Effect of Reagent Concentration on Strength of Lateritic Soil Bio-Treated with <i>Bacillus thuringiensis</i>-Induced Calcite Precipitate Tested with Pocket Penetrometer

The strength of lateritic soil bio-treated with a <i>Bacillus thuringiensis</i> (Bt)-induced calcite precipitate was investigated using a pocket penetrometer (PPT). The effect of bacterial (Bt) and cementation solution concentration (C_s) on the strength of the microbial-induced calcite precipitate (MICP) worked soil was also evaluated. Soil samples were treated with Bt and C_s using three mix ratios (i.e., 25% Bt: 75% C_s, 50% Bt: 50% C_s, and 75% Bt: 25% C_s) based on the natural soil liquid limit (LL = 36.0%). Bt suspension densities of 0, 1.5 × 10⁸, 6.0 × 10⁸, 1.2 × 10⁹, 1.8 × 10⁹, and 2.4 × 10⁹ cells/mL were applied to the soil with four varying C_s concentrations (i.e., 0.25, 0.5, 0.75, and 1 M). The prepared specimens were allowed to homogenise and equilibrate at laboratory conditions. A pocket penetrometer (PPT) was used to test the unconfined compressive strength (UCS) of the prepared specimens at 3, 5, and 7 days after bio-treatment to evaluate the strength of the MICP worked soil at different moisture contents. The results obtained show that the UCS values increased with higher Bt and C_s as well as with a reduction in moisture content as the bio-treated soil equilibrated with the environment. The recorded UCS values for the mix ratios considered were in the order: 50% Bt: 50% C_s > 25% Bt: 75% C_s > 75% Bt: 25% C_s. Therefore, a PPT can be used to determine the approximate unconfined compressive strength of treated soil.