Coupling effect of fiber reinforcement and MICP stabilization on the tensile behavior of calcareous sand

Coupling method of fiber reinforcement and microbial induced carbonate precipitation (MICP) stabilization was adopted to improve the tensile behaviors of calcareous sand. Three types of fibers, including polypropylene fiber (PF), carbon fiber (CF), and basalt fiber (BF), were used to prepare fiber-reinforced MICP-stabilized calcareous sand samples with various fiber contents and lengths. Direct tensile tests were conducted on a specially designed 8-shaped sample to evaluate the tensile behaviors of the samples. Experimental results revealed that the coupled use of fiber reinforcement and MICP stabilization technique could significantly improve the calcareous sand tensile behaviors. By adding the fibers of PF, CF, and BF, the maximum tensile strength of the sample was increased by 234.0%, 111.8%, and 100.7%, and the corresponding maximum residual strength reached 26.8 kPa, 30.63 kPa, and 50.7 kPa, and their maximum failure displacement was increased by 121.2%, 95.4%, and 100%, respectively compared to the MICP stabilized sample. The coupling mechanism was attributed to the fiber inclusion promoting the calcium carbonate precipitation and forming more effective and extra-effective bonding. The interfacial shear resistance between fibers, calcium carbonate, and calcareous sand and the bridging performance of the crossing fibers facilitated the coupling effect. For all three types of fibers, the higher the fiber content, the greater the tensile strength, residual strength, and failure displacement. The tensile behaviors varied due to the difference in surface properties and mechanical properties of the fiber. PF had the best reinforcement performance on tensile behavior. For the sample with PF (0.1% fiber content), the optimal fiber length was 6 and 9 mm. Moreover, a theoretical relationship between the tensile strength increment and fiber content or fiber length was established. The results of this study are of some significance for the stability and safety of coastal engineering.