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

该奖项是NSF 08-519项目征集“小乔治·E·布朗地震工程模拟网络(NEES)研究(NEESR)”竞赛的结果，该奖项包括俄克拉荷马大学(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)、地球力学公司和高级地球解决方案公司。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。桩基础是许多土木工程结构中不可或缺的一部分。桩基础的抗震性能是一个非常复杂的问题，它包含土(固体骨架、孔隙水和孔隙空气)、桩和上部结构之间的相互作用。当软粘土和可液化的松散砂土等软弱土壤包围桩基础时，这种复杂性进一步加剧。液化砂土中桩基础的性状已得到了广泛的研究，但对于软粘土或改良土中桩的地震反应的类似研究还很少。目前的抗震设计实践要求通过限制桩基的横向位移来避免桩基的非弹性行为，因为地震后很难检测到地基的损伤。在肥沃的土壤中，限制桩基的横向位移相对容易实现。对于软弱的土壤，目前的做法是使用更多更具延展性的、直径更大的桩，这些桩很难设计，建造成本也很高。解决这一问题的一个创新的、或许是更具成本效益的解决方案是改善桩基础周围的土壤。对于软弱土层中的既有桩基础进行抗震加固的结构，在某些情况下，改善土体可能是改善基础抗震性能的唯一选择。由于缺乏对改良土和未改良土的特性以及它们之间的相互作用以及在地震期间与桩的相互作用的基本了解，这项技术在地震区并未得到广泛应用。作为了解和改善所有软土中桩基础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十分普遍，但很少受到研究界的关注。以下是在常规设计实践中必须解决的一些悬而未决的研究问题，这些问题必须被用作增强软粘土中桩基础的地震响应的可行选择：(1)对于抗震设计和改造，有什么有效的技术来改善桩基础周围的软粘土？(2)我们如何分析、模拟和设计在有地基加固的软粘土中的桩基以应对地震荷载？(3)单独的桩和群桩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？以及(4)我们如何将我们的理解转化为有用的设计方法，以造福于更广泛的地震工程界？这项工作的学术价值在于，将利用一个由结构和岩土工程师以及在地基加固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的多学科团队，系统地解决上述研究问题。创新的离心机和使用NEES设施和设备的全尺寸现场试验、简化的分析方法和复杂的全耦合模拟技术将被用来了解和改善软粘土中桩基础的抗震性能。研究成果将转化为有用的设计方法和工具，将立即惠及整个地震工程界，并影响长期实践。简单的分析方法将服务于行业的即时需求，而复杂的模拟技术预计将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利用目前正在进行或已经实施的创新课程项目所获得的知识，将拟议的研究纳入K-12、本科生和研究生各级的教育。拟议的教育计划包括面向本科生和研究生的跨多门课程的抗震设计项目，面向高中生的基于网络的模拟比赛，以及面向中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).