Abstract: Calcareous sand have the characteristics of high compressibility, high void ratio and tend to be fragile, which lead to poor mechanical properties and often have adverse effects in the construction of reefs island. In order to improve the mechanical properties of calcareous sand, a new method of strengthening calcareous sand based on microbial induced calcium carbonate precipitation (MICP) combined with Fiber Reinforced （FR）technology is proposed in this paper. The mechanical response and failure mechanism in microcosmic of MICP treated Calcareous sand with different fiber content were analyzed by unconfined compression test and SEM. The main research conclusion is as follows: (1) MICP can effectively solidify the calcareous sand and improve its strength (2) PP Fiber can expand the colonization area of urealysis bacteria, and accelerate the deposition of calcium carbonate, thus enhancing the ductility and toughness of the MICP treated sample. (3) It shows a multi peak characteristic in unconfined compress stress-strain curve that in stress-rising stage, the sand particles and calcium carbonate will be broken locally, but does not affect on the peak strength of the sample; In the stress-decreasing stage, the cementation effect coupling with calcium carbonate, sand particles and fibers could enhances the resistance of the fibers and limits the development of the failure surface. （4）The cementation effect coupling with calcium carbonate, sand particles and fibers contribute majorly to the improvement of the toughness and ductility of the samples.

Key words: microbially induced carbonate precipitation, fiber reinforcement, calcareous sand, unconfined compressive strength;
mechanical properties, compressing failure model