Residual Soil Improvement Using Microbially Induced Cementation of Ureolytic Bacteria

The process of improving the strength of granular soil by utilising microorganisms is known as the biocementation process. This method involves the precipitation of calcium carbonate into the soil matrix structure through microbial calcite precipitation, resulting in the cementation of soil particles and subsequent improvement in strength and reduced permeability. This study investigated the potential of using Sporosarcina pasteurii, a microorganism strain with urease activity, to stabilise residual soil through urea hydrolysis. The cementation reagents used consisted of urea and calcium chloride at various concentrations ranging from 0.25, 0.5, 0.75, and 1.0 to 1.5 M. These concentrations were varied to assess their impact on the precipitation of calcium carbonate driven by ureolysis. Bacterial concentrations of 1.20 × 106 cfu/ml and 1.80 × 106 cfu/ml were employed to evaluate the shear strength of the soil. The study also employed treatment durations of 24, 36, 48, and 60 hours. The findings demonstrated that higher concentrations of bacteria of 1.80 × 106 cfu/ml yielded superior strength compared to lower concentrations of 1.20 × 106 cfu/ml. The optimal concentration of cementation reagents for effective treatment was determined to be 0.5 M, as any concentration exceeding this value resulted in a decline in bacterial activity, leading to a decrease in strength improvement. Furthermore, it was discovered that a more significant number of active bacteria cells during the biocementation process resulted in higher precipitation of calcite, with the highest percentage of 2.2% achieved after 48 hours of treatment. Consequently, the greater the amount of calcite precipitated, the more significant the strength improvement up to a treatment duration of 48 hours. Similarly, the Unconfined Compressive Strength (UCS) of the treated soil exhibited an increase with longer treatment durations, reaching a peak UCS of 71 kPa at 48 hours. Therefore, at the 48-hour mark, the strength of the soil had improved by 120% compared to the untreated sample, representing the highest recorded strength improvement.