Unexpected Cyclic Deformation of Saturated Fine-Grained Soils

摘要饱和细粒土的意外循环变形在1999年Kocaeli地震（M=7.4）期间，土耳其家乐福购物中心的饱和粉土和粘土中发生了意外的强度损失、软化和沉降。 55，000平方米的综合体位于沿着伊兹米特湾，距离断裂断层约5公里。 场地下面是软粘土、粉砂和松散粉砂的交替地层。 地震发生时正在施工，主楼下面的土壤已使用喷射灌浆进行改良，以减轻液化;然而，场地的大部分以及周围的所有财产仍在未经改良的土壤上。 地震发生后，主要研究者（PI）和同事们检查了家乐福和邻近的场地，以比较处理和未处理区域的抗震性能。幸运的是，在地震发生前几周，沉降延伸计已安装在未改善区域（C地块），用于监测3.3 m超载填料下的持续沉降。 这些仪器在地震发生时就已就位，从而能够对土壤剖面内六个高度的地震引起的沉降进行前所未有的测量。 在填充物下方测得约12 cm的地震诱发沉降。 现场工程师最初认为沉降与粉砂地层有关。 直到PI最近重新检查数据时，才发现大部分沉降来自淤泥/粘土和高塑性粘土层。因为这些土壤具有塑性，不符合通常使用的地震易损性准则（如中国标准），它们被归类为“不可验证的，“设计者没有预料到它们会成为地震引起的重大变形的来源。 然而，正如测量结果所表明的那样，这些材料容易受到显著的循环软化和变形的影响。 特别是智力价值，研究结果意味着，目前的液化筛选准则可能不是可靠的预测地震脆弱性。 此外，PI认为，该案例证明了在使用术语“可液化”或“不可液化”时定义土壤行为的重要性。 研究结果消除了人们普遍的误解，即中国标准分类为非粘性土不能产生高孔隙水压力和循环荷载下变形。 正如这里所示，“不可否认”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意识到一些项目涉及含有大面积饱和淤泥和粘土的大型土坝。 这些结构的地震变形和破坏潜力被默认忽略，没有严格的分析，因为粉土和粘土被认为是不可膨胀的-一个潜在的危险的做法。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十字板剪切试验、静态和动态实验室试验以及数值分析。 一个关键的目标是获得高质量的粉土和粘土管样品，以测量它们在动态载荷下的响应。 T医生博阿齐奇大学教授、泽塔斯公司总裁杜尔贡奥卢说。土耳其的Jim Mitchell博士将与弗吉尼亚理工大学名誉教授Jim Mitchell博士一起沿着担任项目顾问。 这项工作还将涉及博士卡迈恩波利托作为一个教师研究助理。 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一所以本科为主的大学。就更广泛的影响而言，这项研究代表了一个前所未有的学习机会，可能对地震工程实践产生重要影响。这是唯一已知的情况下，地震引起的沉降已被测量在不同的土层，特别是在塑性土壤。 研究结果表明，这些土壤的脆弱性被低估，并可能导致改进设计方法和建筑实践。 此外，将有一个研究小组，包括一个少数民族，一个法律上残疾人，高级和初级教师从主要的研究和本科学校，以及国际参与者。 这项工作还将使瓦尔帕莱索的学生接受研究生研究和教育，并培养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的课程，以及PI与美国土木工程师协会（ASCE）和联邦应急管理局（FEMA）经常举办的专业工程短期课程。 最后，该项目将为PI和Polito博士计划的更全面的粉土和粘土地震行为研究奠定基础。

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.