AbstractAlthough vegetable fibers demonstrate great advantages as a reinforcement element, their degradation susceptibility is currently the main concern in geotechnical works and requires further investigation to advance their durability. In recognition of these needs, this study aims to evaluate the effects of silica and polymer surface treatments on sisal fiber biodegradability and on the mechanical behavior of sand matrix reinforced with these fibers exposed to actual environmental conditions for up to eight months. Fourier transform infrared spectroscopy (FTIR), X-ray fluorescence spectrometry (FRX), scanning electron microscopy (SEM), direct tensile and consolidated-drained triaxial tests were performed. Overall, the secondary cell wall of the fiber was the main structure affected by the exposure, reflecting the degradation by natural soil microflora. The strain-hardening behavior acquired by the addition of fiber to the sand at zero time was gradually reduced with natural aging. Increments in cohesive intercept of the composite over the biodegradation of fibers point to biofilm formation. Due to the inorganic base, silica treatment resulted in the most effective barrier against fiber biodegradation, contributing to composite durability along the eight months. Notwithstanding the degradation process, both natural and surface-treated sisal fibers resulted in shear strength parameters superior to those of the nonreinforced soil, encouraging their sustainable applications in engineering works, such as temporary landfills.