NEESR-II: Dynamic Passive Pressure on Full-Scale Pile Caps

NEESR-II：全尺寸桩帽上的动态被动压力

• 批准号: 0421312
• 负责人: Travis Gerber
• 金额: --
• 依托单位: Brigham Young University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-10-01 至 2009-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0421312&HistoricalAwards=false
• 关键词: NEESR; II; Dynamic; Passive; Pressure

For bridges and other structures subject to lateral loadings, passive earth pressure contributessignificantly to overall stability. While the maximum passive pressure acting on pile caps andabutment walls can readily be predicted, the issue of how passive pressure develops as a function ofdeflection is more problematic. Furthermore, essentially all of the load-displacement relationshipscurrently used are derived from static or extremely slow loadings. Under seismic loading conditions,both dynamic and cyclic effects are present which alter the load-deflection relationship. While cyclicloading effects typically reduce the strength and stiffness of the soil, dynamic loading effects tend toproduce an apparent increase in soil strength and stiffness due to material and radiation damping.Faced with the lack of well-defined load-displacement relationships which address the effects of bothstiffness degradation and increased damping, the engineering community has often applied staticload-deflection relationships in seismic design situations. The degree to which this approach isconservative or non-conservative is uncertain. What is certain, however, is that soil stiffness atabutment contacts is a critical factor in overall bridge performance.Our vision for this project is to develop load-displacement relationships for typical soilsadjacent to pile caps and abutments based on full-scale testing, which account for both dynamic andcyclic effects. Incorporation of dynamic effects into passive earth pressure relationships wouldcreate a validated definition of dynamic and cyclic soil response that has previously beenunavailable. Although shake table and centrifuge models can provide important guidance regardingthese issues, a reasonable number of full-scale load tests are necessary to provide ground truthinformation. This research would fulfill this need. In terms of intellectual merit, the proposedtesting and analysis will provide practitioners with dynamic stiffness and damping factors based onfull-scale pile cap tests for a variety of soil types and densities, as well as for a range of vibrationfrequencies and displacement levels. In addition, the test results, archived in the NEES database,will become important benchmarks for researchers interested in calibrating/verifying new computercodes or numerical models.To obtain the data needed to develop dynamic, passive load-displacement relationships, afield testing program will be conducted. This program will consist of laterally loading a concrete pilecap without backfill and then reloading after backfilling with four different soils type. Since theresistance provided by the backfill is a function of density, each backfill will be compacted to twodifferent densities and then tested. Loading will be accomplished by a combined use of a hydraulicload actuator and NEES-UCLA's eccentric mass shakers. Initially, a hydraulic load shaker will loadthe cap to a target deflection, at which point the actuator length will be fixed. Next, eccentric shakersmounted atop the pile cap will be activated to produce a dynamic loading. The combined use ofactuator and shaker will provide levels and rates of strain previously unobtainable in full-scaletesting. The frequency of the shakers will be varied to obtain a range of loading frequencies. Thisloading process will then be repeated at increasing levels of deflection. Subsequent data analysis willquantify the observed passive earth pressure relationships in a series of curves and correspondingequations. Additional analytical work will determine dynamic impedances of the backfill based onspring and dashpot models.The broader impacts of this project include improved design of structural componentsthrough a better definition of the geotechnical component of soil-structure interaction. Additionalimpacts include mentored educational experiences for participating university students as well asincreased awareness of earthquake engineering issues by junior high and high school studentsparticipating in an outreach program centered on the research performed.

对于承受横向荷载的桥梁和其他结构，被动土压力对整体稳定性有重要贡献。虽然作用在桩帽和桥台墙上的最大被动压力可以很容易地预测，但被动压力如何作为挠度的函数发展的问题更成问题。此外，目前使用的载荷-位移关系基本上都是从静态载荷或极慢载荷导出的。在地震荷载条件下，动力和循环效应都存在，这改变了荷载-挠度关系。虽然循环荷载效应通常会降低土壤的强度和刚度，但由于材料和辐射阻尼，动态荷载效应往往会明显增加土壤的强度和刚度。面对缺乏明确定义的荷载-位移关系来解决刚度退化和阻尼增加的影响，工程界经常在抗震设计情况下应用静态荷载-位移关系。这种方法保守或不保守的程度是不确定的。然而，可以肯定的是，桥台接触处的土壤刚度是桥梁整体性能的关键因素。我们对该项目的设想是，根据全尺寸试验，为桩帽和桥台附近的典型土壤建立荷载-位移关系，该试验考虑了动力和循环效应。将动态效应纳入被动土压力关系中，将创建一个有效的动态和循环土壤响应的定义，这在以前是不可用的。虽然振动台和离心机模型可以提供重要的指导，克服这些问题，一个合理的数量的全尺寸载荷试验是必要的，以提供地面真相信息。这项研究将满足这一需求。在智力价值方面，拟议的测试和分析将为从业人员提供动态刚度和阻尼系数的基础上，全尺寸桩帽测试的各种土壤类型和密度，以及一系列的振动频率和位移水平。此外，保存在NEES数据库中的试验结果将成为对校准/验证新计算机代码或数值模型感兴趣的研究人员的重要基准。为了获得开发动态、被动荷载-位移关系所需的数据，将进行现场试验计划。该程序将包括横向加载混凝土桩帽没有回填，然后重新加载后，用四种不同的土壤类型。由于回填土提供的阻力是密度的函数，因此每种回填土将被压实到两种不同的密度，然后进行测试。加载将通过液压负载致动器和NEES-UCLA偏心质量振动筛的组合使用来完成。最初，液压负载振动器将加载帽到目标偏转，在该点处，致动器长度将被固定。接下来，安装在桩帽顶部的偏心振动器将被激活以产生动态载荷。制造器和振动器的组合使用将提供以前在全尺寸测试中无法获得的应变水平和速率。振动器的频率将被改变以获得一定范围的加载频率。这个加载过程将在挠度增加的情况下重复。随后的数据分析将量化所观察到的被动土压力的关系，在一系列的曲线和相应的方程。额外的分析工作将根据弹簧和阻尼器模型确定回填土的动态阻抗。该项目的更广泛影响包括通过更好地定义土-结构相互作用的岩土部分来改进结构部件的设计。其他影响包括参与大学生的指导教育经验，以及初中和高中学生参与以研究为中心的外展计划对地震工程问题的认识提高。