Mechanical characterization of calcareous sand reinforced by EICP multivariate tests and synergistic biochar reinforcement

The foundation bearing capacity in island reef infrastructure construction is often inadequate due to the poor engineering properties of calcareous sand. Enzyme-induced carbonate precipitation (EICP) has emerged as an efficient and environmentally friendly method to improve these properties, significantly enhancing the mechanical behavior of calcareous sand. However, the effectiveness of EICP is constrained by environmental factors that affect free urease activity and the limited availability of nucleation sites. To further improve the mechanical properties of EICP-reinforced calcareous sand, this study explores the addition of biochar to the sand. The characteristics of the samples were evaluated using unconfined compressive strength tests, calcium carbonate content measurements, and scanning electron microscopy. The results indicate that incorporating biochar into EICP significantly increases calcium carbonate production, strengthens the reinforced sand, and reduces surface porosity. This improvement is attributed to biochar’s surface properties, which provide additional nucleation sites for calcium carbonate formation, enhancing calcium carbonate content and sand strength. However, controlling the biochar content at approximately 5% is crucial to avoid excessive porosity, which could weaken the material. These findings offer a theoretical foundation for applying EICP-biochar-reinforced calcareous sand in engineering projects.