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

对于受横向荷载影响的桥梁和其他结构，被动土压力对整体稳定性有重要贡献。虽然作用于桩承台和台壁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有问题。此外，目前使用的所有载荷-位移关系基本上都是从静态或极慢载荷中推导出来的。在地震荷载作用下，动力效应和循环效应同时存在，改变了荷载-挠度关系。虽然循环加载效应通常会降低土壤的强度和刚度，但动力加载效应往往会由于材料和辐射阻尼而使土壤的强度和刚度明显增加。由于缺乏明确的荷载-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中应用静力-挠度关系。这种方法是保守还是非保守的程度是不确定的。然而，可以肯定的是，土刚度是桥梁整体性能的一个关键因素。我们对这个项目的愿景是在全面测试的基础上，为桩承台和桥台附近的典型土壤开发荷载-位移关系，这考虑了动力和循环效应。将动力效应纳入被动土压力关系将创建一个有效的动力和循环土壤响应定义，这在以前是不可用的。虽然振动台和离心机模型可以提供有关这些问题的重要指导，但需要进行合理数量的全尺寸负载试验来提供真实的信息。这项研究将满足这一需求。就智力价值而言，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因子。此外，保存在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。为了获得开发动态被动载荷-位移关系所需的数据，将进行现场测试程序。该方案将包括横向加载一个没有回填的混凝土桩顶，然后在回填四种不同的土壤类型后重新加载。由于回填体提供的阻力是密度的函数，每个回填体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执行器和NEES-UCLA的偏心质量激振器的组合使用来完成。最初，液压负载激振器将阀盖加载到目标挠度，此时执行器长度将固定。接下来，安装在桩帽顶部的偏心振动筛将被激活以产生动态载荷。混合使用振荡器和激振器将提供以前在全尺寸测试中无法获得的应变水平和速率。将改变激振器的频率以获得一系列加载频率。这个加载过程将在不断增加的挠度水平上重复进行。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。额外的分析工作将根据弹簧和阻尼器模型确定回填体的动态阻抗。该项目更广泛的影响包括通过更好地定义土-结构相互作用的岩土工程成分来改进结构部件的设计。其他影响还包括为参与研究的大学生提供指导教育经验，以及提高初中生和高中生对地震工程问题的认识，这些学生参与了以研究为中心的外展项目。

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.