Stress Path and Orientation Effects on Natural Fiber–Reinforced Sand

Triaxial tests were conducted to investigate the effects of stress path and orientation on natural fiber–reinforced sand. Four stress paths were employed, including the conventional triaxial compression (CTC) test, reduced axial pressure triaxial extension (RTE) test, constant principal stress triaxial compression (TC) test, and reduced confining pressure triaxial compression (RTC) test. Deviator stress, volume or void ratio change, shear strength, and reinforcement ratio were analyzed for varying fiber mass contents and aspect ratios. Test results demonstrate that fiber reinforcement significantly impacts sand’s shear strength. However, its effectiveness varies across different stress paths. The reinforcement level decreases from CTC to TC and then to RTC and RTE. A stress path angle, denoted αs, was introduced to characterize the effects of stress path, with the angle inclined between the stress path line and the x-axis. This indicates that the reinforcement level generally decreases as the stress path angle increases. Additionally, fiber reinforcement provided greater shear strength under CTC conditions than under RTE conditions, highlighting the significance of stress orientation. Critical and peak values of samples’ shear strength were also studied. In CTC conditions, fiber reinforcement contributes a more considerable critical shear strength rather than the peak value. This indicates that natural fibers are particularly effective in resisting large strains of sand at compressive stress levels. In details of shear strength parameters, a theoretical framework was proposed to capture fiber reinforcement mechanisms through changes in the peak values of the friction angle and cohesion of fiber–sand mixtures. Moreover, critical values of the friction angle of fibered or bare sand decrease visibly under RTE conditions, resulting in an easier failure in unloading conditions. These findings provide valuable insights for fiber reinforcement applications in geotechnical engineering, such as ground reinforcement or desertification control.