Analysis of Passive Earth Pressure for Unsaturated Retaining Structures

Although commonly ignored in conventional design, cost savings could be possible if a proper assessment of the effect of matric suction in retaining structures with unsaturated backfill were completed. In this study, an analytical method was developed based on the limit equilibrium (LE) method and the assumptions of the plastic soil mass and planar slip surfaces. The proposed method can calculate the distributions of passive earth pressure, passive thrust, and their points of application under unsaturated conditions, which can consider the effect of apparent cohesion and adhesion within the framework of the suction stress-based effective stress approach. Furthermore, an unsaturated interface shear strength model was validated by existing experimental data and used in this method. The parametric studies were performed under the one-dimensional (1D) steady flow condition to investigate the effects of the soil friction angle, surcharge pressure, interface friction angle, backfill inclination angle, and seepage condition on passive earth pressure distributions and the air entry pressure parameter on the coefficient of total passive thrust and its point of action. Both the Coulomb and Rankine methods can be regarded as special cases of the proposed method under certain conditions. The passive earth pressures estimated by proposed method were then verified against the existing experimental data as well as the predictions by the extended Rankine and Coulomb theories.