NEESR-SG: Experimental and Micromechanical Computational Study of Pile Foundations Subjected to Liquefaction-Induced Lateral Spreading

文摘：对斜坡地及滨水地区桩基液化侧向扩展问题进行了深入的研究。这是造成地震损失的主要原因，造成的损失高达数十亿美元。现有的工程方法(极限平衡法和p-y法)对液化砂土作用下的桩基弯曲响应的评价存在较大的知识空白。不确定性的一个来源是必须对群桩施加侧向压力的区域，即单桩面积的总和，或桩间土。这两个假设之间的差异可能会产生三倍变化的桩身弯矩。另一个不确定性的来源是土壤渗透性的重要性，它没有包括在当前的LE方法中，但全尺寸和离心机振动试验表明，它可以产生7倍的弯矩变化。离心机和全尺寸振动台试验，以及现场试验和初步分析表明，液化砂土在单桩和群桩附近的反应是复杂的。包括土-桩相互作用引起的超负孔隙水压力引起的液化土的硬化；最终膨胀的自由场中的水；以及地基附近的液化土以一种复杂的随时间变化的方式软化。将使用一系列现有的实验和分析工具，采用NEES提供的一种基本和全面的办法来解决这一问题。研究的目的是了解桩基础附近横向扩展过程中的砂土液化现象；解决如何在简化条件下以及在为适当的分析平台提供基本理解的情况下设计抗横向扩展的桩基的工程问题；以及澄清在未来的研究和工程应用中使用离心机模型的正确方法。这是通过一项实验研究计划完成的，该计划使用了布法罗大学的1G 2D振动、6米高的层流容器NEES设备和RPI NEES离心机设备，包括使用新型先进传感器测量振动过程中的土壤加速度、变形和压力；以及RPI、加州大学圣地亚哥分校和杜兰大学的微观机械离散元(DEM)数值实验和有限元(FE)分析；所有这些都由适当的识别和分析框架集成在一起，包括系统识别和可视化能力。通过1G和离心机物理试验验证的DEM数值试验，旨在首次在单个土粒和颗粒群的水平上提供详细的信息，根据基本的细观力学原理计算出靠近桩基础的液化土的响应，包括应变局部化、剪切带和桩周围流动。DEM的洞察将被用来改进在有限元分析中使用的本构关系中未解决的重要方面。一个有来自美国、日本(包括日本合作者)和其他国家的广泛工程师和研究人员参加的第三年研讨会将对研究结果进行评估。该项目将演练一套新的研究工具和整合它们的方法。该项目的互动网站将定期更新结果和解释。研究生和本科生参与研究，重点是招收人数不足的学生、顶峰课程和类似课程的学期项目、在项目结束时准备1-2个教育模块，以及针对K-12学生及其教师的具体努力，目的是加强项目的影响。

Abstract: Critical research will be conducted on the problem of pile foundations subjected to liquefaction-induced lateral spreading in sloping ground and near waterfronts. This is a major cause of earthquake damage with costs in the billions of dollars. A large knowledge gap has been identified in current engineering methods to evaluate bending response of pile foundations to the lateral loading applied by the liquefied sand (limit equilibrium (LE) and p-y methods). One source of uncertainty is the area over which the lateral pressure must be applied for pile groups, the sum of the areas of the piles alone, or also soil between the piles. The difference between these two assumptions can produce pile-bending moments varying by a factor of three. Another source of uncertainty is the importance of soil permeability, not included in current LE methods, but which full scale and centrifuge shaking tests indicate can produce a variation of bending moments by a factor of seven. Centrifuge and full-scale shaking table tests, as well as insitu testing and preliminary analyses, suggest a complicated response of the liquefied sand in the neighborhood of individual piles and pile groups. Included is stiffening of the liquefied soil due to excess negative pore water pressures caused by soil-pile interaction; water flowing from the free field with eventual expansion; and, softening of the liquefied soil near the foundation, in a complicated time-dependent manner. A fundamental and comprehensive approach, made possible by NEES, will be used to tackle the problem using a range of available experimental and analytical tools. The objectives of the research are to understand the phenomenon of sand liquefaction during lateral spreading near pile foundations; to solve the engineering questions of how to design pile foundations against lateral spreading both in simplified terms as well as in terms of providing a basic understanding for appropriate analytical platforms; and, to clarify the correct way to use centrifuge modeling in future research and engineering applications. This is done through an experimental research plan using the University at Buffalo 1g 2D shaking, 6m-tall laminar box container NEES facility and the RPI NEES centrifuge facility, including use of novel advanced sensors to measure soil accelerations, deformations and pressures during shaking; and micromechanical Discrete Element (DEM) numerical experiments and finite element (FE) analyses at RPI, UC San Diego and Tulane University; all integrated by an appropriate identification and analysis framework including system identification and visualization capabilities. The DEM numerical experiments, validated by the 1g and centrifuge physical tests, aim at providing for the first time detailed information at the level of individual soil grains and groups of grains, on the response of the liquefied soil close to pile foundations including strain localization, shear banding and flow around the piles, calculated from basic micromechanical principles. The DEM insights will be used to refine important unsolved aspects of the constitutive relations used in the FE analyses. A Year 3 Workshop with the participation of a broad range of engineers and researchers from the US, Japan (including Japanese collaborators) and other countries, will evaluate the results of the research. The project will exercise a new set of research tools and ways to integrate them. The Interactive Web Site of the project will be periodically updated with results and interpretations. Graduate and undergraduate involvement in research, with emphasis on the recruitment of under-represented students, term projects in capstone and similar courses, preparation of 1-2 Educational Modules at the end of the project, and specific efforts toward K-12 students and their teachers, are aimed at intensifying the impact of the project.