Comparison study of silty clay-gravel mixture-geotextile interface without and with polyurethane foam adhesive injection

The interface between geotextile and geomaterials plays a crucial role in the performance of various geotechnical structures. Soil–geotextile interfaces often suffer reduced performance under environmental stressors such as rainfall and cyclic loading, limiting the reliability of geotechnical structures. This study examines the influence of gravel content ( G c ), compaction degree ( C d ), and rainfall duration ( R d ) on the mobilized shear strength at the silty clay–gravel mixture (SCGM)–geotextile interface through a comprehensive series of direct shear tests under both static and cyclic loadings. A novel approach using Polyurethane Foam Adhesive (PFA) injection is introduced to enhance the interface behavior. The results reveal that increasing G c from 0% to 70% leads to a 35–70% improvement in mobilized shear strength and friction angle, while cohesion decreases by 15%–60%, depending on C d . A higher C d further boosts shear strength by 6%–70%, influenced by Gc and normal stress levels. Under cyclic loading, increasing displacement amplitude reduces shear stiffness (K), while having minimal impact on the damping ratio (D); K and D appear unaffected by the number of cycles in non-injected samples. Rainfall reduces mobilized shear strength by 8%–25%, depending on the normal stress, with a 47% drop in friction angle and a 24% increase in cohesion after 120 minutes of rainfall exposure. In contrast, PFA-injected samples exhibit a marked increase in mobilized shear strength under both dry and wet conditions, primarily attributed to enhanced cohesion. Notably, PFA treatment proves particularly effective in maintaining higher shear strength and stiffness in rainfall-affected interfaces, demonstrating its potential in improving geotextile–soil interaction under challenging environmental conditions.