Large-scale spatial characterization and liquefaction resistance of sand by hybrid bacteria induced biocementation

Abstract

Microbially induced calcite precipitation (MICP) has received greater attention of geotechnical engineers recently as a potentially effective and sustainable approach for biocementation of sands. Present study investigates spatial variability and synergistic effects of bacterial hybrids of Sporosarcina (S.) pasteurii and Bacillus (B.) sphaericus for sand biocementation which is important to explore to stimulate real field conditions where multiple urease producing can be present in soil environment. Large-scale testing was conducted in rectangular tank (135 cm × 113 cm × 65 cm) filled with Narmada sand (1.21 Ton), treated using bio-chem distribution system consisting of bioreactor, chemical mixer, circulation tank, injection pipes, treatment tank, and drainage outlets. The design of bioreactor and slotted injection pipes are the major novel aspect of this study for on-site bacterial cultivation and uniform calcite precipitation throughout the treatment depth to increase liquefaction resistance of sand which was not investigated in previous large-scale testing studies. Bacterial hybrid was first cultivated, then inoculum of 10 L was prepared, and finally, 200 L of bacterial solution was cultivated in bioreactor. Sand was bioaugmented with hybrid bacteria in tank, subsequently injected 0.50 M urea and cementation solution periodically up to 8 days. Biosparging was also implemented for 10–12 h daily using air pumps to ensure aerobic conditions. During the treatment, pH, electrical conductivity (EC), and total dissolved solids (TDS) were measured periodically as indicators of biogeochemical reactions occurring during biotreatment. Dynamic cone penetration tests (DCPTs) were performed and DCPT-blows up to 30 cm depth were measured at 20 locations. Contours of dynamic cone penetration index (DCPI) and calcite precipitation were used to assess spatial variability of biocementation. Average DCPT blow count for untreated sand was 5, but it increased up to 50 with 10.6% calcite content, for 30 cm penetration. The DCPT results showed significant improvement in liquefaction resistance. Biocemented samples were collected from various locations to measure calcite content and conduct SEM. Overall, the large-scale testing showed effective and substantial biocementation of sand using the cultivated hybrid bacteria and bio-chem distribution/air sparging system developed in this study. The spacing of injection points and the amounts of injection biotreatment solutions can be optimized to achieve uniform, adequate, and cost-effective biotreatment under actual field applications.