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

该奖项的目的是研究由于堤防长度变化的地质条件而导致的堤防破坏风险评估方法。在洪水期间，由于堤防的负荷，水将开始渗入堤防和堤基。在低流速下，水将穿过土壤基质而不会对其产生明显影响。然而，随着河水水位的上升，渗水量增加，渗水速度和/或渗水压力也相应增加。这增加了从堤坝内部或其基础侵蚀土壤的可能性，这种现象被称为内部侵蚀。如果任其发展下去，内部侵蚀会导致堤坝的不稳定和最终的失败。目前的堤防评估方法在评估渗流行为时假设了相对均匀的地下条件。然而，史前小溪或河道留下的近地表土壤中埋藏的地质结构往往对堤防基础的渗流行为产生巨大影响，往往达到这些结构控制堤防内部侵蚀可靠性的程度。该奖项支持研究提供必要的知识，以开发一种方法来评估具有不同地质特征的防洪堤的下渗风险。新的评估程序将渗流的三维方面考虑到这些特征中，并评估可能引发内部侵蚀的渗流条件发展的可能性。通过绘制这些特征沿长堤防河段发生的位置，并评估每个特征对堤防构成的风险，我们可以通过结合沿其排列的各种地质特征的风险来评估整个堤防河段的可靠性。这项研究的结果将帮助工程师评估哪些地方的堤坝对它们所保护的土地构成了不可接受的风险。由于美国有数千英里的堤坝需要评估和缓解地下渗透，这种方法有可能大大提高现有资金的使用效率，以提高我国基础设施这一关键方面的可靠性。该项目将评估风险的分析策略和方法转变为在不同地质条件下建造的线性岩土工程。该项目是专门为堤坝制定的，但所开发的方法也适用于其他线性结构或项目。分析框架由鲁强可靠性分析（响应面蒙特卡罗分析）组成，应用于个别地貌特征，以估计关键水力参数达到可能引发内部侵蚀的水平的概率。然后将这些概率与内部侵蚀破坏过程中发生的其他步骤的可能性结合起来，以获得由于项目路线沿线的每种地貌特征而导致的破坏概率。由此产生的概率将被统计地结合起来，以获得由于所考虑的失效机制而对项目造成的总风险。这种方法提供了一种可靠的方法来评估线性结构的总风险，以一种考虑长度效应和沿其排列的地质灾害的方式。

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.