Novel and simplified method of producing microbial calcite powder for clayey soil stabilization

Abstract

The green ground improvement technique utilizing microbial-induced calcite precipitation (MICP) has gained increasing interest. However, this technique is generally limited to stabilizing cohesion-less soil, as MICP poorly distributes in fine-grained soil with a low coefficient of permeability. Additionally, the method and information on producing MICP are not widely available, with few researchers reporting on optimum conditions and methods. Therefore, this study identified a new, simple approach for producing an ammonium carbonate supernatant, which can be precipitated into calcite and used to stabilize clayey soil. Based on the test results and optimization analysis, the proposed optimum conditions for producing the supernatant were Sporosarcina pasteurii as the selected bacteria and inoculation and incubation in a specific medium for 34 h to obtain a bacterial density of approximately 2.5 × 109 cells/mL. Then, the active bacteria were transferred to a 4M urea solution and cured for 8 h, after which the produced supernatant was precipitated into calcite and various percentages of admixtures were used to stabilize clayey soil specimens. The compressive strength of stabilized soil samples was investigated through a series of unconfined compression tests with various curing times and percentage of admixtures. The soil stabilized with calcite had higher ultimate compressive strengths than the untreated soil and its strength development over time was expressed by exponential function where the strength was expected to increase even after 56 days of curing. Microstructural analysis results indicated that the calcite particles filled the voids between soil particles and resulted in denser package of soil. However, the strength development of the calcite-treated soil was slow, with some elements of calcium silicate hydrate found in the calcite-treated soil specimens after curing for 56 days. The research outcomes will lead to potential implications for sustainable geotechnical and pavement engineering.