Abstract: Stability factors, failure mechanism and strength characteristics of expansive soil slope are expounded in this paper from aspects of fractures, strength characteristics and failure mode with the middle route project of South-to-North Water Diversion, North Hubei water transfer project, water diversion from Yangtze River to Huaihe River and other large-scale water diversion projects as research background.Fractures in expansive soils can be classified as swelling and shrinking type and non-swelling and shrinking type, of which the latter is protogenesis, and the former belongs to secondary fractures generated by dry-wet cycles.Rainfall and strength reduction are major factors of shallow slide of most expansive soil slopes, while the dip direction, dip angle and strength of non-swelling nor shrinking cracks dominate the overall stability of expansive soil slopes.The instability of expansive soil slope can be summarized as shallow slide induced by expansive deformation and global slide controlled by fracture strength.Therefore, the expansive deformation and fracture distribution should be taken into account in stability analysis, and the strength parameters of expansive soil cannot be regarded simply as the comprehensive strength, but need to be depicted using soil mass strength parameters and fracture strength parameters according to actual geological conditions.On this basis, expansive slopes should be treated in accordance with its specific failure mechanism.

Key words: expansive soil, cracks, soil strength, slope stability, the Middle Route Project of South-to-North Water Diversion