Abstract: To enhance the strength of red clay reinforced by microbial-induced calcium carbonate deposition (MICP) in a single mixing process, it is necessary to increase the concentration of the cementing solution in order to enhance the amount of calcium carbonate deposited and address the resulting decrease in microbial activity. The nonionic surfactant D-sorbitol increases the permeability of cell membranes, allowing the exocytosis of intracellular urease and enhancing enzyme activity, thereby promoting calcium carbonate synthesis. The three factors of D-sorbitol dosing, cementing solution concentration, and cementing solution-to-bacteria ratio were selected to determine the optimal dosing for the maximum calcium carbonate deposition, and the red clay soil was mixed and reinforced with this optimal dosing. The results showed that (1) The addition of D-sorbitol to bacillus subtilis enhances the urease activity; (2) Response surface tests were conducted to determine the optimal conditions for the maximum calcium carbonate deposition: D-sorbitol concentration of 0.48 g/L, cementing solution concentration of 3.72 mol/L, and cementing solution-to-bacteria ratio of 1.03:1; (3) After indoor curing for 14 days, the unconfined compressive strength of the red clay soil compacted and reinforced to 95% compaction was 979, 1 675 kPa, and 1 931 kPa for the untreated, microbial, and D-sorbitol-added groups, respectively; (4) The addition of D-sorbitol led to an increase in the calcite content in Bacillus subtilis from 14.4% to 38.8%, as determined by X-ray diffraction (XRD) analysis.

Key words: microbially induced calcium carbonate deposition, red clay, D-sorbitol, mixing method, response surface methodology