Effects of Fiber Length and Content on the Enhancement of Spray-Applied Substrate in Soil Spray Seeding

(1) Background: Soil stability is essential for hydroseeding applications, particularly in erosion-prone areas. This study examines the effects of coir fiber reinforcement on soil properties and optimizes fiber length and content for improved performance. (2) Methods: Triaxial tests, soil physical measurements, and cracking experiments were conducted on sandy and silty soils using five fiber lengths (1–5 cm) and three fiber contents (0.2–0.6%). Principal component analysis (PCA) and Response Surface Methodology (RSM) were applied for optimization. (3) Results: The results show that coir fiber increases soil cohesion, shear strength, porosity, and permeability while reducing bulk density. The best reinforcement occurred at a 3–4 cm fiber length and 0.4–0.6% content, enhancing both the shear strength and crack resistance. Correlation analysis indicated a positive relationship between porosity and shear strength and a negative correlation between crack ratio and shear strength, confirming fiber reinforcement benefits. RSM analysis identified 3.051 cm + 4.07% as optimal for sandy soil and 3.376 cm + 0.456% for silty soil. (4) Conclusions: The optimal coir fiber combination significantly improves soil stability, providing theoretical support for optimizing spray substrates.