A Limit Analysis Model for Active Earth Pressure in Unsaturated Soils Considering Capillary‐Adsorptive Stress and Composite Tensile–Shear Failure Mechanism

ABSTRACT Saturated‐based approaches relying on shear‐failure assumptions inadequately describe rainfall‐induced active earth pressure in unsaturated soils. This study presents a three‐dimensional (3D) upper‐bound analytical framework for active earth pressure in unsaturated soils by coupling a unified suction‐stress formulation with a composite tensile–shear failure mechanism. Both adsorptive and capillary contributions to suction stress are explicitly incorporated, and hydraulic hysteresis effects are considered through differentiated wetting and drying strength evolution. A two‐segment rotational failure mechanism, consisting of an upper tensile‐dominated zone and a lower shear‐dominated zone, is formulated while maintaining velocity continuity along the entire failure surface. Soil–wall roughness is explicitly included to account for interface friction effects. Closed‐form solutions for the active earth pressure coefficient are derived. A parametric analysis examines the influences of suction‐stress parameters, rainfall intensities and patterns, wall roughness, and 3D geometry. The results demonstrate distinct time‐dependent earth pressure responses governed by adsorption–capillarity partitioning, hysteresis, and interface conditions. This research provides a theoretical framework for evaluating active earth pressure on retaining structures under unsaturated states under rainfall infiltration.