Effect of polypropylene fiber length and content on the wet–dry durability of xanthan gum-stabilized clayey sand: mechanical response and ultrasonic monitoring

Biopolymer soil stabilization offers environmental benefits, but susceptibility to moisture cycling may limit long-term durability. This study assessed the effect of polypropylene (PP) fiber reinforcement on the wetting–drying (W–D) durability of clayey sand stabilized with 0.25% xanthan gum (XG). Specimens containing 0.4% or 0.8% PP fibers, with lengths of 6–12 mm, were cured for 7 days and exposed to up to six W–D cycles. Degradation was evaluated using cumulative mass loss (CML), relative axial height change, unconfined compressive strength (UCS), and ultrasonic pulse velocity (UPV), while microstructural changes were examined by scanning electron microscopy (SEM). The XG-stabilized specimen without fibers disintegrated about 1 h after initial immersion, whereas adding 0.8% 6-mm fibers extended specimen integrity beyond 280 h. After six W–D cycles, this specimen exhibited a 7.84% wetting-induced height increase, a 2.2% drying-induced height reduction, a CML of 10.3%, and reductions of 62.5% and 88.9% in UCS and UPV, respectively. SEM images showed that fiber bridging and particle retention delayed disintegration. Overall, selecting an appropriate PP fiber length and content can improve the W–D durability of XG-stabilized soils.