Abstract: The microbially induced carbonate precipitation (MICP) treated calcareous sand tends to be brittle and has low tensile strength. To solve this problem, a special “8” shaped mold was designed and a series of direct tensile tests were conducted to investigate the improvement of fiber reinforcement, the mechanism of fiber-reinforced MICP-treated, and influencing factors such as fiber content and fiber length. The results showed that fiber can improve tensile strength, peak displacement, and residual strength and reduce brittle failure. Overall, the tensile strength of fiber-reinforced MICP-treated calcareous sand samples was influenced by the fiber content and fiber length, the tensile strength tended to increase and then decrease with increasing fiber content and fiber length. Compared to the unreinforced sample, the tensile strength of the sample with 0.60% fiber content (the optimum fiber content) increased by172.40%，the peak displacement increased by 158.1%. The mechanism of fiber reinforcement can be explained by the fact that the fiber increased the amount of adsorption of Sporosarcina pasteurii, promoted the precipitation of calcium carbonate between the fibers and calcareous sand and the surface on the fibers, increased the interfacial forces between the fiber and the calcareous sand, improved the tensile strength of MICP-treated calcareous sand. Fiber can significantly alter the failure characteristics of samples, the tensile stress-displacement curve of the unreinforced sample had only two phases, and an initial phase, and an elastics phase, the curves of fiber-reinforced samples could be characterized by four phases including an initial phase, an elastic phase, a damage phase, and a residual phase. The effect of fiber content was mainly related to the interfacial forces between the fibers and calcareous sand, and the spatial disturbution of fibers. The effect of fiber length was mainly related to the number of fibers near the failure surface and the tensile stress per unit length of fiber could bear. Furthermore, the results of this study have certain guiding significance for the stability and safety of oceanic projects.

Key words: calcareous sand, microbial induced carbonate precipitation (MICP), fiber reinforcement, calcium carbonate content;
tensile strength