Cyclic Stress-Strain Characteristics of EPS GeoFoam

该奖项是根据2009年《美国复苏和再投资法案》(公共法律111-5)资助的。块状模压聚苯乙烯(EPS)土工合成材料是一种多孔土工合成材料，在岩土工程中有着长期的成功应用历史。EPS地泡沫具有密度极低的特点，已成为各种岩土问题的首选材料，这些问题需要使用轻质填料来稳定斜坡、软土上的路堤、挡土结构、桥梁引道、桥台和埋地管道。近年来，人们越来越多地考虑EPS土工泡沫的可压缩包裹体功能，因为它的低刚度，这使得土工泡沫成为一种理想的材料，用于减少刚性不可屈服围护结构(例如，低于坡面的建筑墙、桥台或约束墙)的地震侧向土压力。安装在这种结构上的垂直EPS板可以起到地震缓冲的作用，在地震期间减少地震墙的推力。最近的数值模拟和物理振动台试验结果表明，与不含土泡沫的刚性墙体相比，具有EPS缓冲层的地下结构墙体的动态墙体推力显著减小。尽管在实验室中已经通过三轴压缩试验对EPS土工泡沫在单调加载条件下的行为进行了广泛的研究，但对这种材料的循环应力-应变行为的研究直到现在还很少。目前已发表的EPS土工泡沫的动力特性是由应变控制的共振柱和循环单轴试验获得的，通常被研究者用于数值地震分析，其特征是随着循环剪切应变幅值的增加，动剪切模退化，阻尼比增大。然而，基于应力控制循环单轴试验的初步实验室研究结果表明，EPS土工泡沫的阻尼比随轴向应变幅值的增加而对数减小。此外，对于大于约0.8%的循环轴向应变幅值，材料似乎表现出与永久塑性应变的发生相关的粘弹塑性行为。这些实验结果表明，目前对EPS土工泡沫的循环应力-应变特性知之甚少。在此背景下，本研究的主要目的是利用实验室三轴试验来详细研究循环荷载下土工泡沫的行为。这项实验研究将基于应力控制的循环三轴试验，并将重点放在岩土应用中常用的非弹性EPS材料上。将特别强调以下参数的影响：围压、初始(静态)偏应力、循环偏应力幅值、加载频率、EPS密度和试件尺寸。智力价值：这项研究将更好地了解EPS土工泡沫在循环荷载下的基本应力-应变行为，这对于优化这种材料在岩土地震工程应用中的地震缓冲功能具有非常重要的价值。需要解决的具体问题包括：1)开始弹塑性应力-应变行为的临界循环轴向应变幅值是什么？2)围压和初始偏应力对EPS土工泡沫的动态特性有何影响？3)随着加载循环次数的增加，永久塑性应变以何种方式累积？4)试件尺寸如何影响EPS土工泡沫的循环应力-应变行为？更广泛的影响：本项目将极大地扩展我们对EPS土工泡沫在各种材料密度、初始应力条件和加载频率下的循环应力-应变行为的认识。将开发一个参考网站，概述从拟议的实验室工作中获得的地球泡沫动态特性，并向参与评估EPS地球泡沫在各种岩土地震工程应用中的地震行为的调查人员宣传。该项目将为犹他大学的研究生提供实验室三轴测试方面的培训经验，并将使本科生接触到各种岩土研究活动。

This award is funded under the American Recovery and Reinvestment Act of 2009(Public Law 111-5).Block-molded expanded polystyrene (EPS) geofoam is a type of cellular geosynthetic with a long history of successful applications in geotechnical engineering. Characterized by an extremely low density, EPS geofoam has become the material of choice in a variety of geotechnical problems requiring lightweight fill for slope stabilization, embankments on soft soils, earth retaining structures, bridge approaches, bridge abutments, and buried pipes. During recent years, increasing consideration has been given to the compressible inclusion function of EPS geofoam associated with its low stiffness, which makes geofoam an ideal material for reducing the seismic lateral earth pressures against rigid non-yielding retaining structures (e.g., below grade building walls, bridge abutments, or restrained walls). Vertical EPS panels installed against such structures may act as seismic buffers reducing the seismic wall thrust during an earthquake. Recent findings from numerical simulations and physical shaking table tests addressing the seismic behavior of buried structure walls with EPS buffer are quite promising indicating significant reduction in the dynamic wall thrust compared to the rigid case with no geofoam inclusion. Although the behavior of EPS geofoam under monotonic loading conditions has been extensively studied in the laboratory using triaxial compression tests, little research has been done until present on the cyclic stress-strain behavior of this material. Currently published dynamic properties of EPS geofoam obtained from strain-controlled resonant column and cyclic uniaxial tests, and commonly employed by investigators in numerical seismic analyses, are characterized by degradation of dynamic shear modulus and increase in damping ratio with increasing cyclic shear strain amplitude. However, results from a preliminary laboratory study based on stress-controlled cyclic uniaxial tests indicate a logarithmic decrease in the damping ratio of EPS geofoam with increasing axial strain amplitude. Furthermore, for cyclic axial strain amplitudes greater than about 0.8%, the material seems to exhibit a visco-elasto-plastic behavior associated with the occurrence of permanent plastic strains. These experimental outcomes demonstrate that the cyclic stress-strain characteristics of EPS geofoam are currently poorly understood. In this context, the main objective of this research is to use laboratory triaxial tests to investigate in detail the behavior of geofoam under cyclic loading. This experimental investigation will be based on stress-controlled cyclic triaxial tests, and will focus on non-elasticized EPS materials commonly used in geotechnical applications. Particular emphasis will be placed on the influence of the following parameters: confining pressure, initial (static) deviator stress, cyclic deviator stress amplitude, loading frequency, EPS density, and specimen size. Intellectual Merit: This research will provide a better understanding of the fundamental stress-strain behavior of EPS geofoam under cyclic loading that is invaluable in optimizing the seismic buffer function of this material in geotechnical earthquake engineering applications. Specific questions to be addressed include: 1) What is the threshold cyclic axial strain amplitude for the onset of the elasto-plastic stress-strain behavior? 2) How are the dynamic properties of EPS geofoam affected by the confining pressure and initial deviator stress? 3) In what manner the permanent plastic strains accumulate with increasing number of loading cycles? 4) How does the specimen size influence the cyclic stress-strain behavior of EPS geofoam?Broader Impacts: This project will significantly expand our knowledge on the cyclic stress-strain behavior of EPS geofoam for a variety of material densities, initial stress conditions, and loading frequencies commonly encountered in practice. A reference website summarizing the geofoam dynamic properties obtained from the proposed laboratory work will be developed and advertised to the investigators involved with the assessment of the seismic behavior of EPS geofoam in various geotechnical earthquake engineering applications. The project will provide a University of Utah graduate student with training experience in laboratory triaxial testing and will expose an undergraduate student to various geotechnical research activities.