Experimental Investigation of the Effects of Moisture Levels on Geocomposite Drainage–Geomembrane Interface Shear Behavior

Engineered landfill covers are vital for environmental sustainability. This study investigates the shear behaviors of geocomposite drainage (GCD) and geomembrane (GM) interfaces—smooth (GMS), impinged texture (GMTI), and embossed texture (GMTE)—under 10, 30, and 50 kPa of normal stress and 0%, 50%, and 100% moisture levels using large-scale direct shear tests. All interfaces showed strain-softening behavior. At 50 kPa and 0% moisture, GCD–GMTI had the highest peak strength (28 kPa), whereas GCD–GMS had the lowest (10 kPa) at 100% moisture. Moisture and normal stress showed a coupling effect, reducing strength and friction angle. At a 0% moisture level, the strength of the GCD–GMS and GCD–GMTI interfaces under 50 kPa of normal stress was 500% and 250% of that at 10 kPa, respectively; at a 100% moisture level, these proportions decreased to 310% and 230%, respectively. For GCD–GMTE, the ratio slightly increased from 3.0 to 3.2, indicating better wet performance. Texture significantly affected strength: peak strength at 50 kPa was reduced by 41% (GCD–GMS), 16% (GCD–GMTI), and 26% (GCD–GMTE) as moisture increased from 0% to 100%. Large displacement (LD)-to-peak ratios were 0.8–0.9 (GCD–GMS), 0.7–0.8 (GCD–GMTI), and up to 1.0 (GCD–GMTE). Friction angles were reduced from 18° to 9°, 23° to 18°, and 18° to 14° for GCD–GMS, GCD–GMTI, and GCD–GMTE, respectively. Vertical deformation was <0.6 mm. Shear mechanisms depended on texture and moisture. Microscopic and 3D scans revealed moisture-induced GMTI smoothing, reducing interlocking and strength.