The impact of different levels of uncertainty in geosynthetic clay liner (GCL) shear strength on landfill stability is evaluated in this study. The uncertainty in shear strength of the interface governing landfill stability is defined using statistical parameters obtained from a significant database of GCL internal and GCL - geomembrane interface large-scale direct shear tests. Uncertainty in GCL shear strength arises from material variability within a single product type and between different product types. For a given product type, uncertainty arises from material differences among specimens from the same manufacturing lot as well as among specimens from different manufacturing lots. In addition, uncertainty was observed to arise from differences in test procedures such as hydration, consolidation and shear displacement rate.