Efficiency of microbially-induced calcite precipitation in natural clays for ground improvement

Abstract

Microbially-induced calcite precipitation (MICP) is a promising technique to improve the engineering properties of soft soil in a sustainable, environmentally friendly, and energy saving manner. This study determined the suitable chemical condition to induce MICP activity from alkaliphilic urease-producing Sporosarcina pasteurii bacterium, and to explore the MICP activity’s effectiveness in improving the mechanical properties of three types of natural fine-grained soils (Kaolin clay, Laterite, and Bangkok clay). Untreated and treated clays were evaluated by the free-free resonant frequency test, unconfined compression test, and their microstructures and chemical compositions. The results showed that a urea-Ca2+ input rate of 7.5 mmol/h efficiently promoted the precipitation of calcite by bacteria, as indicated by the highest degree of media clogging in sand column tests. A higher urea-Ca2+ input rate created soil samples that exhibited a high degree of supersaturation, which impacted the precipitation pattern at the microscale, resulting in less contact between soil particles; this was caused by the increase in salinity of the solution to a level that inhibited bacterial activity and retarded calcite precipitation. Once the optimal urea-Ca2+ input rate was determined, bacterial cultures were introduced into samples of each clay, and fed with the optimal chemical mixture while being maintained at room temperature (25–30 °C) for 7 d. Increases in the samples’ stiffness and shear strength coincided with increased amounts of precipitated calcite. Enhancements to each clay type’s strength parameters were dependent on the natural characteristics of clay. Specifically, liquid media carrying the necessary nutrients for MICP activity could not efficiently permeate through the clayey soil with a high plasticity, resulting in a lower observable degree of MICP activity to improve the soil’s characteristics.