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

这项研究是国家科学基金会07-506项目征集“小乔治·E·布朗地震工程模拟网络(NEES)研究”竞赛的成果。该项目是国家科学基金会0530478奖“NEESR-GC：港口系统的地震风险缓解”的有效载荷，并将利用加州大学戴维斯分校NEES土工离心机中由0530478奖进行的测试。该项目由佛蒙特大学领导，并包括给新汉普郡大学的一个子奖。长期以来，人们一直观察到，饱和砂土在受到冲击或地震荷载时，强度会急剧下降，表现为重流体，随着内部水压力的消散，强度逐渐恢复。只要液化状态持续存在，土壤就会从山坡上流下来，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模拟这一行为，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力的增加和随后的消散而演变。目前还没有这些强度变化的全面现场测量来指导此类模型的开发；现有的现场案例记录仅限于观察液化过程产生的最终损害。受控的实验室测量将是可取的，但液化的开始伴随着如此大的应变，以至于传统实验室测试中的土壤样本变得如此剧烈，以至于不能再进行可靠的强度测量。作为测量液化砂演变行为的第一步，设想可以在NEES岩土离心机模型中使用水平拉动土壤模型的薄试件(板，约25毫米乘25毫米乘1.5毫米)来测量液化砂的剪切强度，其主要尺寸与模型底部平行。因此，与液化流动失效相关的大应变和应变率可以通过在摇动过程中和摇动之后相对于砂子移动试件来模拟，直到多余的孔压消失。通过测量试件上的阻力，可以观察到土壤抗剪强度在降低到最小值(残余强度)时的演变，然后随着孔压的消散而增加。离心机模型将提供真实的场级应力和边界条件，密集的仪器阵列将有助于观察从模拟地震开始到结束时液化沙子的强度变化。研究结果还将用于验证相伴环剪试验和改进的循环三轴试验。一系列离心机和小规模实验室试验的综合结果将为如何在较小的实验室设备中模拟大型试验提供指导，从而更容易研究其他类型的土壤，如粉土和粘性砂，在液化过程中的行为，用于一般研究和特殊工程设计目的。建立了一个简单而合理的模型，用于预测颗粒土的抗剪强度随孔压的增加和土体变形的速率变化规律。这将允许更准确地模拟诸如估计液化土壤对桩支撑结构等障碍物施加的力，以及总体上预测水流滑动行为等问题。这些结果有望使设计者更好地理解如何选择剩余强度值来修复土结构。进行有效载荷测试所需的设备将由佛蒙特大学的一群本科生设计和建造，作为他们的高级顶石设计项目，并由一名土木工程本科生在安装前进行校准。新汉普郡大学的一名土木工程研究生将进行相伴的环剪切试验和改进的循环三轴试验。该项目的数据将存档在NEES数据储存库(http://www.nees.org).

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).