Unexpected Cyclic Deformation of Saturated Fine-Grained Soils

饱和细粒土的意外循环变形在1999年土耳其家乐福购物中心发生的7.4级地震中，饱和粉土和粘土发生了意外的强度损失、软化和沉降。这个55,000平方米的综合体位于伊兹米特湾沿岸，距离破裂的断层约5公里。该场地由软粘土、粉砂和松散的粉砂组成的交替地层覆盖。地震发生时，施工正在进行，主楼下面的土壤已经用喷射灌浆进行了改良，以减轻液化；然而，大部分场地以及周围的所有财产仍然留在未经改良的土壤上。地震发生后，首席调查员(PI)和同事们检查了家乐福及其邻近地区，以比较处理地区和未处理地区之间的抗震性能。幸运的是，在地震前几周，在一个未改善的地区(C地块)安装了沉降延伸仪，用于监测3.3米超载填充物下正在进行的沉降。这些仪器在地震期间就位，使对土壤剖面内六个海拔高度的地震引起的沉降进行前所未有的测量成为可能。在填土下面测量到了大约12厘米的地震引起的沉降。工地工程师最初认为这些定居点与粉砂地层有关。直到最近PI重新审查数据，才发现大部分沉降来自粉土/粘土和高塑性粘土层。由于这些土是可塑性的，不符合通常指示地震易损性的准则(如中国标准)，它们被归类为“不可液化”，设计者没有预料到它们会造成显著的地震变形。然而，正如测量表明的那样，这些材料容易受到显著的循环软化和变形的影响。这些发现具有特别的学术价值，表明目前的液化筛选指南可能不是地震易损性的可靠预测指标。此外，PI认为，这个案例表明，当使用可液化或不可液化一词时，定义什么是土壤行为是重要的。这些发现消除了普遍的误解，即按中国标准划分为不可液化的土壤不会在循环荷载下产生高孔压和变形。如图所示，“不可液化”并不意味着“没有问题”。作为这个问题特别关键的一个例子，PI了解到一些涉及大型土坝的项目，这些土坝包含大片饱和粉土和粘土。在没有进行严格分析的情况下，这些结构的地震变形和破坏潜力被默认地忽略了，因为粉土和粘土被认为是不可液化的--这是一种潜在的危险做法。将对家乐福网站进行详细分析，以解释观察结果并传播调查结果。这项工作将包括现场叶片剪切试验、静态和动态实验室试验以及数值分析。一个关键目标将是获得高质量的粉土和粘土试管样本，以测量它们在动态载荷下的反应。土耳其Bogazici大学教授兼Zetas公司总裁T·杜古诺格鲁博士将与弗吉尼亚理工大学名誉教授吉姆·米切尔博士一起担任项目顾问。这项工作还将邀请卡明·波利托博士担任教职员工研究助理。波利托博士是瓦尔帕莱索大学的助理教授。瓦尔帕莱索大学是印第安纳州的一所本科院校。就更广泛的影响而言，这项研究代表着一个前所未有的学习机会，可能对地震工程实践具有重要影响。这是已知的唯一一个在不同的土层中测量到地震引起的沉降的案例，特别是在塑料土壤中。这些发现表明，这些土壤的脆弱性被低估了，并可能导致改进设计方法和建筑实践。此外，还将有一个研究小组，包括一名少数民族，一名法律残疾人，来自主要研究和本科学校的高级和初级教职员工，以及国际参与者。这项工作还将让瓦尔帕莱索的学生接受研究生研究和教育，并发展波利托博士的研究能力。结果将提交给顶级期刊，纳入弗吉尼亚理工大学和瓦尔帕莱索大学的课程，以及PI与美国土木工程师学会(ASCE)和联邦紧急事务管理署(FEMA)频繁举办的专业工程短期课程。最后，该项目将为PI和Polito博士计划更全面地研究粉土和粘土的地震行为奠定基础。

ABSTRACTUnexpected Cyclic Deformation of Saturated Fine-Grained SoilsUnexpected strength loss, softening, and settlement occurred in saturated silts and clays at the Carrefour Shopping Center in Turkey during the 1999 Kocaeli Earthquake (M=7.4). The 55,000 m2 complex is located along Izmit Bay, about 5 km from the ruptured fault. The site is underlain by alternating strata of soft clays, silts, and loose silty sands. Construction was ongoing when the earthquake struck, and the soil beneath the main building had been improved using jet grouting to mitigate liquefaction; however, most of the site, as well all of the surrounding properties, remained on unimproved soil. Following the earthquake, the principal investigator (PI) and colleagues inspected Carrefour and neighboring sites to compare seismic performance between treated and untreated areas.Fortuitously, settlement extensometers had been installed in an unimproved area (Lot C) a few weeks prior to the earthquake for monitoring ongoing settlements beneath a 3.3-m surcharge fill. These instruments were in place during the earthquake and made possible the unprecedented measurement of earthquake-induced settlements at six elevations within the soil profile. Approximately 12 cm of earthquake-induced settlement was measured beneath the fill. Site engineers initially thought the settlements were associated with the silty sand strata. It was not until the data were recently re-examined by the PI that it was discovered that most of the settlement was from silt/clay and high-plasticity clay strata.Because these soils were plastic and did not meet commonly-used guidelines that indicate seismic vulnerability (such as the Chinese criteria) , they