Unconfined Compressive Strength of Compacted Tropical Soil Bio-treated with Bacillus Megaterium

The environmental problem associated with the manufacturing and use of cement and other chemicals for soil stabilisation has led to an innovative and a more environmentally friendly technique called microbial induced calcite precipitation (MICP). MICP utilizes a biological process in soil improvement. Specimens of soil were bio-treated with 1/3 pore volume of stepped Bacillus megaterium (B. megaterium) suspension density 0, 1.5 × 10⁸ cells/ml, 6 × 10⁸ cells/ml, 12 × 10⁸ cells/ml, 18 × 10⁸ cells/ml and 24 × 10⁸ cells/ml, respectively. The specimens were prepared at −2, 0, +2 and  +4% moulding water content (MWC) relative to optimum moisture content (OMC) and compacted with British Standard light, BSL (or standard Proctor) energy. 2/3 pore volume of cementation reagent was injected into the compacted specimens in 3 cycles at 6 h interval and allowed to flow by gravity until partial saturation was achieved. The results obtained indicate that the unconfined compressive strength (UCS) increased with increase in B. megaterium suspension density and with decrease in MWC relative to OMC. Typically for the specimens prepared at OMC, the UCS of specimens treated with stepped B. megaterium suspension density of 1.5 × 10⁸ cells/ml, 6 × 10⁸ cells/ml, 12 × 10⁸ cells/ml, 18 × 10⁸ cells/ml and 24 × 10⁸ cells/ml increased by 33.83%, 40.76%, 52.24%, 58.06%, 59.06%, respectively, compared to the UCS of the untreated specimen (i.e., with 0 cells/ml). Overall, higher calcite content precipitated in specimens resulted in increased UCS and dry density values. The phase structure, composition and morphology characterized using the scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS) and Fourier transformation infra-red (FTIR) spectroscopy, indicated that calcite print was induced within the soil matrix. Specimens treated with B. megaterium suspension densities of 6 × 10⁸ cells/ml, 12 × 10⁸ cells/ml, 18 × 10⁸ cells/ml and 24 × 10⁸ cells/ml and prepared compacted at MWC −2, 0 and  +2 OMC, respectively, satisfied the minimum 200 kN/m² design criterion for the use of materials in municipal solid waste (MSW) containment application.