Evaluating Slope Stability of an Embankment Dam Using Three-Dimensional Limit Equilibrium Analysis

Embankment risk assessments informed by two-dimensional slope stability analyses often do not consider three-dimensional effects or address them only qualitatively. However, three-dimensional effects can have a large impact on embankment stability, particularly for dams in narrow valleys, dams where cross sections or material properties change over short distances, and sites where weak layers are present. Factors of safety from 2D analyses are generally conservative and are, therefore, typically considered acceptable for use in evaluating slope stability. However, ignoring 3D effects can result in significant overestimates of risk when assessing potential for slope instability and dam failure. This can lead to overconservative designs, increased costs for rehabilitation, and incorrect prioritization of funding for dam safety programs. To demonstrate the potential influence of 3D effects on predicted dam stability, a case study is presented in which post-seismic stability of an embankment dam was evaluated using both 2D and 3D limit equilibrium as part of a risk assessment. Factors of safety based on 2D analysis were much lower than 1, suggesting post-seismic instability and deformations at the dam could be significant. In contrast, factors of safety from 3D analysis were all near 1 or greater, suggesting post-seismic instability and deformation may be limited. This difference could significantly alter predicted performance of the dam depending on available freeboard, seismic loading, and other factors. This case history demonstrates the importance of considering 3D effects when assessing risks related to dam instability, particularly for dams in narrow valleys where assumptions required for 2D analysis are invalid.