In existing studies, most slope stability analyses concentrate on conditions with constant temperature, assuming the slope is intact, and employ the Mohr-Coulomb (M-C) failure criterion for saturated soil to characterize the strength of the backfill. However, the actual working temperature of slopes varies, and natural phenomena such as rainfall and groundwater infiltration commonly result in unsaturated soil conditions, with cracks typically present in cohesive slopes. This study introduces a novel approach for assessing the stability of unsaturated soil stepped slopes under varying temperatures, incorporating the effects of open and vertical cracks. Utilizing the kinematic approach and gravity increase method, we developed a three-dimensional (3D) rotational wedge failure mechanism to simulate slope collapse, enhancing the traditional two-dimensional analyses. We integrated temperature-dependent functions and nonlinear shear strength equations to evaluate the impact of temperature on four typical unsaturated soil types. A particle swarm optimization algorithm was employed to calculate the safety factor, ensuring our method's accuracy by comparing it with existing studies. The results indicate that considering 3D effects yields a higher safety factor, while cracks reduce slope stability. Each unsaturated soil exhibits a distinctive temperature response curve, highlighting the importance of understanding soil types in the design phase.

Sedimentary soils usually have an anisotropic structure, and they are generally unsaturated, especially at shallow depths. Existing models for anisotropic and structured soils mainly focus on saturated conditions, while the unsaturation effects are not considered. In this study, a bounding surface model for anisotropic and structured soils under both saturated and unsaturated conditions is developed. The model incorporates the anisotropy and structure effects on the mechanical behaviour (e.g., the loading collapse (LC) bounding surface) and considers the structure degradation and anisotropy evolution. Furthermore, based on experimental results in the literature, the increase in water retention capacity with an increasing degree of anisotropy is incorporated by a new anisotropy and void ratio -dependent soil water retention equation. The proposed hydro -mechanical model is validated against extensive experimental data. Comparisons between experimental and calculated results show that the behaviour of anisotropic and structured soils under both saturated and unsaturated conditions can be well captured.