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Reactivation of a Huge, Deep-Seated, Ancient Landslide: Formation Mechanism, Deformation Characteristics, and Stability

In this study, an investigation on the formation mechanisms, deformation characteristics, and stability of the Outang landslide, composed by three independent blocks (O1, O2, and O3), is performed by integrating site surveys, multi-technique monitoring data, and numerical simulation. The results show that the formation mechanism for blocks O1 and O3 is slide-bulking, and is planar slide for block O2. These three blocks slide along the incompetent layers (ILs): IL1 is the slip surface of block O1 and O2, and IL3 is the slip surface of block O3. Furthermore, the west local fast movement zone might evolve into deep failure. The slope surface movement is step-like, characterized by the alternation of rapid displacement followed by imperceptible displacement over each hydrological year. The surface displacement velocities increased upslope. Based on the numerical simulation, both precipitation and reservoir water are believed as the major factors driving the slope behaviors, and the slope stability would be decreased gradually under the effect of the periodic variation of water level and seasonal precipitation infiltration. As a result of this study, some countermeasures of landslide and long-term monitoring are recommended.