content formed was found to decrease with an increase in mixing water content from 2% dry to 4% wet of optimum moisture content. Finally, the results from the field emission scanning electron microscope (FESEM) and energy dispersive X-ray (EDX) analyses confirmed the formation of calcite crystals in the EICP treated soil.