Abstract: Landslide often exhibits characteristics of multi-stage destruction in practical engineering. However, the most dangerous slip surface and corresponding minimum safety factor are only concerned in general computing and design, this often leaves security risk. Considering the softening characteristics of geomaterial, a theoretical framework of effective simulation and evaluation of landslide multi-stage destruction is established with FLAC3D and Matlab software platform. Taken landslide in low-rent housing area of Dangjiaba in Xunyang county as an example, the process of forming multi-slip surfaces is revealed by progressive evolution of plastic shear strain, plastic tensile strain and shear strain increment and so on. It is shown that time and space sequences are not necessarily corresponding sequence. The temporal sequences of multi-slip surfaces are first-class main slip surface, second-class main slip surface and sub-slip surface; the spatial sequence is first-class master slip surface, sub-slip surface and second-class main slip surface. The number of slip surfaces is equal to the number of tension cracks in collecting on-site. The entry location of first-class main slip surface is in excellent agreement with the tension crack in the frontal part of landslide, but the positions of second-class master slip surface and sub-slip surface have little error with the tension crack on-site. The distribution and magnitude of strength parameters in the slip surfaces gradually change with the development of slip surface from peak strength to residual strength, this is the root reason why the multi-slip surfaces of landslide can be simulated effectively. The evolution of vector sum safety factors according the temporospatial distribution of material parameters in the multi-slip surfaces is obtained. It turns out that there are three different sequences of safety factors in the process of forming the multi-slip surfaces of landslide. It illustrates the active and passive relationships among all slip surfaces in the formation process.

Key words: softening characteristics, multi-slip surfaces, progressive evolution, vector sum method, safety factor