Investigation of the Effects of Calcite Precipitate from Locally Sourced Microbes on some Engineering Properties of a Silty Clayey Soil

Abstract

Microbially Induced Calcite Precipitation (MICP) is a bio-geochemical process that employs ureolytic bacteria to precipitate calcium carbonate within soil matrices, thereby enhancing engineering properties. This study examined the effect of locally sourced bacterial strains processed with cementation solutions on the plasticity, strength, and permeability properties of a silty clayey soil. The MICP treatment of the soil involved the use two bacterial concentrations (10⁷ and 10⁹ cfu/ml) with cementation solutions (0.25M, 0.45M and 0.50M concentrations). Bacillus anthracis was identified as a viable bacterial strain based on its high urease activity and high amounts of CaCO₃ precipitates. The soil was classified as A-2-6 under the AASHTO classification system. The untreated soil exhibited moderate plasticity (LL = 31.18%, PL = 19.13%, PI = 12%), low unsoaked CBR and UCS values of 24.36% and 0.74 kN/m² respectively, and high permeability coefficient (k) of 1.1×10^-5m/s, indicating poor subgrade quality. Microbial treatment significantly improved the properties of the soil: PI was reduced to 4% with bacteria and cementation of 10⁹ cfu/ml and 0.50M respectively. The CBR improved from 24.36% to a maximum value of 36.05% with no curing, and a maximum value of 74.44% at 10⁹ cfu/ml, 0.50M cementation, and 96 days curing. A maximum UCS value of 1.25 kN/m² at at 10⁹ cfu/ml, 0.50M cementation, and 60 days curing, over the 0.74 kN/m² of the untreated soil was obtained. There was a reduction in the permeability of the treated soil, with gradual increase of parameter as the bacterial and cementation concentrations increased. Based on the CBR values, the treated soil can be adopted for subgrade or sub-base course applications. This study showed that the calcite precipitate from Bacillus anthracis (a local bacteria strains) and 0.50M cementation solution effectively improved the engineering properties of silty/clayey soil for subgrade and sub-based applications, with adequate protection from water intrusion.