NEESR Payload: Measurement of the Strength of Liquefied Soil in Physical Models

NEESR 有效负载：物理模型中液化土强度的测量

• 批准号: 0724080
• 负责人: Mandar Dewoolkar
• 金额: --
• 依托单位: University of Vermont & State Agricultural College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-09-01 至 2010-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0724080&HistoricalAwards=false
• 关键词: NEESR; Payload; Measurement; Strength; Liquefied

This research is an outcome of the National Science Foundation 07-506 program solicitation "George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) Research" competition. This project is a payload to National Science Foundation award 0530478, "NEESR-GC: Seismic Risk Mitigation for Port Systems," and will utilize the tests being conducted by award 0530478 in the NEES geotechnical centrifuge at the University of California, Davis. This project is led by the University of Vermont and includes a subaward to the University of New Hampshire. It has long been observed that saturated sands subjected to shock or earthquake loading experience drastic loss of strength and behave as heavy fluids, gradually regaining strength as internal water pressures dissipate. As long as the liquefied state persists, the soil will flow down slopes, producing destructive landslides and large drag forces on obstacles such as piled foundations. Modeling this behavior for risk studies and engineering design, however, requires adequate measurements of how shearing strength loss and its eventual recovery evolve as internal water pressures build up and subsequently dissipate. There are currently no full-scale field measurements of these strength changes to guide development of such models; existing field case histories are limited to observing the final damage produced by the liquefaction process. Controlled laboratory measurements would be desirable, but the onset of liquefaction is accompanied by such large strains that soil samples in conventional laboratory tests become so drastically deformed that reliable strength measurements can no longer be made. As a first step in measuring the evolving behavior of liquefied sands, it is envisioned that the shear strength of liquefying sand can be measurable in-flight in the NEES geotechnical centrifuge model using a thin coupon (plate, about 25 millimeters by 25 millimeters by 1.5 millimeters) pulled horizontally through the soil model, with its major dimensions parallel to the base of the model. The large strains and strain rates associated with liquefaction flow failures would thus be simulated by moving the coupon relative to the sand, through and after the shaking until the excess pore pressures dissipate. By measuring the drag force on the coupon, it will be possible to observe the evolution of the soil shear strength as it decreases to a minimum (residual strength) and subsequently increases as pore pressures dissipate. The centrifuge models will provide realistic field-scale stresses and boundary conditions, and the dense array of instrumentation will facilitate observations to be made on the strength changes in the liquefying sand from beginning to end of simulated earthquakes. The results would also be used to validate companion ring shear and modified cyclic triaxial testing. The combined results of a series of centrifuge and small-scale laboratory experiments will provide guidance on how to simulate the large-scale tests in smaller laboratory apparatus, thus making it easier to study the behavior of other soil types, such as silty and clayey sands, during liquefaction both for general studies and for specific engineering design purposes. A simple yet rational model for predicting the rate-dependent evolution of shearing strength of granular soils as pore pressures build up and the soil mass deforms will be developed. This will permit more accurate simulation of such problems as estimating the forces exerted by liquefied soil on obstacles like pile-supported structures, and the prediction of flow slide behavior in general. These results are expected to give designers enhanced understanding of how to choose residual strength values for remediation of earth structures. Equipment required to conduct the payload tests will be designed and built by a group of undergraduate mechanical and electrical engineering students at the University of Vermont as their senior capstone design project, and calibrated before installation by a civil engineering undergraduate student. The companion ring shear and modified cyclic triaxial tests will be carried out by a civil engineering graduate student at the University of New Hampshire. Data from this project will be archived in the NEES data repository (http://www.nees.org).

这项研究是国家科学基金会07-506计划招标的结果“乔治·布朗，小地震工程模拟网络（NEES）研究”竞赛。 该项目是国家科学基金会奖的有效载荷0530478，“ NEESR-GC：港口系统的地震风险降低风险”，并将利用奖励0530478在加利福尼亚大学戴维斯分校的NEES GEOTECHNICAL CERIFUGE进行的测试。 该项目由佛蒙特大学（University of Vermont）领导，其中包括新罕布什尔大学（University of New Hampshire）的子宣告。 长期以来，人们一直观察到，受到冲击或地震载荷经历强度的饱和沙子的饱和砂，作为沉重的液体，随着内部水压力消失，逐渐恢复了强度。只要液化状态持续存在，土壤就会向下流下斜坡，在堆积地基等障碍物上产生破坏性的滑坡和大型阻力。但是，为风险研究和工程设计进行建模，需要对剪切强度损失及其最终恢复的充分测量，随着内部水压的累积并随后消散。目前尚无这些强度变化的全面现场测量来指导这种模型的开发；现有的现场病例历史仅限于观察液化过程产生的最终损害。受控的实验室测量是可取的，但是液化的发作伴随着如此巨大的应变，以至于传统实验室测试中的土壤样品变得如此明显，以至于无法再进行可靠的强度测量。作为测量液化砂不断发展的行为的第一步，可以预见，在Nees Geotechnical离心机模型中，可以使用薄优惠券（Plate，约25毫米乘25毫米乘25毫米乘以1.5乘以1.5乘以1.5毫米，可以测量液化砂的剪切强度，可以测量。毫米）水平拉动土壤模型，其主要尺寸与模型的基础平行。因此，通过将优惠券相对于沙子移动，通过摇动和之后，将模拟与液化流量失败相关的较大应变和应变速率，直到多余的孔压力消失为止。通过测量优惠券上的阻力，可以观察到土壤剪切强度的演变，因为剪切强度降低到最小（残留强度），然后随着孔隙压力消散而增加。离心机模型将提供现实的场尺度应力和边界条件，仪器的密集阵列将有助于从模拟地震的开始到结尾对液化砂的强度变化进行观察。结果还将用于验证伴随环剪切和修饰的环状三轴测试。一系列离心机和小规模实验室实验的综合结果将为如何模拟较小的实验室设备中的大规模测试提供指导，从而更容易研究其他土壤类型的行为，例如硅和克莱伊砂。 ，在液化过程中，无论是用于一般研究还是用于特定工程设计的目的。一个简单而合理的模型，用于预测颗粒土壤剪切强度的速率依赖性演变，因为孔隙压力积累并开发土壤质量变形。这将允许更准确地模拟诸如估计液化土壤在诸如桩支撑结构之类的障碍物上施加的力以及一般而言的流动幻灯片行为的预测。 预计这些结果将使设计师对如何选择剩余强度值的理解来补救地球结构。 进行有效载荷测试所需的设备将由佛蒙特大学的一组本科机械和电气工程专业的学生设计和建造，并将其作为其高级盖石设计项目，并在土木工程本科生安装前进行校准。新罕布什尔大学的土木工程研究生将​​进行伴侣戒指剪切和修饰的环状三轴测试。 该项目的数据将在NEES数据存储库（http://www.nees.org）中存档。