Abstract: The sliding surface of the expansive soil slope affected by fracture surface, always locates in the severe and mild weathered layer interface and approximates to a straight line. The trailing edge of landslide is generally vertical tension crack. The lower main slip bed is nearly level. Based on the above characteristics, a simplified model of three-broken line rigid bodies is proposed, where strength of soil in sliding body uses stable values after repeated wet-dry cycles. According to the static equilibrium of the whole rigid body, the solution formula of the safety factor of the slope is established. Taking the Qinhuai River expansive soil slope as an example, dry and wet cycling laboratory test of expansive soil is carried out. The formula is used to calculate the long-term stability coefficient of the slope, and the result is close to that of the FLAC strength reduction method. And it can also reflect the adverse effect of the fracture on the force of the sliding body. The influence of the thickness of the severe weathered layer and the thickness of the expansive soil layer on the value of the safety factor of the slope is analyzed by designed program. It is found that when the former two thicknesses increase, safety factor of the slope firstly decreases sharply and then tends to slow. And considering the shallow soil layer strength and the severe weathering depth changes, the safety factor of severe weathering layer interface in different stages of dry-wet cycles is analyzed. It is discovered that the safety factor exhibits the trend of exponential decay; and the slope loses stability mainly after six cycles of wetting and drying. The depth of sliding surface is about 1.83 m.

Key words: expansive soil, rigid body equilibrium, three-broken line model, dry-wet cycles, slope stability