Mobilization of Sand Particles and Erosion Progression Under Various Permeating Fluids

众所周知，管道会导致防洪堤和土坝的灾难性破坏，人们对管道的研究已经持续了近一个世纪。在典型的土壤侵蚀实验研究中，渗透流体采用自来水或去离子水；因此，对土壤侵蚀行为的认识大多局限于以相对纯水为渗透流体的情况。在现场，通过土坝或堤防的渗透流体可能含有浆液（在浆液墙安装的情况下）或细粒，这些浆液是由上游土壤侵蚀的渗透流体携带的。细粒含量可能具有不同的大小、浓度和其他物理化学特性。我们在恒压头条件下进行了可重复的室内冲蚀试验，结果表明，不同渗透流体对砂体的管程有显著的影响。膨润土浆液和含有1%无黏性细粒的水在较长时间内不会引起管道进展，而含有0.1%细粒的水和水在不到40分钟的时间内使预制管孔扩大到土柱的一侧。在无黏性细粒流体的情况下，这一观察结果与直觉相反，根据目前的知识，密度和粘度较高的渗透流体往往会施加更高的水力剪切应力，造成更多的侵蚀。目前的知识和最近的实验室观察之间的差距是由于缺乏对悬浮液-流体-土壤相互作用的基本理解。本研究的假设是，渗透流体的物理化学特性（粘度、流体密度、离子强度、pH值，可能还有悬浮液的表面电势）和流动条件共同导致了砂粒的侵蚀。本研究的主要目的是确定悬浮-流体-土壤相互作用的物理化学机制，为不同渗透流体和水力条件下砂土的不同侵蚀行为提供基础解释。该研究项目将回答以下三个问题：(1)哪些力主导粒子位移：粒子间的伦敦-范德华力和电双层力，还是流体动力力？(2)具有不同物理化学特性（颗粒浓度、粒径、粘度、流体密度、静电表面电位、离子强度、pH值）的各种渗透流体对排位力的影响如何？哪个是主导因素？(3)流速（即层流和湍流）如何改变上述各因素在颗粒位移中的作用？对11种渗透流体的物理化学特性进行了实验量化。利用这11种流体进行微观流动实验和室内孔蚀实验，揭示不同水力条件下上述特性在颗粒位移过程中的各自作用。这项研究将产生变革性的知识，因为它将提供对土坝和堤防管道进展的现实理解。它还将指导我们现实地评估和修复许多自然和人造基础设施，例如在自然河岸栖息地保护中，在河床或桥梁基础冲刷中，水流可以以不同的流速携带不同浓度的各种颗粒，在管道工程中，雨水、废水或石油可能具有不同的粘度或携带不同的颗粒负荷。这笔资金还将为PI提供教育机会，指导研究生成为未来独立和自信的研究人员，及时将多学科研究方法和成果整合到研究生和本科课程中，并让K-12学生和代表性不足的本科生参与研究，以吸引未来的人才进入STEM领域。在这个项目中发展的知识将通过网站、在大学和高中的演讲以及PI和学生撰写的出版物来传播。

Piping is known to cause catastrophic failures of levees and earthen dams, and has been studied for nearly a century. In the typical experimental studies on soil erosion, tap water or de-ionized water is used as permeating fluid; therefore, the understanding of soils' erosion behavior is mostly limited to situation where relatively pure water is the permeating fluid. In the field, permeating fluids through earthen dams or levees may contain slurry (in the case of slurry wall installation) or fines that are carried in the permeating fluid from the upstream soil erosion. The fines content may have various sizes, concentrations, and other physicochemical characteristics. Our repeatable laboratory experiments in hole-erosion tests under constant head have revealed that the piping progression of a sand differed significantly with different permeating fluids. Bentonite slurry and water that contains 1% cohesionless fines did not induce piping progression during a prolonged period, while water and water with 0.1% fines caused the pre-formed piping hole to enlarge to the side of the soil column in less than 40 minutes. This observation is counter-intuitive in the case of fluid with cohesionless fines, based on the current knowledge that permeating fluid with higher density and viscosity tends to exert higher hydraulic shear stress and cause more erosion. The gap between the current knowledge and the recent laboratory observation is due to the lack of fundamental understanding of the suspension-fluid-soil interaction. The hypothesis of this research is that the physicochemical characteristics (viscosity, fluid density, ionic strength, pH, and possibly suspension's surface electric potential) of the permeating fluids and the flow conditions accumulatively contribute to the erosion of sand. The main objective of this research is to identify the physicochemical mechanisms of suspension-fluid-soil interactions and provide fundamental explanations for the different erosion behaviors of sand under various permeating fluids and hydraulic conditions. The research project will answer the following three questions: (1) Which forces dominate the particle dislodging: inter-particle London-van der Waal forces and electrical double layer forces, or the hydrodynamic forces? (2) How do the various permeating fluids with different physicochemical characteristics (particulate concentration, size, viscosity, fluid density, electrostatic surface potential, ionic strength, pH) contribute to the dislodging forces? Which is a dominating factor? (3) How does the flow rate (i.e., laminar and turbulent flows) change the role of each of the aforementioned factors in particle dislodging? The physicochemical characteristics of eleven permeating fluids will be experimentally quantified. Microscopic flow experiments and laboratory hole erosion experiments using the eleven fluids will be conducted to reveal the individual roles of the aforementioned characteristics in the particle dislodging process at different hydraulic conditions. This research will yield transformative knowledge because it will provide realistic understanding of the piping progression in earthen dams and levees. It will also guide us to realistically evaluate and remediate many natural and built infrastructures, such as in natural riparian habitat protection, in riverbed or bridge foundation scouring where flows can carry various particles with different concentrations at different flow rate, and in pipeline engineering where storm water, wastewater, or oil can have different viscosity or carry various particle loadings. This funding will also provide educational opportunities for the PI to mentor graduate students to become future independent and confident researchers, to timely integrate multidisciplinary research methodologies and outcomes into graduate and undergraduate courses, and to outreach to and involve K-12 students and underrepresented undergraduate students in research in order to attract future talents to the STEM fields. The knowledge developed in this project will be disseminated through websites, presentations in university and high schools, and publications written by the PI and students.