Shear Strength of GMX/GCL Composite Liner under High Normal Stress

An experimental program of large-scale direct shear tests was conducted to evaluate shear strength of a composite liner consisting of a textured geomembrane (GMX) over a hydrated needle-punched geosynthetic clay liner (GCL) for a wide range of normal stress, including high-stress conditions (>4 MPa). Tests were conducted with specimens placed between hard backing plates (single-interface) as well as between sand and gravel layers (multi-interface) to investigate the effects of local stress concentrations from coarse soil particles. Single-and multi-interface specimens experienced similar transitions from GMX/GCL interface failure at low normal stress to GCL internal failure at high normal stress. Failure surfaces for the single-interface specimens were essentially planar. Failure surfaces for the multi-interface specimens exhibited out-of-plane deformation (i.e., dimpling) that occurred because of variation in contact stress from the overlying gravel layer and increased with increasing normal stress. This deformation yielded modest percentage increases in peak shear strength and large percentage increases in large displacement shear strength and produced changes in local thickness and local bentonite dry mass per unit area for the GCL specimens. Direct shear tests conducted on single-interface GMX/GCL composite liners placed between hard backing plates are unlikely to reveal geomembrane out-of-plane deformation and damage effects but are likely to yield conservative (low) shear strengths for GMX/GCL composite liners that are placed in close proximity to coarse soils in the field. (C) 2016 American Society of Civil Engineers.