Mobilization of Sand Particles and Erosion Progression Under Various Permeating Fluids

众所周知，管道会导致堤坝和土坝的灾难性破坏，人们对管道的研究已有近一个世纪的历史。在典型的土壤侵蚀实验研究中，通常采用自来水或去离子水作为渗透介质，因此对土壤侵蚀行为的认识大多局限于相对纯净的水为渗透介质的情况。在野外，穿过土坝或堤坝的渗透流体可能含有来自上游土壤侵蚀的渗透流体中携带的泥浆(在安装泥浆墙的情况下)或细粉。细粒含量可以具有不同的大小、浓度和其他物理化学特性。我们在恒定水头下进行的可重复的室内冲蚀试验表明，不同的渗透液，砂岩的管涌过程有很大的不同。膨润土浆和含有1%无粘性细粉的水在较长时间内不会引起管涌进展，而水和含有0.1%细粉的水在不到40分钟的时间内使预制的管孔扩大到土柱一侧。根据目前的知识，在含有无粘性细粉的流体中，这种观察是违反直觉的，因为渗透密度和粘度较高的流体往往会施加更高的水力剪应力，并导致更多的侵蚀。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了解。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，可能还有悬浮液的表面电位)和流动条件累积起来对沙子的侵蚀起作用。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，并为不同渗透流体和水力条件下沙子不同的侵蚀行为提供基本解释。研究项目将回答以下三个问题：(1)哪种力主导着颗粒的移位：颗粒间的London-van der Waal力和电双层力，还是流体动力？(2)具有不同物理化学特性(颗粒浓度、尺寸、粘度、流体密度、静电表面电位、离子强度、pH)的各种渗透液对移位力有何贡献？哪一个是主导因素？(3)流速(即层流和湍流流)如何改变上述每个因素在颗粒移位中的作用？将对11种渗透液的物理化学特性进行实验量化。利用这11种流体进行了微观流动实验和室内孔蚀实验，以揭示上述特性在不同水力条件下颗粒排出过程中的单独作用。这项研究将产生变革性的知识，因为它将提供对土坝和堤坝中管道进展的现实理解。它还将指导我们切实评估和修复许多自然和已建基础设施，例如在天然河岸栖息地保护中，在河床或桥梁基础冲刷中，水流可以在不同流速下携带不同浓度的各种颗粒，以及在管道工程中，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。这笔资金还将为该研究所提供教育机会，以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纳入研究生和本科课程，并接触和参与K-12学生和未被充分代表的本科生参与研究，以吸引未来的人才进入STEM领域。在这个项目中开发的知识将通过网站、在大学和高中的演讲以及由国际和平协会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.