Three-dimensional active earth pressure under transient unsaturated flow conditions

Abstract

The active earth pressure evaluation was typically conducted assuming dry backfills and/or two-dimensional collapse mechanisms. In practice, retained slope collapse usually shows a three-dimensional (3D) feature and soils are usually unsaturated. The present work develops a framework for evaluating the 3D active earth pressure under transient unsaturated flow conditions. The framework is implemented by using the kinematic approach of limit analysis, in conjunction with a closed-form solution for the transient unsaturated flow. Based on a 3D horn-like failure mechanism, the active earth pressure coefficient is determined. Results for three hypothetical soils of fine sand, silt, and clay are calculated. It emerges that the inclusion of 3D effects and unsaturated transient flow leads to a smaller active earth pressure coefficient. The 3D effects are mainly influenced by the width-to-height ratio of walls; several certain cases analyzed in this paper show that the difference in the active earth pressure coefficient with and without considering 3D effects can be as much as about 27% when the width-to-height ratio equates to 2, whereas this difference decreases to about 10% and 5% when the ratio reaches to 5 and 10. The contribution of transient flow is mainly affected by the soil types; considering the unsaturated flow effect in clay can approximately reduce the active earth pressure of 67%, whereas this difference corresponds to 34% and 22% in silt and fine sand. The obtained results can help to quantitatively understand changes in the active earth pressure due to 3D effects and the presence of transient infiltration.