Effect of bacterial concentration and moisture saturation on strength of BC soil using S. pasteurii and B. licheniformis for bio cementation

Extensive growth in urban infrastructure necessitates construction in expansive clayey soils that lead to the development of innovative and sustainable ground improvement techniques such as bio cementation. This technique is affected by its particle size, pore space, and the choice of bacterial strain in clayey soils. In the present study, two bacterial strains, S. pasteurii (SP) and B. lechinoformis (BL), in improving the strength of BC soil under controlled conditions have been attempted. The study builds upon the previous work to investigate the effects of bacterial concentration, moisture saturation, and exposure period by adopting a novel hand wet-mixing technique of bacterial strains into BC soil to optimize bacterial concentration that increases short-term gain in shear strength. Bacterial concentrations (4%, 8%, and 12%) with a maximum exposure period of 28 days and at different moisture saturations (dry side-63%, OMC-94.5%, and wet side-96%) were the dominant variables. The unconfined compressive strength (UCS) of untreated control specimens remolded at OMC, dry side of OMC, and wet side of OMC was 75.69, 102.42, and 63.26 kPa, respectively. The corresponding maximum UCS at 28 days exposure was 156.77, 267.43, and 148.12 kPa for specimens treated with 8% SP and 136.67, 258.38, and 120.12 kPa for specimens treated with 8% BL. When bacterial concentration increased to 12%, both the bacterial strains exhibited a marginal reduction in UCS. The highest increase in strength ratio of 2.61 and 2.52 was observed for BC soil remolded with 8% S. pasteurii and B. lecheniformis, respectively, with remolding moisture content on the dry side of OMC at 28 days. Hence, 8% bacterial concentration was considered to produce optimal results for both the bacterial strains.XRD and SEM analyses indicated calcite precipitation in cavities and on the surface of fine particles, causing an increase in the strength of BC soil.