Geosynthetics, in particular geomembranes (GMs) and geosynthetic clay liners (GCLs), are seeing increased usage in mining applications such as tailings impoundments and heap leach pads. These applications can involve extreme conditions. For instance, heaps can now exceed 200 m in height and impose normal stresses approaching 4 MPa on a leach pad liner system. Such loads exceed the limits of most standard laboratory testing devices, making it difficult to properly evaluate the behavior of geosynthetic materials for these applications. Direct shear testing of geosynthetics is typically limited to normal stresses less than 690 kPa, which falls far short of the loads expected in many heap leach pads and high solid waste landfills. To address this need, two series of interface shear tests between textured GMs and needle-punched GCLs were performed at normal stresses ranging from 348 to 4146 kPa. The first testing series measured the individual interface shear strengths by using a rigid backing plate on both sides of the liner system. For the second series of tests, soil layers were added to allow more realistic out-of-plane deformations of the geomembrane. Results indicate that the effect of local GM deformations is to modestly increase the peak shear strength and dramatically increase the residual shear strength of the GM/GCL interface.