Rock-like behavior of biocemented sand treated under non-sterile environment and various treatment conditions

Microbially induced calcite precipitation (MICP) is a recently developed technique for microbiological ground improvement that has been applied for mitigating various geotechnical challenges. However, the major challenges, such as calcite precipitation uniformity, presence of different bacteria, cementation solution optimization for cost reduction, and implementation under non-sterile and uncontrolled field environment are still not fully explored and require detailed investigation before field application. This study aims to address these challenges of MICP to improve the geotechnical properties of sandy soils. Several series of experiments were conducted using poorly graded Narmada River (India) sand, which were subjected to various biotreatment schemes and tested for unconfined compressive strength (UCS), split tensile strength (STS), ultrasonic pulse velocity (UPV), hydraulic conductivity (after 6 d, 12 d, and 18 d of treatment), and calcite content. The microstructure of sand was examined through a scanning electron microscope (SEM). Initially, the sand was individually augmented with two non-pathogenic bacterial strains, i.e. Sporosarcina (S.) pasteurii and Bacillus (B.) sphaericus. The stopped-flow injection method was adopted to provide cementation solutions at three different durations (treatment cycle) of 12 h, 24 h, and 48 h and three different pore volumes (PVs) of 1, 0.75, and 0.5. The pore volume here refers to the porosity which is expressed as a ratio, i.e. a porosity of 50% was used as 0.5. The results showed rock-like behaviors of biocemented sand with the UCS, STS, and UPV enhancement up to 2333 kPa, 437 kPa, and 2670 m/s, respectively. The hydraulic conductivity reduction of 96.6% was achieved by 12% of calcite formation after 18 d of treatment using Sporosarcina pasteurii, 12-h treatment cycle, and one pore volume of cementation media in each cycle. Overall, a 24-h treatment cycle and 0.5-pore volume cementation solution were found to be the optimal treatment which was effective and economical to achieve heavily cemented, rock-type biocemented sand using both bacteria.