NEESR-SG: Understanding and Improving the Seismic Behavior of Pile Foundations in Soft Clays

该奖项是NSF 08-519项目“小乔治·e·布朗地震工程模拟（NEES）研究网络（NEESR）”竞赛的结果，参赛单位包括俄克拉何马大学（牵头机构）、爱荷华州立大学、圣何塞州立大学（主要是本科生机构）、地球力学公司和Advanced GeoSolutions公司。该项目将利用加州大学戴维斯分校和加州大学洛杉矶分校的NEES设备站点。桩基是许多土木工程结构的组成部分。桩基础的抗震性能是一个非常复杂的问题，涉及土壤（实体骨架、孔隙水和孔隙空气）、桩和上部结构之间的相互作用。当软弱的土壤，如软粘土和可液化的松散砂包围桩基础时，这种复杂性进一步加剧。液化砂土中桩基的受力特性已经得到了广泛的研究；然而，对软粘土或改良土中桩的地震反应进行类似的研究却很少。目前的抗震设计实践要求通过限制桩基础的侧向位移来避免桩基础的非弹性行为，因为地震后地基的损伤很难检测到。在肥沃的土壤中，限制桩基础的侧向位移相对容易实现。在软弱土壤的情况下，目前的做法是使用更多的延展性更强、直径更大的桩，这些桩设计难度大、施工成本高。解决这一问题的一种创新的、或许更具成本效益的方法是改善桩基周围的土壤。在某些情况下，对软弱地基中现有桩基进行抗震加固的结构，改善地基的抗震性能可能是唯一的选择。由于缺乏对改良土和未改良土的基本行为以及它们之间的相互作用以及地震中与桩的相互作用的基本认识，这种技术在地震区没有广泛应用。作为了解和改善所有软弱土壤中桩基抗震性能的长期目标的第一步，拟议的研究将集中在软粘土上。软粘土在美国地震多发地区相当普遍，但很少受到研究界的关注。以下是一些尚未解决的研究问题，在常规设计实践中，在将地基改善作为一种可行的选择来增强软土中桩基的地震反应之前，必须解决这些问题：(1)在抗震设计和改造中，改善桩基周围软土的有效技术是什么？(2)如何分析、模拟和设计地震荷载下的软土桩基础？(3)在有或没有地基改善的情况下，单个桩和桩群在地震事件中的表现如何？我们如何验证我们的分析和模拟工具和设计？(4)我们如何将我们的理解转化为一种有用的设计方法，以使更广泛的地震工程界受益？这项工作的智力价值在于，上述研究问题将由一个多学科团队系统地解决，该团队由结构和岩土工程师以及在地基改善技术和桩基抗震设计方面拥有丰富经验的工业合作伙伴组成。采用NEES设施和设备的创新离心机和全尺寸现场试验，简化的分析方法和复杂的全耦合模拟技术将用于了解和改进软粘土桩基的地震行为。研究成果将转化为一种有用的设计方法和工具，使整个地震工程界立即受益，并影响长期实践。简单的分析方法将满足行业的直接需求，而复杂的模拟技术有望显示简单分析方法的局限性，并影响长期的行业实践。除了使地震工程界受益外，拟议项目的更广泛影响还包括将拟议的研究整合到K-12、本科和研究生的教育中，利用从目前正在进行或已经实施的创新课程项目中获得的知识。拟议的教育计划包括一个跨越本科生和研究生多个课程的地震设计项目，一个面向高中生的基于网络的模拟竞赛，以及一个面向中学生和小学生的基于冒险场景的学习模块。该项目的数据将通过NEES数据存储库（http://www.nees.org）提供。

This award is an outcome of the NSF 08-519 program solicitation "George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) Research (NEESR)" competition and includes the University of Oklahoma (lead institution), Iowa State University, San Jose State University (a predominately undergraduate institution), Earth Mechanics, Inc., and Advanced GeoSolutions, Inc. This project will utilize the NEES equipment sites at the University of California, Davis and the University of California, Los Angeles. Pile foundations are an integral part of many civil engineering structures. The seismic behavior of pile foundations is a very complex problem with interactions between soils (solid skeleton, pore water, and pore air), piles, and superstructure. This complexity is further exacerbated when weak soils such as soft clays and liquefiable loose sands surround the pile foundation. The behavior of pile foundations in liquefiable sands has been studied extensively; however, similar investigations for soft clays or seismic response of piles in improved soils have been rarely performed. The current seismic design practice calls for avoiding inelastic behavior of pile foundations by restricting their lateral displacements because it is difficult to detect damage to foundations following an earthquake. Limiting the lateral displacement of a pile foundation is relatively easy to achieve in competent soils. In the case of weak soils, the current practice is to use an increased number of more ductile, larger diameter piles that are difficult to design and expensive to construct. An innovative, and perhaps more cost-effective, solution to this problem is to improve the soil surrounding the pile foundation. For structures undergoing seismic retrofit with existing pile foundations in weak soils, in certain instances, improving the soils may be the only option to improve the seismic behavior of the foundation. This technique is not widely used in seismic regions due to lack of fundamental understanding of the behavior of improved and unimproved soils and the interactions between them as well as with the piles during earthquakes. As a first step in a long term objective of understanding and improving the seismic behavior of pile foundations in all weak soils, the proposed research will focus on soft clays. Soft clays are quite prevalent in earthquake prone areas of the U.S., but have received little attention from the research community. Following are some of the unanswered research questions that have to be addressed before ground improvement can be used as a viable option to enhance the seismic response of pile foundations in soft clays in routine design practice: (1) What are the effective techniques for improving soft clays around pile foundations for both seismic design and retrofit? (2) How can we analyze, simulate, and design pile foundations in soft clays with ground improvement for earthquake loads? (3) How do individual piles and pile groups, with and without ground improvement, behave during seismic events and how can we validate our analysis and simulation tools and designs? And (4) how can we translate our understanding into a useful design methodology to benefit the broader earthquake engineering community? The intellectual merit of this work is that the above mentioned research questions will be systematically addressed using a multidisciplinary team consisting of structural and geotechnical engineers and industrial partners who have extensive experience in ground improvement techniques and seismic design of pile foundations. Innovative centrifuge and full-scale field tests using NEES facilities and equipment, simplified analysis methods, and sophisticated fully coupled simulation techniques will be utilized to understand and improve the seismic behavior of pile foundations in soft clays. The research results will be translated into a useful design methodology and tools that will benefit the entire earthquake engineering community immediately as well as influence the long term practices. Simple analysis methods will serve the immediate needs of the industry while sophisticated simulation techniques are expected to show the limitations of the simple analysis methods and impact the long term industry practices. In addition to benefiting the earthquake engineering community, the broader impacts of the proposed project include the integration of the proposed research into education at K-12 and undergraduate and graduate levels using the knowledge gained from innovative curriculum projects currently underway or already implemented. The proposed education plan includes a seismic design project that spans multiple courses for undergraduate and graduate students, a web-based simulation competition for high school students, and an adventure scenario based learning module for middle and elementary school students. Data from this project will be made available through the NEES data repository (http://www.nees.org).