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

对于横向荷载作用下的桥梁和其他结构，被动土压力对整体稳定性有很大贡献。虽然作用在桩帽和桥台墙上的最大被动压力是可以很容易预测的，但被动压力如何作为挠度的函数来发展的问题则更成问题。此外，目前使用的荷载-位移关系基本上都是从静态或极慢的荷载中推导出来的。在地震荷载作用下，既存在动力效应，又存在循环效应，从而改变了荷载-挠度关系。虽然循环荷载效应通常会降低土体的强度和刚度，但由于材料和辐射阻尼，动载效应往往会导致土体强度和刚度的明显增加。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统计荷载-挠度关系。这种方法是保守的还是非保守的程度是不确定的。然而，可以肯定的是，桥台接触处的土体刚度是影响桥梁整体性能的关键因素。我们对该项目的愿景是在全尺寸试验的基础上，建立与桩帽和桥台相邻的典型土体的荷载-位移关系，其中考虑了动力和循环效应。将动力效应结合到被动土压力关系中，将创建以前无法获得的对动态和循环土壤响应的有效定义。虽然振动台和离心机模型可以提供有关这些问题的重要指导，但为了提供地面真实信息，必须进行合理数量的全尺寸载荷试验。这项研究将满足这一需求。就智能优点而言，拟议的测试和分析将为从业者提供基于各种土壤类型和密度以及一系列振动频率和位移水平的全尺寸桩帽测试的动态刚度和阻尼系数。此外，存档在NEES数据库中的测试结果将成为对校准/验证新的计算机代码或数值模型感兴趣的研究人员的重要基准。为了获得建立动态、被动载荷-位移关系所需的数据，将进行现场测试计划。该计划将包括横向加载一个没有回填的混凝土桩帽，然后在四种不同类型的土壤回填后重新加载。由于回填材料提供的阻力是密度的函数，因此每个回填材料将被压实成两种不同的密度，然后进行测试。加载将通过组合使用液压负载执行器和NEES-UCLA的偏心质量振动器来完成。最初，液压负载振动器会将盖子加载到目标偏转，此时执行机构的长度将固定。下一步，安装在桩帽顶部的偏心振动器将被激活，以产生动态荷载。组合使用振荡器和摇床将提供以前在全尺寸测试中无法获得的应变水平和速率。振动器的频率将改变，以获得一定范围的加载频率。然后，随着挠度的增加，将重复这一加载过程。随后的数据分析将把观测到的被动土压力关系量化为一系列曲线和相应的方程。更多的分析工作将根据弹簧和仪表盘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-结构相互作用的土工组件来改进结构组件的设计。其他影响包括为参与的大学生提供指导的教育经验，以及初中和高中学生参加以所进行的研究为中心的外展计划时对地震工程问题的认识增加。

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.