Ring shear tests on slip zone soils of three giant landslides in the Three Gorges Project area

Reactivation of large-scale landslides along the reservoir banks of the Three Gorges Project (TGP) is recognized as the most likely form of geohazards threatening safety and operation of the dam and the navigation. This study presents an investigation into the naturally drained shear properties of slip zone soils of such landslides. Twenty-seven specimens of slip zone soils of three giant landslides are tested at three shearing rates (0.1, 1 and 10 mm/s) by means of a large ring shear apparatus accommodating abundant coarse particles in the specimens. It is observed that a) soils with higher plasticity index or liquid limit tend to have lower residual shear strength; this influence of the Atterberg limits on the residual shear strength weakens as the shearing rate increases; b) even small variations in the particle size distribution (PSD) cause notable differences in shear properties. The PSD parameters, such as the coefficient of curvature, sand content, ratio of gravel content to the sum of remaining contents, and ratio of coarse fraction to fine fraction, have close correlations with the residual strength; and c) the residual strength is clearly affected positively by particle symmetry (quantified by elongation) and negatively by surface smoothness (quantified by convexity).