Collaborative Research: Novel Measurement of Shear Strength Evolution in Liquefied Soil and Calibration of a Fluid Dynamics-based Constitutive Model for Flow Liquefaction

无数的水力填土和尾矿坝、堤坝和其他斜坡含有内部粘聚力很小或没有内聚力的松散土壤。这种状态使得它们在地震事件中很容易发生故障，在地震事件中，土壤变得“液化”。虽然这种土体结构的大流量故障并不常见，但这种故障的灾难性后果极大地提高了相关的风险。对这种行为进行建模需要可靠的估计，即在地震期间产生内部水压力时，液化土壤的强度如何演变到最小。同样重要的是了解一旦震动停止，土壤如何在水压消散时恢复其强度。到目前为止，还没有这种强度演变的全面现场测量来指导模型开发；因此，工程师必须依赖从稀疏的、记录不充分的流动故障案例历史中计算出来的事后强度估计。如果土壤中含有大量的细小颗粒，选择剩余强度设计值的不确定性就会加剧，因为每种经验方法都建议了一种不同的方法来解释细小颗粒。该项目将使执业工程师更好地了解可液化土壤的大应变行为，改进对剩余抗剪强度的估计，并提供新的校准建模工具，这些工具是在涉及数千万甚至数亿美元的高风险、高后果项目中做出关键决策所必需的。该项目的教育和技术转移计划包括培训和指导来自多个学科(岩土工程、机械和电气工程以及流体力学)的研究生和本科生。这个跨学科项目的中心是在一个比传统实验室测试更现实的环境中，对可液化土壤的抗剪强度(包括在孔压产生、剩余强度动员和孔压消散过程中的演变)有潜在的变革性的理解。具体地说，新型离心机模型将利用在摇动前、摇动中和摇动后通过可液化土壤的薄金属片(盘)来直接测量土壤的剩余强度和强度恢复。通过在试件中嵌入压力传感器，将首次有可能直接在液化土壤的剪切面上测量孔压。大量的测量将使研究人员能够探索细粒含量(土壤可压缩性)、有效应力和应变率对残余强度的影响。锥体穿透阻力和剪切波速度将在离心机模型中进行现场测量，使研究人员能够验证或改进经验剩余强度关联式。反过来，离心机测试项目的物理测量将使研究人员能够校准基于流体动力学的多学科应力-应变本构模型，用于评估液化问题，如滑坡、泥石流和桩基周围的横向扩散。该项目将利用加州大学戴维斯分校的NHERI离心机设施。

Countless hydraulic fill and tailings dams, levees, and other slopes contain loose soils with little or no internal cohesion. This state renders them vulnerable to failures under seismic events in which the soil becomes "liquefied." Although large flow failures of such earth structures are uncommon, the catastrophic consequences of such failures dramatically elevate the associated risk. Modeling this behavior requires reliable estimates of how the strength of a liquefying soil evolves to its minimum as internal water pressures are generated during an earthquake. Of equal importance is understanding how the soil subsequently regains its strength as water pressures dissipate once shaking ceases. To date, there are no full-scale field measurements of this strength evolution to guide model development; therefore, engineers must rely on strength estimates calculated after the fact from sparse, poorly documented flow failure case histories. The uncertainty in selecting design values of residual strength is exacerbated if the soil contains significant amount of fine particles, as each empirical method suggests a different approach to account for fines. This project will provide practicing engineers a better understanding of the large-strain behavior of liquefiable soils, improved estimates of residual shear strength, and new calibrated modeling tools that are required to make crucial decisions on high-risk, high-consequence projects involving tens or even hundreds of millions of dollars. The project's education and technology transfer plan includes training and mentoring graduate and undergraduate students from multiple disciplines (geotechnical, mechanical, and electrical engineering, as well as fluid dynamics).This interdisciplinary project centers on developing a potentially transformative understanding of the shearing resistance of liquefiable soils (including its evolution during pore pressure generation, residual strength mobilization, and pore pressure dissipation) in an environment with greater realism than available in conventional laboratory tests. Specifically, novel centrifuge models will utilize a thin metal coupon (plate) pulled through a liquefiable soil before, during, and after shaking to directly measure the soil's residual strength and strength recovery. By embedding a pressure transducer in the coupon, it will be possible for the first time to measure pore pressures directly on the shearing surface in a liquefied soil. The large number of measurements will allow the researchers to explore the effects of fines content (soil compressibility), effective stress, and strain rate on residual strength. Cone penetration resistance and shear wave velocity will be measured in-situ in the centrifuge models, allowing the researchers to validate or improve empirical residual strength correlations. In turn, the physical measurements from the centrifuge testing program will enable the researchers to calibrate a multi-disciplinary, fluid dynamics-based stress-strain constitutive model for evaluating liquefaction problems such as flow slides, debris flows and lateral spreading around pile foundations. This project will utilize the NHERI Centrifuge facility at the University of California, Davis.