Influence of Abacá Fiber Treated with Sodium Hydroxide on Undrained Shear Strength in Organic Silt

Highly decomposed organic soils exhibit low strength and stability, which pose challenges for geotechnical engineering. This study evaluates the effectiveness of abacá natural fibers treated with 5% NaOH to prevent biodegradation and reinforce organic silt. An experimental program was conducted to investigate the effects of fiber content (1, 1.5, and 2%) and length (5, 10, and 15 mm) on the undrained shear strength (Su), elastic modulus (E50), maximum dry density (MDD), and optimum water content (OWC). The results revealed a slight reduction in MDD and OWC, while Su increased significantly, reaching 104.13% for 1.5% fiber content and 15 mm fiber length. E50 decreased by up to 52.61%, indicating a transition toward more ductile behavior and variability due to the inherent heterogeneity of the soil. ANOVA and post hoc Tukey analyses confirmed the statistical significance of fiber content and length on Su, with optimal performance observed at 1.5% content and 15 mm length. These findings demonstrate that chemically treated abacá fibers provide effective and sustainable soil reinforcement and that chemical treatment is essential to maintain short-term durability in biologically active organic soils.