Limitations of Limit Equilibrium Methods for Seismic Slope Stability Analysis

Dam safety analysis of earth embankments often involves the use of limit-equilibrium slope stability analysis to evaluate static and/or seismic stability, the results of which can be used in simplified methods (e.g., Makdisi-Seed) to estimate seismic performance. For larger, higher risk projects, numerical (finite-difference/finite-element) modeling methods are often used to provide a better understanding of the failure mechanism(s) and estimate the static and/or seismic performance (e.g., deformations).
Both methods of analysis (limit-equilibrium and numerical) often provide comparable results. However, numerical models provide a much better understanding of the failure mechanism(s). While limit-equilibrium methods rely on a single failure surface, numerical models are able to predict multiple connected or separate failure surfaces and failure zones. Knowledge regarding potential failure mechanisms allows the engineer to better design the embankment dam and adjacent structural components.
This paper present the results of several recent embankment dam analyses using both limit-equilibrium and numerical analysis methods:
The first is a seismic case history analysis of the Lexington Dam during the Loma Prieta Earthquake. The numerical model results are compared to the pseudo-static limit-equilibrium analysis of the embankment. The results of the analysis are comparable to the measured post-seismic performance of the embankment.
The second involves the static, seismic, and post-seismic analyses of a 125 foot high embankment dam in Utah. The embankment dam included a concrete cutoff wall through the centerline of the embankment and embedded into the foundation rock. The numerical model showed a much more complex failure surface behvavior than the limit-equilibrium method. The numerical model results were then used in the structural design the cutoff wall. These cases provide insight into both failure mechanisms and the benefits of using numerical modeling, and the limitations of using limit-equilibrium analysis methods for the static and seismic design of embankment dams. 19 pp., 2 tables, 10 figures, 17 references.