Smart soil grouting using innovative urease-producing bacteria and low cost materials.

Abstract

Calcium carbonate is a biomineral whose precipitation could be mediated by ureolytic bacteria and contributes in strengthening of sandy soils. The type of bacteria and grade of reagents have significant influence on microbially induced calcite precipitation (MICP). In the present study, factorial experiments based on these two factors were designed to determine their potential on MICP process, taking into consideration the economic advantages while giving quality results as well. For the first time, Alkalibacterium iburiense strain EE1 (GenBank accession no. MF355369.1) is reported for its biogrouting activity. Optimum growth conditions for MICP treatments were pH (9·56±0·021), EC (44·7±0·057mScm-1 ), OD600 (2·04±0·015), NH4 + concentration (487·06±1·021mmoll-1 ), and urease activity (20·0±0·75mmoll-1 urea hydrolysed min-1 ) after 72-h incubation. Statistical analysis comparing the growth in technical-grade medium prepared in tap water and analytical-grade medium prepared in deionized water showed no significant differences (P=1·0) in biomass and urease activity. In contrast to previous studies, the current approach could reduce the bacterial culture and cementation solution ratio by about 50%, using a simple surface percolation method with staged injection instead of parallel injection to treat different sand columns. Using fixation solution could immobilize the bacteria over the full length of columns for better strength improvement. The unconfined compressive strength ranged between 0·64 to 2·11kgcm-2 , and the corresponding CaCO3 contents 5·7-38·5%. The scanning electron microscope images indicated that the precipitated CaCO3 by bacteria was stable calcite. Alkalibacterium iburiense and technical-grade reagents under nonsterile conditions are satisfactory consolidating agents for sandy soils. This approach is considered eco-friendly and cost-effective for future scale-up applications in various geotechnical engineering.