A series of large-scale direct shear tests were performed to measure the interface shear strength between a textured geomembrane (GMX) and a needle-punched geosynthetic clay liner (NP GCL) for large ranges of normal stress (σn,s = 13 to 2071 kPa) and shear displacement rate (R = 0.1 to 29,000 mm/min.). The experimental data indicate that failure mode of a GMX/GCL specimen depends on both σn,s and R. Tests conducted at low to moderate normal stress levels (σn,s < 692 kPa) produced interface failures for all displacement rates. At higher normal stress levels, partial or complete internal GCL failures occurred for low displacement rates (R < 100 mm/min.) and interface failures occurred for high displacement rates (R > 10,000 mm/min.). Peak shear strengths were largely unaffected by displacement rate except at very low (13 kPa) and very high (2071 kPa) normal stress levels. Displacement rate had a significant effect on large-displacement shear strengths at higher normal stresses (σn,s > 692 kPa), partly due to the effect of R on specimen failure mode. Financial support for this investigation was provided in part by Grant No. CMMI-0800030 from the Geotechnical Engineering Program of the U.S. National Science Foundation and by a grant from CETCO of Hoffman Estates, Illinois, USA. Geosynthetic materials were provided by CETCO and Agru America of Georgetown, South Carolina, USA. This support is gratefully acknowledged. The writers also thank Alexander Stern, a senior at The Ohio State University, for his assistance with the experimental work.