This study proposes a comprehensive framework, based on an upper-bound approach, for assessing how vegetation enhances wall stability through two primary mechanisms. The two mechanisms are reinforcement from root systems and hydrological reinforcement through transpiration-induced soil suction. Both contributions are integrated as additional internal energy dissipation terms within a logarithmic-spiral failure model. New expressions of earth pressure in unsaturated soil are derived, considering the influence of vegetation. The active earth pressure acting on the retaining wall is obtained using sequential quadratic programming. The proposed method is validated against classical non-vegetated solutions, confirming its accuracy. The results show that vegetation significantly reduces active earth pressure, with the extent of reduction depending on soil type, root distribution, and transpiration rate. In clay soils, both mechanical and hydrological effects are important, while in sandy soils, mechanical root reinforcement plays the dominant role. The effectiveness of vegetation is influenced by root depth, density, and diameter, with practical design insights provided through parametric charts. This work offers a theoretically consistent and design-oriented tool for evaluating vegetated retaining walls, emphasizing the coupled hydro-mechanical interactions between plants and soil.

Rainfall infiltration can alter matric suction and significantly affect the active earth pressure acting on retaining structures with unsaturated backfills. This effect becomes more complex when hydraulic layering and finite-width failure mechanisms are involved. In this study, a coupled analytical–numerical framework is developed for a vertical retaining wall with a two-layer unsaturated backfill subjected to time-varying rainfall. A one-dimensional transient seepage formulation based on the Gardner-type hydraulic model is combined with a three-dimensional kinematic upper-bound mechanism generated by double-logarithmic-spiral curves. The transient pressure head and effective saturation are converted into a suction-induced resisting contribution and introduced into the work-rate balance. The effects of initial water-content distribution, bottom hydraulic boundary condition, normalized wall width, rainfall intensity, and layer thickness ratio are investigated. The results show that the calculated active earth pressure coefficient is controlled by the coupled influence of transient suction loss, hydraulic contrast between layers, and finite-width confinement. Clay-related profiles are more sensitive to the initial hydraulic state and finite-width effect than sand-dominated profiles. The bottom boundary condition may either increase or decrease the active pressure depending on the hydraulic contrast. For some rainfall intensities, an intermediate layer thickness ratio produces a larger calculated active pressure than the corresponding homogeneous limits. These findings should be interpreted within the assumptions of one-dimensional vertical seepage, hydraulically and mechanically continuous layer interfaces, a smooth wall-back contact, and the adopted admissible three-dimensional failure mechanism.

Temperature and seepage are critical factors influencing the stability of unsaturated retaining walls, as they modulate soil shear strength through alterations in matric suction. This study proposes a three-dimensional analytical framework for evaluating active earth pressure under thermal and seepage conditions. With a kinematic upper-bound approach, temperature-dependent suction evolution and steady-state seepage are incorporated into a horn-shaped failure mechanism. The proposed method is validated against published analytical/numerical solutions, confirming its reliability. A systematic parametric study is conducted to examine how temperature, seepage velocity, wall geometry, and soil pore characteristics affect the active earth pressure behavior. The results reveal distinct behavioral trends depending on soil type: for sand, the active earth pressure increases with rising temperature, indicating reduced stability; conversely, for clay, it decreases with temperature elevation, suggesting enhanced stability. While seepage has minimal impact on sand, it exhibits a clear directional dependence in clays, with infiltration increasing active thrust and evaporation promoting stability through suction recovery. Three-dimensional analysis yields substantially lower earth pressure values compared with conventional two-dimensional approaches, highlighting potential design economies. The proposed method provides engineers with a practical tool for coupled thermal hydraulic mechanical analysis of retaining walls in unsaturated fills, facilitating more realistic and cost-effective designs under varying environmental conditions.