The high-strength method of reinforcing sand particles using MICP combined with coconut fiber under unsaturated conditions in marine and coastal regions

This study addresses the cracking issue of airport foundations in marine and coastal regions by proposing an unsaturated reinforcement method based on Microbially Induced Calcium Carbonate Precipitation (MICP) combined with coconut fibers. Composite sand columns incorporating coconut fiber and bioslurry were prepared, and the effects of fiber length and content on the mechanical properties of MICP-treated sand columns were investigated. Experimental results revealed that the addition of short fibers (1–5 mm) significantly improved the unconfined compressive strength and ductility of the MICP-treated sand columns. As the bioslurry content decreases in the sand columns, the enhancement effect of short fibers on the unconfined compressive strength becomes more pronounced, with fiber addition improving compressive strength by up to 98 %. However, the inclusion of medium fibers (5–10 mm) and long fibers (10–15 mm) negatively affected the mechanical properties of the sand columns. Microstructural analysis further confirmed the synergistic reinforcement effect of short fibers and calcium carbonate precipitation. Short fibers acted as bridges, forming additional contact points between sand particles, which facilitated calcium carbonate precipitation at critical contact points, thereby enhancing the overall stability and strength of the sand columns. This characteristic was more pronounced under unsaturated conditions. This study provides a feasible technical solution for the effective reinforcement of airport foundations and demonstrates potential in unsaturated reinforcement and improving the ductility of sandy soil foundations. • Short fibers improve the calcium carbonate content (CCC) and unconfined compressive strength (UCS) of sand columns, while medium/long fibers reduce performance due to entanglement. • Fiber content influences UCS, with lower bioslurry content requiring higher fiber content for optimal enhancement. • Fiber-reinforced specimens exhibit elastoplastic behavior, with short fibers promoting shear failure and medium/long fibers causing bulging failure. • MICP-fiber composites under unsaturated conditions outperform both fiber-free unsaturated and fiber-reinforced saturated systems.