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This paper focuses on the effect of fiber orientation on the resistance of seepage-induced erosion in fiber-reinforced sand. To clarify the discrepancy and mechanism of different-oriented fibers improving the resistance of the sand matrix, a series of DEM-Darcy coupling simulations were conducted. The microscopic parameters of fiber-reinforced sand were confirmed by the rigorous calibration procedure. The fibers perpendicular to the seepage direction were found to increase the difficulty of moving fluid through the specimen and significantly reduce the erosion rate of the specimen. These macroscopic behaviors acquired corresponding explanations at the mesoscopic scale, including the evolution of fiber-sand contact orientation, coordination number, average normal contact force, tensile force, and energy dissipation. According to the simulation results, it is found that the highest proportion of tensile force in perpendicular fibers can reach 80%, while the parallel fibers are only 40%, which indicates that the perpendicular fibers have a significant netting effect. The mesoscopic behaviors reasonably revealed the role of the fibers with different orientations on the sand matrix during the seepage. This study is beneficial for further understanding the mechanical behaviors of fiber-reinforced sand under seepage-induced erosion in safety engineering. [ABSTRACT FROM AUTHOR]

Turfy soil is a special soil with numerous undecomposed plant fibers and humus, which can be considered a soil-fiber mixture. In this paper, the macro-mesoscopic dynamic responses of turfy soil subjected to multilevel cyclic loading with different waveforms (triangular, square, sinusoidal) were investigated employing the discrete element method (DEM). The microscopic parameters of the DEM model were confirmed by laboratory tests and calibration procedures. The hysteresis between the applied and measured stress on the loading plates was discussed to determine the appropriate frequency before simulation. Macroscopically, the discrepancies, including the velocity of the loading plates, the deformation of the specimen, and sample size effect were investigated under these three waveform loadings. Microscopically, the differences in energy, coordination number, average normal contact force between pure and turfy soil, internal force alternation and bending degree of fibers in turfy soil were studied to disclose the discrepancies in macroscopic behaviors with different waveforms. Simulation results illustrate that square waveforms can excite the support of the soil matrix by plant fibers in turfy soil to a greater extent. This study is beneficial for further understanding the mechanical behaviors of soil-fiber mixtures under the action of different forms of cyclic loads in safety engineering. [ABSTRACT FROM AUTHOR]