were classified as "non-liquefiable," and the designers did not anticipate them being a source of significant earthquake-induced deformation. As the measurements suggest however, these materials were vulnerable to significant cyclic softening and deformation. Of particular intellectual merit, the findings imply that current liquefaction screening guidelines may not be reliable predictors of seismic vulnerability. Further, the PI believes this case demonstrates the importance of defining what soil behavior is being referred to when the term liquefiable or non-liquefiable is used. The findings dispel the common misconception that soils classified non-liquefiable by the Chinese criteria cannot generate high pore pressures and deform under cyclic loading. As shown here, "non-liquefiable" does not translate to "non-problematic." As an example of where this issue is especially critical, the PI is aware of a number of projects involving major earth dams that contain large zones of saturated silts and clays. The seismic deformation and failure potential of these structures was tacitly ignored without rigorous analysis because the silts and clays were considered non-liquefiable-a potentially dangerous practice. A detailed analysis of the Carrefour site to explain the observations and disseminate the findings will be undertaken. The work will include field vane shear testing, static and dynamic laboratory testing, and numerical analysis. A key objective will be obtaining high-quality tube samples of the silts and clays to measure their response under dynamic loading. Dr. T. Durgunoglu, professor at Bogazici University and president of Zetas, Inc. in Turkey will serve as a project advisor, along with Dr. Jim Mitchell, professor emeritus at Virginia Tech. The work will also involve Dr. Carmine Polito as a faculty research associate. Dr. Polito is an assistant professor at Valparaiso University, a predominantly undergraduate institution in Indiana. In terms of broader impact, the study represents an unprecedented learning opportunity that could have important implications for earthquake engineering practice. This is the only known case where earthquake-induced settlements have been measured within distinct soil strata, especially in plastic soils. The findings suggest an under-appreciated vulnerability of these soils and could lead to improved design methods and building practices. Also, there will be a research team, including an ethnic minority, a legally disabled person, senior and junior faculty from major research and undergraduate schools, and international participants. The work will also expose students at Valparaiso to graduate research and education, and develop Dr. Polito's research capabilities. The results will be submitted to top journals, incorporated into courses at Virginia Tech and Valparaiso and the PI's frequent professional engineering shortcourses with the American Society of Civil Engineers (ASCE) and the Federal Emergency Management Agency (FEMA). Finally, this project will lay the foundation for a more comprehensive study of the earthquake behavior of silts and clays planned by the PI and Dr. Polito.