Geocomposite clay liners (GCLs) are used in environmental, transportation and geotechnical engineering applications. Determination of the internal and interface shear strength parameters of GCLs has a huge importance for stability analyses. Therefore, the study of the internal shear strength of one type of nonreinforced GCLs was performed. Laboratory testing programme consisted of five series of direct shear tests. Special attention was given to the influence of the specimen hydration procedure and horizontal displacement rate on the shear test results. The analysis of the direct shear tests, presented in the paper, clearly demonstrate that the measured values of internal shear strength depend on the way of performing the laboratory tests. The internal shear strength envelopes for nonreinforced GCLs are proposed on the basis of obtained results.