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Based on a laboratory study of drained direct shear tests of remoulded, pre-cut and polished specimens and drained ring shear tests of uncut and remoulded specimens of slip surface materials of five landslide soils, significant correlations of the mobilized shear strength parameters, cohesion (c, c r ) and internal friction angle (Φ, Φ r ), are proposed. The investigated soils consisted of the slip surface material belonging to tuffaceous clay, mudstone, loess and siltstone. Most of the previous studies on residual shear strength measured by reversal direct shear and ring shear devices have obtained significantly different results using the two devices, even when pre-cut and polished specimens were used in the direct shear device. In this study, the residual shear strength values of remoulded specimens measured by a ring shear device are shown to significantly correlate with the drained large displacement shear strength values of remoulded specimens, which were measured using pre-cut and polished specimens in a direct shear device. The correlation between the cohesion measured in the two shear devices is expressed by the linear relationship, c r ¼ 0:7394c À 6:6857, while the correlation between the friction angle measured in the two devices is expressed by the linear relationship, F r ¼ 1:0852F À 6:0247. The proposed linear correlations for effective cohesion (c′) and effective friction angle (Φ′) have yielded significant coefficients of determination within an effective normal stress range of 30-150kN/m 2 .

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