Enzyme-induced carbonate precipitation (EICP) is a new biogeotechnical ground improvement technique that uses calcium carbonate (CaCO₃) formed by biochemical processes to increase soil strength and stiffness. In this paper, crude urease extracted from soybeans was employed to catalyze the precipitation of CaCO₃ in sand. To optimize the urease extraction efficiency, factors affecting the soybean crude urease extraction, including the powdered soybean particle size, concentration, soaking time, and soaking temperature, were addressed. This paper also provided further insight regarding the impact of the urease activity of soybean crude extract on the chemical conversion efficiency and the biocementation performance in EICP. The findings revealed that the powdered soybean concentration and the particle size were the two most important factors affecting the urease activity of the soybean crude extract. The enzyme activity utilized in the EICP process might further lead to different reactant efficiencies of urea-CaCl₂ solution, and consequently, the improvement in the physical and mechanical properties of biocemented sand. Considering the chemical conversion efficiency and the biocementation performance, 60 g/L of powdered soybean was concluded as the preferred quantity for extracting the crude urease, with an enzyme activity of 6.62 mM urea min⁻¹. Under this condition, a chemical conversion efficiency of approximately 95% for 0.5 M urea-0.5 M CaCl₂ could be obtained in merely 12 h, and the unconfined compressive strength (UCS) of the EICP-treated sand exceeded 4 MPa with a CaCO₃ content of ~8%. As a high-efficient cost-effective alternative to the purified enzyme for carbonate precipitation, the soybean crude urease showed great potential for ground improvement.

Microfluidics is a new experimental approach that investigates the processes related to fluids flowing through small channels or porous media. Microfluidic chips, the core elements of the microfluidic technique, consist of quasi-2-dimensional pore structures etched and sealed in transparent materials. Fluids are allowed to flow through the chips, and real-time observations can be made about the transport and reactions of fluids at the pore scale. In this study, we test the feasibility of using the microfluidic technique to visualise the microbially induced carbonate precipitation (MICP) process. The experimental results obtained in this study show that the generation, evolution, and morphology of calcium carbonate, the effective product in MICP, can be clearly observed during the entire spatial and temporal range of the tests. Comparative tests with different testing variables, including the concentration of treatment solutions, activity of bacterial suspensions, and particle size, were also carried out. The results show that the influence of these variables on the treatment effects, such as the production rate of the reaction, spatial distribution of calcium carbonate, type and morphology of crystals, amount of ammonia gas produced, and presence/absence of clogging issues, can be visualised in microfluidic chips. The results presented in this paper prove that the microfluidic technique could be a helpful tool for unravelling the mechanisms of the MICP process at the pore scale and understanding the factors influencing the MICP process.