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To clarify the sliding mechanism of a landslide, the shear behavior characteristics of slip zone soil are always vital. In the Northwest of China, thousands of landslides generally develop in three main types of slip zone soil, i.e., Quaternary loess, weathered Mesozoic-Cenozoic mudstone, and weathered Mesozoic-Paleozoic phyllite. In the present study, the shear behavior characteristics of these three kinds of slip zone soil were investigated through direct shear tests and their implications for the sliding mechanisms of landslide were also discussed. It was found that, in general, an abrupt failure of loess landslide could be related to the strong strain-softening behavior of loess in the direct shear test, while the ductile shear behavior of weathered mudstone and phyllite coincided with the creep deformation of the corresponding landslides. The residual friction angle of slip zone soil varied with the soil type and its mineral composition, and the lowest residual friction angle about 11.5° could be found in weathered phyllite (SET) with graphite. Compared with the previous research, it is interesting to find that the residual friction angles of mudstone and phyllite in the present study accorded reasonably with the empirical correlations proposed by Wen et al. (2007) for weak rocks in the Three Gorges area, China.

Re-compaction process after previous liquefaction exerts significant effects on subsequent cyclic behaviour of soil. Due to the complexities in environmental loading conditions, the relevant studies on this topic are limited. In this paper, we collected natural submarine silts from non-liquefied and post-liquefied seabed in the Yellow River subaqueous delta via offshore hydraulic sampling method, respectively. Monotonic and cyclic triaxial tests were conducted between them to investigate the re-compaction effect after previous wave-induced liquefaction on the undrained static and cyclic shear behaviours of seabed soil. Our experiments indicated that due to re-compaction process, the static and cyclic strengths of seabed silt in post-liquefied area are both enhanced in comparison to non-liquefied soil. A transformation in static shear failure mode from plastic to brittle after wave-induced liquefaction could be identified in seabed silt. Furthermore, the developments of pore water pressure and cyclic axial strain with number of cycles for seabed silts could be characterized by the hyperbolic and power models, respectively, and an empirical relationship between pore water pressure and cyclic axial strain is proposed for natural seabed silt. Finally, compared with non-liquefied seabed silt, the threshold cyclic stress ratio (CSR) for post-liquefied seabed silt is also elevated due to re-compaction effect.