Reliability Based Underseepage Analysis in Levees Using Monte Carlo Simulation

Current methods for assessing the potential for unsatisfactory levee performance due to underseepage consist of deterministic seepage analyses and simplified reliability methods. The most common simplified reliability approaches currently being used apply the First-Order Second-Moment Taylor Series method using the U.S. Army Corps of Engineers Blanket Theory Equations as the performance functions. These methods do not reflect a realistic state of nature of the levees and the uncertainty associated with their performance. A new method is proposed that allows modeling the initiation of erosion process with more accurate failure mechanism and more complex subsurface geometry. The method uses finite element analyses to develop a series of equations that define the relationship between the variables and the Factor of Safety (FS). Using these equations, Probability Density Functions (PDF) for variables, and the computer program @Risk, a Monte Carlo analysis is performed to calculate the probability of unsatisfactory performance. Several examples are presented to illustrate the proposed procedure. Multiple regression analyses are performed to assess the relative effect that changes in the input variables have on the FS for the various analyses. The results suggest that uncertainty in the levee geometry has a greater effect on the variation of the FS.