Abstract: Complex landslide is the combination of load-caused landslide and retrogressive landslide, with the characteristics of both load-caused landslide and retrogressive landslide. Through research on the formation mechanism of complex landslide, we reveal that complex landslide successively experiences the failure modes of retrogression in the front side, load in the backside, penetration of slip zone in the middle, and finally global sliding instability. Correspondingly, we divide complex landslide into three zones start area in the leading edge (tractive effect), main slide area in the middle, and start area in the trailing edge (loading effect). According to the failure modes and mechanical characteristics of different areas, as well as the interaction among these areas, we establish a calculation model of geomechanics, and derive the thrust of the three areas in detail. The thrust calculation for different areas should be specific. For the calculation of leading edge start area and trailing edge start area, their interactions with the main sliding area should be considered. And the stability of the main sliding area plays a dominant role in the whole landslide. It is necessary to consider the absence of slide-resistance of the leading edge start area and also the loading effect of the trailing edge start area. Calculation example of a typical complex landslide proves the rationality of the thrust calculation method. This research provides a reference for the prevention and control of complex landslide.

Key words: complex landslide, deformation and failure mode, geomechanical model, calculation of thrust, stress analysis of slices and blocks