A new method for assessing the potential for unsatisfactory levee performance due to underseepage is presented. Specifically, the method assesses the potential for the initiation of piping (the internal backward erosion of the foundation or embankment caused by seepage). Current assessment methods consist of deterministic seepage analyses and simplified reliability methods. Deterministic methods produce either a maximum hydraulic exit gradient or a Factor of Safety against piping but they do not account for high levels of uncertainty in soil properties and subsurface geometry that are inherent to many levee analyses. The most common simplified reliability approaches that are currently being used to analyze levees with regard to underseepage apply the First Order Second Moment (FOSM) Taylor Series method using the US Army Corps of Engineers "Blanket Theory" equation as the performance function. These methods take into account the uncertainty of the soil properties but are limited to simplified subsurface geometries and often do not model the actual mechanism responsible for levee failure due to underseepage piping. The proposed new method uses a Monte Carlo simulation to calculate the probability of unsatisfactory levee profile performance and can take into account complexities in subsurface geometry that cannot be assessed using the simplified reliability methods. The relationships between uncertainty of the soil parameters, the subsurface geometry, and the Factor of Safety against piping are defined through parametric variation analyses of a finite-element seepage model. The results of the parametric analyses are used to develop a series of equations that define the relationship between the various input parameters and the factor of safety. Using these equations, probability density functions for the various input parameters, and the computer program @Risk which interfaces with Excel, a Monte Carlo analysis is performed to calculate the probability of unsatisfactory performance which represents the probability of initiating erosion given a river flood level. The results of the analysis represent a single node of the event tree. In order to assess failure potential, other points in the event tree will need to be assessed with calculations or judgment since it is only the first phase that is currently considered to be in the process of piping. The new method is demonstrated using actual data of levee profiles from the Natomas Basin in Sacramento, California as a case study. The case study highlights the benefits of reliability-based analyses over the Factor of Safety and demonstrates the importance of subsurface geometry in reliability calculations.

The events of Hurricane Katrina in 2005 prompted the US Army Corps of Engineers (USACE) to commission studies to identify the failure mechanisms of levees and I-walls. This involves updating of the current USACE Engineering Manual (EM) 1110-2-1913, "Design and Construction of Levees," which uses Blanket Theory for seepage analysis. Blanket Theory entails analytical methods for calculating seepage pressures and flows beneath levees. The revision of the manual will address the design seepage criteria for levees, with a focus on incorporating new seepage analysis procedures besides Blanket Theory. Finite element analysis is one such method that has more recently become the method of choice for general seepage analyses in geotechnical engineering. The focus of this research is mainly on underseepage analyses of levees in the lower Mississippi valley using numerical modeling, with a goal of helping engineers in making the transition from current Corps methods to finite element analysis. General guidelines are provided to conduct seepage analysis using finite element analysis for pre-defined Blanket Theory cases as well as for the design of seepage berms. In addition, the 3D finite element modeling is conducted for a full-scale field load test involving complex geometry and stratigraphy, which is useful in better understanding the response of levees and I-walls. / Ph. D.

It has been estimated that approximately fifty percent of the United States’ population lives behind levees. Because these earth structures are typically long, subjected to seasonal changes and spatial variability, it is logical to analyze them in an uncertainty-based approach. This research is focused on assessing the potential of internal erosion related failure due to underseepage with the general objective of assessing the failure potential of individual geomorphic features while considering length effects. The project team was granted $204,000 from the National Science Foundation and $10,000 from the United States Society on Dams which resulted in research collaboration within graduate students and University of Delaware faculty as well as several presentations in prestigious conferences (in the U.S and Europe) and publication of journal papers. Findings from this research should be easily transferrable to other linear earth structures (such as dams, construction excavations, detention ponds, road embankments, etc.), and should significantly enhance reliability analysis across a wide array of structure types and geologic settings allowing a broad impact on future research, as well as geotechnical engineering practice.