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Expansive soils are widespread in the world and coincide with areas of high human activity. The main cause of deep instability of expansive soil slopes is due to their softening caused by excavation and seepage. By developing a comprehensive numerical model based on the theory of unsaturated soil, this study examines the characteristics of stress and displacement distribution of expansive soil slopes through hydraulic‐mechanical coupled numerical simulation. This study analyzes the evolution patterns of slopes with excavation unloading and seepage of water storage to reveal the mechanisms of deep‐seated instability of expansive soil slopes. The findings demonstrate that: The instability of expansive soil slopes begins at the foot of the slope and propagates along the interlayer, affecting the entire slope. Excavation leads to the softening of the expansive soil interlayer and the transfer of shear stress. During water storage, the weakening of the soil strength results in slope instability along the weak interlayer slip. Softening of the expansive soil interlayer facilitates the redistribution of shear forces in the slope and alters the distribution law of the plastic zone in the deep layer. Overly slowing down the slope leads to significant excavation unloading, which is detrimental to the slope's stability. [ABSTRACT FROM AUTHOR]

Expansive soils exhibit significant swelling-shrinkage characteristic under the cyclic moisture changes. The expansive soil slopes with high designing safety factor (gentle slope and low rainfall level) will also fail, which could not happen in an analysis ignoring expansive deformations. In this paper, the basic pore surface fractal model is derived and user-defined subroutines performing swelling-shrinkage deformation are developed to simulate the deformation features of expansive soil slopes under moisture variations. First, Fractal model is verified experimentally for accuracy in describing expansive soil issues. A hydro-mechanical coupling model based on the Fractal model in ABAQUS is then established to enable the numerical analysis of expansive soil slopes. In addition, the effects of swelling deformation, desiccation and slope ratio on deformation features are investigated. The results show that the consideration of swelling-shrinkage deformation causes the slope deformation features change, promotes larger additional displacements, and enhances water sensitivity. The simulated slopes can exhibit typical gentle sliding, shallow and progressive damage after several moisture cycling. This research provides a design reference for expansive soil slopes and expansive soil associated catastrophic issues. [ABSTRACT FROM AUTHOR]