Initial static shear stress is commonly sustained in slopes, embankments, and backfills, where saturated soils are prone to severe liquefaction-induced damage under seismic loading. Although randomly distributed fibers have been widely used to improve the liquefaction resistance of sand, their effectiveness under varying initial stress states remains insufficiently understood. In this study, stress-controlled undrained cyclic triaxial tests were performed on unreinforced and fiber-reinforced loose sand under isotropic and anisotropic consolidation conditions. The results show that fiber inclusion suppresses excess pore pressure generation and delays axial strain accumulation, with the improvement becoming more pronounced as fiber content increases. Additionally, the reinforced specimens exhibit markedly higher cyclic resistance under initial compressional stress, whereas initial extensional stress promotes rapid strain accumulation and earlier failure. The initial static shear stress also governs the deformation and failure modes of unreinforced and fiber-reinforced sand, resulting in flow liquefaction, cyclic mobility, or residual deformation accumulation. These findings emphasize the need to consider in-situ initial stress states when assessing fiber reinforcement for seismic slope design.

Liquefaction; Fiber-reinforced sand; Initial static shear stress; Peak axial strain