The tensile deformation of fibers is often overlooked in traditional analyses of fiber reinforcement mechanisms, with pullout failure being considered as the primary failure mode in fiber-reinforced soil. In recent years, flexible fibers have increasingly been used in fiber-reinforced soil. However, their failure modes have not yet been revealed. In this study, plastic fibers are used for pullout tests conducted by a modified horizontal tensile testing apparatus. The mechanical characteristics of the fiber-soil interface and the deformation characteristics of plastic fibers have been analyzed. It has been found that the failure modes of plastic fibers in reinforced soil can be categorized into three cases: pullout failure with elastic tensile deformation, pullout failure with plastic tensile deformation, and fracture failure with plastic tensile deformation. A theoretical calculation method is proposed to describe the progressive pullout behavior, and the pullout force-displacement relationship can be determined. The pullout force calculated using this method is less than that obtained from traditional methods due to the incorporation of the fiber's deformation characteristics. Through a comparison between the pullout test results and the predicted results, the effectiveness of the proposed method in capturing the pullout force-displacement relationship of flexible fibers in soil is verified.
