Multiscale Experimental and Computational Investigations of Erosion-Induced Failure of Levee Systems

岩土工程系统涉及许多元素，如由土粒和孔隙流体组成的多孔介质，它们与基础或围护结构等结构系统相互作用。因此，由此产生的系统非常复杂，如果它受到极端加载条件的影响，例如地震或飓风期间遇到的条件，它就会变得越来越复杂。包括结构元素的岩土系统失败的一个主要例子是路易斯安那州新奥尔良卡特里娜飓风过后防洪堤I形墙的坍塌。现有的工程工具和程序在模拟流水、土壤介质和堤防结构之间相互作用的动力学方面非常有限。为了解决这个问题，目前的工作将开发一种高保真的多尺度方法，以提供尽可能多的合理可行的信息，关于洪水上升时结构和土壤之间可能发生的分离，这种分离对结构周围水压力分布的影响，以及结构附近土壤介质中可能发生的管道和冲刷。这项跨学科研究的活动围绕以下目标：为极端荷载条件下的岩土系统的分析和设计提供一个经过实验调整和验证的、从而具有预测性的计算研究工具；解决建立堤防洪水侵蚀机理模型所需的流固耦合项，探索典型洪水情景下堤防系统洪水破坏的复杂响应。一项新的实验计划将研究流体与嵌入颗粒之间的相互作用。利用嵌入在匹配折射率流体中的透明颗粒，将使用数字粒子图像测速仪(DPIV)和嵌入颗粒的直接成像相结合的方法来测量流体和密集填充颗粒的组合运动。以颗粒作为沙粒和/或土壤颗粒的替代物，实验结果将有助于构建颗粒上的升力模型-这是精确模拟所必需的-并验证计算方法。将构建一种结合基于连续介质的技术和微尺度模型的最佳特征的多尺度计算方法，以理想化代表堤坝的土-流体-结构系统。洪水将使用计算流体动力学(CFD)方法进行模拟，该方法使用任意拉格朗日欧拉(ALE)移动网格技术来适当地模拟流体自由面和流固耦合作用，远场土壤介质将使用有限元方法理想化，近场土壤介质将使用离散元方法和CFD技术相结合来建模。不同尺度的子域将通过在其界面上适当传递联系信息来耦合。拟议工作的结果将提高对岩土系统复杂行为的理解，并有助于飓风风险评估。建议的计算框架同样适用于其他重要的岩土工程应用，例如码头墙的地震响应、风暴潮对近海结构的影响，以及围护结构和深基础的动力响应。该项目的外展活动利用南卫理公会大学现有的项目，如愿景工程和SMU女生工程夏令营，向当地达拉斯-沃斯堡大都会的初中生和高中生推广跨学科工程。

Geotechnical systems involve a number of elements such as the porous media, composed of soil grains and pore-fluids, interacting with a structural system such as a foundation or a retaining structure. The resulting system is thus very complicated and becomes increasingly complex if it is subjected to extreme loading conditions such as those encountered during earthquakes or hurricanes. A prime example of failure of geotechnical systems that includes a structural element is the collapse of the flood-protection levee I-wall in the aftermath of Hurricane Katrina in New Orleans, LA. Current engineering tools and procedures are very limited in modeling the dynamics of the interactions between the flowing water, the soil media and the levee structure. To address this issue, the present work will develop a high-fidelity multiscale approach to provide as much information as reasonably feasible about the possible separation between the structure and the soil as floodwater rises, the impact of such separation on the distribution of water pressure surrounding the structure, and the potential occurrence of piping and scour in the soil media near the structure.The activities of this interdisciplinary research effort are centered around the following goals: to provide an experimentally tuned and validated, and thus predictive, computational research tool for the analysis and design of geotechnical systems subjected to extreme loading conditions; to resolve the fluid-particle interaction coupling term needed to model the mechanism of flood-induced erosion of levees; and to explore the complex response of flood-induced failure of levee systems under representative flooding scenarios. A novel experimental program will investigate the interaction between a fluid and embedded particles. Using transparent particles embedded in a matched-index-of-refraction fluid, the combined motion of the fluid and densely-packed particles will be measured using a combination of digital particle image velocimetry (DPIV) and direct imaging of the embedded particles. Taking the particles as surrogates for sand and/or soil particles, the experimental results will serve to construct a model for the lift force on particles - which is necessary for accurate simulations - and validate the computational method. A multi-scale computational approach that combines the best features of continuum-based techniques and microscale models will be constructed to idealize the soil-fluid-structure system that represents a levee. Floodwater will be modeled using a computational fluid dynamics (CFD) approach that utilizes the Arbitrary Lagrangian Eulerian (ALE) moving mesh technique for proper modeling of the fluid free-surface and fluid-structure interaction, far-field soil media will be idealized using the finite element method, and near-field soil media will be modeled using the discrete element method coupled with a CFD technique. The different scale subdomains will be coupled through proper transfer of contact information at their interfaces.The outcome of the proposed work will improve the understanding of the complex behavior of geotechnical systems and contribute to hurricane risk assessment. The proposed computational framework will equally benefit other important geotechnical engineering applications such as the seismic response of quay walls, impact of storm surge on offshore structures, and the dynamic response of retaining structures and deep foundations. The outreach activities in this project utilize existing programs at Southern Methodist University such as Visioneering and SMU Engineering Summer Camps for Girls to promote interdisciplinary engineering to Middle and High School students from the local Dallas-Fort Worth Metroplex.