Shear Strength Characteristics of Polyester Fiber Strip–Reinforced Sand with Different Fines Contents

Abstract

Sand–fines mixtures are widely encountered in northern Algeria, where their inherent heterogeneity and anisotropy can significantly affect the stability of geotechnical structures, particularly under wet conditions. This study investigates the combined influence of low-plasticity fines content and polyester fiber strip reinforcement on the mechanical behavior of sand–silt mixtures. A series of direct shear tests was conducted on specimens with fines contents (FC) ranging from 0% to 40% (0, 10, 20, 30, and 40%), prepared at a medium dense state (relative density Id = 0.5). Reinforcement was provided using polyester fiber strips of 60 mm length. Key mechanical parameters, including shear strength, friction angle (φ), cohesion (c), dilatancy angle (ψ), deformation modulus (Es), and California Bearing Ratio (CBR), were evaluated. The experimental results indicate that increasing fines content leads to a reduction in shear strength of Chlef sand. The friction angle decreases from 48.13° to 35.85° with increasing fines content, while the inclusion of polyester fiber strips significantly enhances soil cohesion, increasing from 0 to 12.85 kPa for unreinforced specimens and up to 21.1 kPa for reinforced specimens. Additionally, CBR results show a marked improvement due to reinforcement, with values increasing from 19.94 for unreinforced sand to 28.12 for sand containing 10% fines reinforced with centrally placed fiber strips. Overall, the findings highlight the beneficial role of polyester fiber reinforcement in improving the mechanical performance and bearing capacity of sand–fines mixtures, particularly at intermediate fines contents.