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

岩土工程系统涉及许多元素，如多孔介质，由土壤颗粒和孔隙流体组成，与结构系统（如基础或挡土结构）相互作用。因此，由此产生的系统非常复杂，如果它受到极端载荷条件的影响，如地震或飓风期间遇到的情况，则会变得越来越复杂。包括结构元素在内的岩土工程系统失效的一个主要例子是在卡特里娜飓风过后发生在洛杉矶新奥尔良的防洪堤i型墙的倒塌。目前的工程工具和程序在模拟流动的水、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，本研究将开发一种高保真的多尺度方法，以提供尽可能多的信息，如洪水上涨时结构与土壤之间可能的分离，这种分离对结构周围水压分布的影响，以及结构附近土壤介质中可能发生的管道和冲刷。这项跨学科研究工作的活动主要围绕以下目标：为极端载荷条件下岩土系统的分析和设计提供实验调整和验证，从而预测计算研究工具；求解洪水侵蚀堤防机制模型所需的流-粒耦合项；探讨代表性洪涝情景下堤防系统洪涝破坏的复杂响应。一个新的实验程序将研究流体和嵌入粒子之间的相互作用。将透明颗粒嵌入到匹配折射率的流体中，使用数字颗粒图像测速（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堆积颗粒的联合运动。将颗粒作为沙子和/或土壤颗粒的替代品，实验结果将有助于构建颗粒升力模型-这是精确模拟所必需的-并验证计算方法。结合连续体技术和微尺度模型的最佳特征，将构建一种多尺度计算方法来理想化代表堤坝的土壤-流体-结构系统。洪水将使用计算流体动力学（CFD）方法进行建模，该方法利用任意拉格朗日欧拉（ALE）移动网格技术对流体自由表面和流固相互作用进行适当的建模，远场土壤介质将使用有限元方法进行理想化，近场土壤介质将使用离散单元方法结合CFD技术进行建模。不同尺度的子域通过适当的接口传递联系信息实现耦合。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。所提出的计算框架将同样有利于其他重要的岩土工程应用，如码头墙壁的地震反应，风暴潮对海上结构的影响，以及挡土结构和深基础的动力反应。该项目的推广活动利用了南卫理公会大学现有的项目，如Visioneering和SMU女生工程夏令营，向来自当地达拉斯-沃斯堡大都会区的中学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.