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

这项研究是美国国家科学基金会07-506项目招标“小乔治·e·布朗地震工程模拟网络（NEES）研究”竞赛的结果。该项目是国家科学基金会0530478奖“NEESR-GC：港口系统地震风险降低”的有效载荷，并将利用由加州大学戴维斯分校的NEES岩土离心机进行的0530478奖的测试。该项目由佛蒙特大学领导，其中包括新罕布什尔大学的分项目。长期以来，人们一直观察到饱和砂在受到冲击或地震载荷时强度急剧下降，表现为重流体，随着内部水压的消散逐渐恢复强度。只要液化状态持续，土壤就会顺着斜坡流下来，产生破坏性的山体滑坡，并对桩基等障碍物产生巨大的阻力。然而，为风险研究和工程设计建立这种行为模型，需要充分测量剪切强度损失及其最终恢复如何随着内部水压的增加和随后的消散而演变。目前还没有对这些强度变化进行全面的现场测量，以指导此类模型的开发；现有的现场案例历史仅限于观察液化过程产生的最终损害。可控的实验室测量是可取的，但液化的开始伴随着如此大的应变，以致常规实验室测试中的土壤样品变得如此剧烈变形，以致无法再进行可靠的强度测量。作为测量液化砂演化行为的第一步，设想液化砂的抗剪强度可以在NEES岩土离心模型中测量，使用一个薄板（约25毫米乘25毫米乘1.5毫米）水平拉过土壤模型，其主要尺寸与模型基座平行。大应变和应变率与液化流动失效相关，因此可以通过相对于沙子移动接头，通过和之后的震动，直到超孔隙压力消散来模拟。通过测量粘块上的阻力，可以观察到土抗剪强度的演变，因为它降低到最小值（残余强度），随后随着孔隙压力的消散而增加。离心机模型将提供真实的现场尺度应力和边界条件，密集的仪器阵列将有助于观察液化砂从模拟地震开始到结束的强度变化。该结果也将用于验证伴随环剪切和改进循环三轴试验。一系列离心机和小型实验室实验的综合结果将为如何在较小的实验室设备上模拟大型试验提供指导，从而使研究其他土壤类型（如粉质和粘土砂）在液化过程中的行为更容易，无论是用于一般研究还是用于特定的工程设计目的。本文将建立一个简单而合理的模型，用于预测孔隙压力增加和土体变形时颗粒土抗剪强度的速率依赖性演变。这将允许更准确地模拟诸如估计液化土对诸如桩支撑结构等障碍物施加的力以及一般的流滑行为预测等问题。这些结果有望使设计者更好地理解如何选择土结构修复的剩余强度值。进行有效载荷测试所需的设备将由佛蒙特大学的一组机械和电气工程本科生设计和建造，作为他们的高级顶点设计项目，并在安装前由一名土木工程本科生进行校准。伴随环剪和修正循环三轴试验将由新罕布什尔大学土木工程研究生进行。该项目的数据将存档在NEES数据存储库（http://www.nees.org）中。

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).