This paper describes two large-scale constant-load (creep) shear testing devices that were developed to evaluate the long-term shearing behavior of geosynthetic clay liners (GCLs) and interfaces between GCLs and other geosynthetics or soils. One device was designed to simulate loading conditions that typically occur on a GCL deployed in a landfill cover system. The other device was designed to simulate loading conditions that typically occur on a GCL deployed in a landfill lining system. A needlepunched GCL was selected for evaluation of its long-term shearing behavior under these two types of loading conditions and the test results are presented in terms of time-displacement curves and shear rate-displacement curves. The results to date show that the GCL has undergone relatively small shear displacements and that the shear displacement rates within the GCL and/or at the test interface have been continuously decreasing with time. For the conditions used in this testing program, it is believed that the GCL's behavior can be considered stable. Further testing is planned to moreaccurately define the time-dependent internal and interface shear behavior of the GCL.