The ring shear apparatus is most suitable for measuring the residual strength of soils, but it is difficult to use conventional ring shear tests to measure the peak strength of low-permeability, high-plasticity clay because shear rates that completely satisfy drained conditions are unknown. Moreover, regarding residual strength, an efficient method to complete the residual state has not been ... [Show full abstract] established, although it is known that much deformation is necessary. Therefore this study first demonstrates the effectiveness of the newly developed ring shear apparatus through comparison with the results of triaxial compression tests. Shear-rate effects are then examined in ring shear tests using samples containing various proportions of bentonite. Finally, using results obtained using various remoulded soils, the effects of plasticity index in saturated soils on maximum and residual strength are demonstrated.