Abstract: Different from plane sliding and circular sliding, the sliding direction of landslide with composite bottom sliding interface varies because the sliding direction must adapt to the dip angle of the bottom sliding interface. When the sliding mass passes through the turning part of the bottom sliding interface, the sliding direction changes abruptly. Such abrupt change induces the shear fracture surface in the sliding mass; but the shear resistance in the sliding mass hinders the formation of such shear fracture surface, which is beneficial to the overall stability of the sliding mass. For composite sliding slope with gentle front and steep back, a rigid body limit equilibrium analysis method with three sliding planes considering the internal shear fracture surface of sliding mass is proposed to reflect the constraint mechanism of internal shear resistance on slope stability. A detailed case analysis on Vajont landslide is presented. Results reveal an objective internal shear constraint mechanism in the composite landslide as reflected by the rigid body limit equilibrium analysis of three sliding surfaces.

Key words: slope stability, composite bottom sliding interface, internal shear fracture of sliding mass, three slip surfaces, rigid body limit equilibrium