Effect of carbonate precipitating bacteria on strength and hydraulic characteristics of loess soil

Abstract

Microbial-induced calcite precipitation (MICP) is one of the environmentally friendly techniques that has recently become popular amongst geotechnical engineers. Two bacterial species of Bacillus family, i.e., B. pasteurii and B. megaterium have been used to improve the loess soil properties. A set of unconfined compressive, permeability, ultrasonic, and collapse potential tests have been applied to assess the characteristics of natural soil compared to those of MICP-treated ones. The effects of curing time (1, 3, and 7 days), bacterial optical density (OD = 0.5, 1, and 1.5), and soil density (13, 14, and 15 $$ \frac{kN}{m^3} $$ ) have been investigated. Results indicate that biological enhancement has improved the engineering properties of the loess soil. MICP-treated soil using B. megaterium provides higher strength improvement ratios (1.15–4.4 times) rather than B. pasteurii-treated samples (1.05–3.4 times). Correspondingly, specimens containing B. megaterium have greater permeability reduction ratios (3.9–93.7%) compared with those of B. pasteurii ones (2–95%). Moreover, scanning electron microscope (SEM) analysis has been employed to confirm the findings. It is worth noting that various bacteria concentrations, curing periods, and soil densities can affect the stress-strain curve considerably. The results indicated that MICP reduced the collapse potential between 24 and 54.8% and increased the longitudinal wave velocity between 1.1 and 2.4 times more than the untreated soil.