A Reliability-Based Evaluation of the Effects of Geometry on Levee Underseepage Potential

Levee foundations along meandering rivers are often modeled in seepage analyses with simplified models consisting of two layers having uniform thickness and soil properties. While this may simplify the analysis and allow for use of simplified reliability methods, it is important to realize the limitations of these simplifying assumptions. Due to the complex geomorphic regime that is often encountered in the fluvial environment, levee foundation geometry can range from simple to very complex. Variations in layer thickness and continuity affect the pore pressure and seepage regime and, consequently, affect the potential for internal erosion of the foundation soils. A study has been performed to assess the effects of complex subsurface geometry on the susceptibility of levees to underseepage related internal erosion. Reliability-based internal erosion analyses have been performed on eight hypothetical levee profiles using two analytical methods. The first-order second-moment-blanket theory (FOSM-BT) method applies a simplified reliability assessment technique to the US Army Corps of Engineers “Blanket Theory” equations. This method requires that the analysis profile be simplified to meet the requirements of the equations and places limitations on how the uncertainty of the various input parameters is modeled. The more complex response surface-Monte Carlo simulation method is capable of modeling more complex geometries and multiple failure modes and is flexible in how uncertainty of the various input parameters are modeled. The results of the two methods are compared to evaluate the effectiveness of the FOSM-BT in evaluating internal erosion potential in increasingly complex levee foundation conditions.

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