Active Earth Pressures for Unsaturated Retaining Structures

Although commonly ignored in classical earth pressure methods, precipitation-induced failures in retaining structures highlight a need for explicit consideration of negative pore pressures or matric suction. Changes in the degree of saturation in the backfill can significantly affect active earth pressures. This paper presents an analytical framework for calculating the thrust of active earth pressures under unsaturated steady flow conditions. The proposed method addresses limitations associated with the collapse mechanism and effective stress representation in alternative analytical methods. The formulation was derived by implementing an analytical solution for one-dimensional (1D) steady flow into a limit equilibrium-based effective stress analysis. A closed-form solution for the thrust was obtained by employing a log spiral surface, considering the effect of tension cracks, backslope inclination, and batter for c'-phi' soils with variable backfill saturation. A unified effective stress representation was used to describe the state of stress in partially saturated soils. The method was evaluated for different hypothetical soil types, and the effect of infiltration on lateral earth pressures was demonstrated. This approach presents a useful tool for explaining the true service state behavior of walls and evaluating the forensics of failed structures. (C) 2015 American Society of Civil Engineers.