Collaborative Research: Assessing the Reliability of Levees in Changing Geologic Conditions

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This award is to investigate methods of assessing the risk of levee failure as a result of variable geologic conditions along the length of the levee. As levees are loaded during floods, water will begin to seep through both the levee and through the levee foundation. At low flow velocities the water will pass through the soil matrix without significantly affecting it. However, as the river water rises, the amount of seepage increases and the velocity of the seeping water and/or the pressure of the seeping water increases in response. This increases the potential for erosion of soil from within the levee or its foundation, a phenomenon known as internal erosion. If allowed to continue, internal erosion can lead to instability and eventual failure of the levee. Current levee evaluation methods assume relatively uniform subsurface conditions for assessing seepage behavior. However, buried geologic structures in the near-surface soils left behind by prehistoric creek or river channels often have a huge effect on the seepage behavior in the levee foundation, often to the point where these structures control the reliability of the levee with respect to internal erosion. This award supports research to provide needed knowledge for the development of a method for assessing the underseepage risk to levee reaches that have variable geologic features in their foundation. The new assessment procedures takes into account the three-dimensional aspects of the seepage flow into these features and assesses the probability of developing seepage conditions where internal erosion is likely to initiate. By mapping where these features occur along a long levee reach and evaluating the risk each of these features poses to the levee, we can evaluate the reliability of the entire levee reach by combining the risks of the various geologic features located along their alignment. The results of this research will assist engineers in assessing where levees present unacceptable risk to the land they protect. Because there are thousands of miles of levees in the United States needing evaluation and mitigation for underseepage, this methodology has the potential to greatly improve the efficiency at which available funds are spent to increase the reliability of this critical aspect of our nation's infrastructure.This project transforms the analysis strategy and methodology for assessing risk to linear geotechnical projects constructed over varied geologic conditions. The project is formulated specifically for levees but the methods developed will also be applicable to other linear structures or projects. The analysis framework consists of robust reliability analyses (response surface Monte Carlo analyses) that are applied to individual geomorphic features to estimate the probability of key hydraulic parameters reaching levels where the initiation of internal erosion is likely. These probabilities will then be combined with the likelihood of other steps in the internal erosion failure process occurring to obtain the probability of failure due to each geomorphic feature along the project alignment. The resulting probabilities will then be statistically combined to obtain the total risk to the project due to the failure mechanisms considered. This methodology provides a robust means for assessing the total risk to linear structures in a way that accounts for length effects and the geologic hazards located along their alignment.