The large deformation mechanical properties of slip zone soil play an important role in the stability evolution of landslide. The traditional landslide stability evaluation method can only be used to calculate a single stability factor, which cannot dynamically evaluate the landslide stability as it evolves. The large deformation mechanical properties of slip zone soil from Outang landslide in the Three Gorges Reservoir area are investigated by the indoor repeated direct shear test. Based on the damage theory, the shear damage behavior of slip zone soil with large shear displacement is analyzed, and a mechanical model describing the relationship between shear stress and shear displacement in accordance with the mechanical mechanism of landslide is established. Then, the stability of Outang landslide is dynamically evaluated by skillfully combining the mechanical model and the residual thrust method. The results show that the slip zone soil has obvious softening behavior and constant residual strength under the condition of large deformation. The model with clear physical meaning can reflect the large displacement shear mechanical properties of slip zone soil, which is consistent with the test results. The stability factor of Outang landslide gradually decreases and tends to be constant as landslide moves. The mechanical mechanism of the landslide stability evolving with deformation is the strain softening behavior of the slip zone soil, and the mechanical mechanism of the landslide stability evolving with water level is the reduction of effective stress in anti-sliding section under the influence of reservoir water. It is suggested that active measures should be taken in time in the prevention and control of landslide, and the construction of drainage engineering should be paid attention to for large-scale bank landslides.

Current stability evaluation methods are unable to consider the progressive developing process of the bulking landslide and lack progressive evaluation method. Based on the strain-softening constitutive model and energy conservation theory, this study proposes a progressive evaluation method for buckling landslide stability considering the shear mechanical characteristics of the slip zone with large displacement, enabling progressive evaluation of landslide stability as the buckling landslide deforms. Proposed method was validated based on the good agreement between evaluation result and observation from the critical displacement of Baiyangou landslide in the Three Gorges Reservoir area in China, as a case study. The results also suggest a displacement distribution along the slip zone presenting a two-step growth feature from the front edge to rear edge; in contrast, the shear strength distribution presents a two-step decline feature. In addition, the parameter sensitivity analysis of slip zone and slide mass of buckling landslide shows that buckling landslide stability is directly proportional to the thickness of slip zone and elastic modulus of slide mass and inversely proportional to the length of slide mass.