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

这项奖励是为了研究评估堤防因堤防长度变化的地质条件而导致的堤防失事风险的方法。当洪水期间堤坝被淹没时，水将开始从堤坝和堤基中渗出。在低流速下，水将通过土壤基质，而不会对其产生明显影响。然而，随着河水的上升，渗流量增加，渗水的速度和/或渗水的压力也相应增加。这增加了堤坝或堤基内部土壤被侵蚀的可能性，这种现象被称为内部侵蚀。如果任其继续，内部侵蚀可能会导致堤防不稳定，最终导致堤防坍塌。目前的堤防评价方法采用相对统一的地下条件来评价渗流行为。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基的渗流行为产生巨大的影响，往往控制着堤防在内部侵蚀方面的可靠性。该奖项支持研究，以提供所需的知识，为开发一种方法来评估堤段的地下渗漏风险，这些堤段的基础具有不同的地质特征。新的评估程序将渗流的三维方面考虑到这些特征中，并评估了可能发生内部侵蚀的渗流条件的可能性。通过绘制这些特征出现在长堤段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路线定位的各种地质特征的风险来评估整个堤段的可靠性。这项研究的结果将帮助工程师评估堤坝对他们保护的土地构成不可接受的风险。由于美国有数千英里长的堤坝需要评估和缓解渗漏，这种方法有可能极大地提高可用资金的使用效率，以增加我国基础设施这一关键方面的可靠性。该项目改变了评估在不同地质条件下建造的线性岩土工程风险的分析战略和方法。该项目是专门为堤防制定的，但所开发的方法也将适用于其他线性结构或项目。分析框架包括稳健的可靠性分析(响应面蒙特卡罗分析)，这些分析应用于各个地貌特征，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相结合，以获得由于项目路线沿线的每个地貌特征而导致的失败概率。然后将所得到的概率进行统计组合，以获得由于所考虑的失败机制而对项目造成的总风险。这种方法为评估线性结构的总风险提供了一种可靠的方法，这种方法考虑了长度效应和沿其路线定位的地质灾害。

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.