Controlling CaCO3 spatial distribution in microbially induced carbonate precipitation-improved sandy soils with biochar: Experimental evaluation and model validation

Sandy soils often suffer from low geotechnical stability and require effective reinforcement. Microbially induced carbonate precipitation (MICP) has emerged as a promising method, yet spatial heterogeneity of CaCO 3 precipitation restricts uniform reinforcement. To alleviate this issue, this study leveraged the bacterial-adsorption capacity of biochar (BC) by stratifying the soil matrix and incrementally increasing the BC dosage along the depth direction. Meanwhile, an existing transport-reactive model was refined by revising the urea hydrolysis kinetics equation to incorporate bacterial attachment, detachment, and decay, and was validated through solution tests and sand column experiments. Results show BC enhanced bacterial retention and CaCO 3 precipitation, whereas uniform BC incorporation intensified the vertical heterogeneity of bacteria and CaCO 3 , leading to a typical “dense-top, loose-bottom” pore structure. In contrast, the gradient BC configuration promoted a more balanced vertical distribution of bacteria and CaCO 3 , thereby improving the coordination of porosity and permeability evolution. Among all tested conditions, the gradient BC configuration with a total dosage of 3 wt% (1.5, 3, and 4.5 wt% in the upper, middle, and lower layers, respectively) exhibited the most favorable overall performance, with the highest CaCO 3 uniformity index of 0.71. Measured and simulated bacterial concentration, CaCO 3 content, and permeability showed good agreement, supporting the applicability of the proposed model. The study demonstrates that gradient BC incorporation is an effective internal regulation strategy for enhancing the uniformity, while the revised transport-reaction model showed satisfactory predictive capability and can support MICP parameter selection and process design for sandy soil reinforcement engineering.