Soil-Foundation-Structure Interaction Effects on Cyclic Failure Potential of Silts and Clays

在最近几次地震中，地震引起的地面破坏造成了数十亿美元的损失。例如，坎特伯雷地震序列导致新西兰克赖斯特彻奇400亿美元的损失（超过新西兰GDP的20%），其中大部分损失归因于松散饱和沙土的液化。地震发生后，整个社区都被重新安置，因为在高度液化危险的地区被宣布为“红色区域”。就地震时的强度损失而言，土壤可以被广泛地描述为表现出“砂状”或“粘土状”的行为。“砂状”土壤的液化是由于颗粒间接触应力的减少，以及应力从土壤颗粒转移到占据颗粒之间空间的流体。大量的“沙状”土壤液化的历史案例使工程师能够开发预测其发生的程序。相比之下，“粘土状”土壤的破坏机制知之甚少，尽管在某些情况下，这种破坏导致了结构的破坏。事实上，“细粒”土壤的循环破坏通常局限于结构下方的区域，而不是远离结构的自由场土壤。有必要更好地了解“粘土状”土壤在地震中可能失效的条件。要想获得这样的理解，就需要对“粘土状”土壤的基本循环特性有新的认识，并结合地震震动时施加在结构下土壤上的循环应力知识。这个项目将使工程师能够更好地预测“粘土状”土壤在地震期间会发生或不会发生周期性破坏的条件。这将通过减少未来结构失效的风险来加强公共安全。该研究项目将产生一个包含所有实验数据的可公开访问的数据库，以便未来的研究人员能够从实验中受益。此外，项目团队将通过加州大学洛杉矶分校工程与多样性卓越中心（CEED），为工程新生开设以实践为导向的项目课程，并通过risup（面向代表性不足人群的工程研究密集系列）项目，吸引代表性不足的学生。该项目将通过了解土壤-基础-结构相互作用（SFSI）对循环地面破坏潜力的影响来实现其技术目标。项目活动包括：(1)开发弹性动力解决方案，量化SFSI引起的应力振幅和阶段，并将这些解决方案部署在一个基于网络的工具中供公众使用；(2)对细粒土样品进行实验室测试，以评估液化敏感性随土壤塑性和更高级的土壤行为指标（即强度归一化）的变化，以便设计离心机建模实验；(3)涉及可变塑性细粒土沉积物上的单自由度结构的离心机建模研究；(4)简化了循环破坏触发及沉降分析程序。目前，在评估接地失效可能性时，由于缺乏对关键行为方面的理解，以及工程师缺乏将SFSI影响纳入其中的工具，SFSI的影响几乎完全被忽视。该项目的主要智力价值将是加强对这些基本问题的理解。

Earthquake-induced ground failure has resulted in billions of dollars of damage during recent earthquakes. For example, the Canterbury earthquake sequence resulted in $40 billion in losses (over 20% of New Zealand GDP) in Christchurch, New Zealand, with much of this loss attributed to liquefaction of loose saturated sandy soil. Entire communities were relocated following the earthquake sequence as "red-zones" were declared in regions of high liquefaction hazard. Soils can be broadly characterized as exhibiting either "sand-like" or "clay-like" behavior with respect to strength loss during earthquakes. Liquefaction of "sand-like" soils is due to reductions of interparticle contact stresses, and transfer of stress from the soil particles to fluid occupying the space between particles. A large number of case histories of liquefaction of "sand-like" soils has enabled engineers to develop procedures for predicting its occurrence. By contrast, the mechanisms of failure of "clay-like" soils are more poorly understood, though there are cases in which such failures have resulted in damage to structures. In fact, cyclic failure of "fine-grained" soils are often constrained to the regions beneath structures, and not in the free-field soils away from the structures. There is a significant need to better understand the conditions for which "clay-like" soils may fail during earthquakes. Forging such an understanding will require new knowledge in the fundamental cyclic behavior of "clay-like" soils combined with knowledge of the cyclic stresses imposed on the soil beneath structures during earthquake shaking. This project will enable engineers to better predict conditions for which cyclic failure of "clay-like" soil will and will not occur during earthquakes. This will enhance public safety by reducing the risk of future structural failures. The research project will result in a publicly accessible database containing all of the experimental data so that future researchers will be able to benefit from the experiments. Furthermore, the project team will engage under-represented students through the UCLA Center for Excellence in Engineering and Diversity (CEED) through a hands-on project-oriented course for engineering freshman, and through the RISE-UP (Research Intensive Series in Engineering for Under-represented Populations) program.The project will achieve its technical objectives by developing an understanding of the influence of Soil-Foundation-Structure Interaction (SFSI) on cyclic ground failure potential. The project activities include: (1) development of elasto-dynamic solutions quantifying the amplitude and phase of stresses induced by SFSI, and deployment of these solutions in a web-based tool for public use; (2) laboratory testing of fine-grained soil samples to evaluate variations of liquefaction susceptibility with soil plasticity and more advanced indicators of soil behavior (i.e., strength normalization) for the purpose of designing the centrifuge modeling experiments, (3) centrifuge modeling studies involving single-degree-of-freedom structures resting atop variable-plasticity fine-grained soil deposits, and (4) a simplified procedure for analyzing triggering of cyclic failure and the resulting settlements. Currently, the influence of SFSI is nearly entirely neglected when assessing ground failure potential due to a lack of understanding of key aspects of behavior, and a lack of tools available for engineers to incorporate SFSI effects . The primary intellectual merit of the project will be to enhance understanding of these fundamental issues.