Characterization of Liquefaction Resistance of Aged Soils

与年轻土壤的抗液化能力相比，人们对年老土壤的抗液化能力知之甚少。古老（超过1万年）的土壤没有得到太多研究的一个原因是，它们经常被错误地认为不容易液化。这个假设很多时候是不正确的。例如，在1886年南卡罗来纳州查尔斯顿的地震中，有20万年历史的土壤液化了。研究老化土壤的另一个困难是，所有的采样方法都会对土壤造成某种干扰，而这种干扰的影响是未知的。本研究旨在通过评估与年龄相关的现有案例历史，对不同年龄的未受干扰的土壤样本进行循环三轴试验和显微照片分析，并表征美国东南部沿海平原不同年龄的六个（三个古液化和三个非液化）地点的原位特性，在对老化土壤抗液化性的认识和理解方面取得开创性进展。美国东南部沿海平原的地质和高地下水条件为研究年龄对土壤液化阻力的影响和原位测量提供了理想的地点。调查将由克莱姆森大学、南卡罗来纳大学和标普公司合作进行。这些研究将为理解土壤年龄对液化阻力、穿透阻力和剪切波速的影响提供重要的进展。它们还将提供对导致土壤老化的物理和化学过程的更多了解。在此基础上，提出了评价老化土抗液化能力的准则。我们所知道的一点表明，较老的土壤比较年轻的土壤更能抵抗液化。因此，当前的设计实践在较老的土壤中可能过于保守。研究结果还将用于减少南卡罗莱纳海岸平原（SCCP）古地震震级和加速度反演的不确定性。这项研究有四个广泛的含义。首先，研究结果可能会用于修改美国地质调查局的地震危险地图，其中包含来自SCCP的特定地点数据。其次，该结果将允许对SCCP以及世界上其他具有老化土壤的地区的液化潜力进行准确评估。第三，研究结果将为研究老化土壤对其岩土力学特性的影响，特别是土壤年龄对降低其液化敏感性的影响提供详细的数据库。第四，研究结果将提供方法，可以减少世界上其他板内地区地震危险性评估程序的不确定性，这些地区很少有历史地震，很少或没有强震记录。此外，六个调查地点将被开发用于未来可能的大型现场振动筛试验、爆炸诱发液化研究和全尺寸桩荷载试验。德克萨斯大学开发的George E. Brown, Jr.地震工程模拟网络（NEES）大型移动振动台（NEES@UTexas）提供了一种新的、有前途的方法来原位测量浅层土壤的液化阻力和动力行为。这项研究还将为本科生和研究生提供机会，包括来自代表性不足的群体和以本科生为主的机构的学生，参与岩土工程领域和实验室研究。

Relatively little is known about the liquefaction resistance of aged soils compared to what is known of young soils. One reason why aged (older than 10,000 years) soils have not been studied much is that they are often incorrectly assumed to be non-susceptible to liquefaction. This assumption many times is not true. For example, soils as old as 200,000 years liquefied during the 1886 Charleston, South Carolina earthquake. Another difficulty with studying aged soils is that all sampling methods result in some disturbance to the soil, with the impact of disturbance unknown. This research seeks to make seminal advances in the knowledge and understanding of the liquefaction resistance of aged soil by evaluating existing case histories with respect to age, conducting cyclic triaxial tests and micrograph analyses on undisturbed soil samples of various ages, and characterizing the in situ properties of six (three paleoliquefaction and three no-liquefaction) sites of various ages in the Coastal Plain of the Southeastern United States. The geology and high groundwater conditions in the Coastal Plain of the Southeastern United States provide an ideal location to study the effects of age on soil liquefaction resistance and in situ measurements. The investigations will be performed collaboratively by Clemson University, the University of South Carolina and S&ME, Inc. The investigations will provide significant advances in understanding the effects of soil age on liquefaction resistance, penetration resistance, and shear-wave velocity. They will also provide an increased understanding of physical and chemical processes contributing to soil aging. Based on the results, guidelines for evaluating liquefaction resistance of aged soils will be developed. The little that is known suggests that older soils are more resistant to liquefaction than younger soils. Thus, current design practice could be excessively conservative in older soils. The results will also be used to decrease the uncertainty in back-calculated paleoearthquake magnitudes and accelerations in the South Carolina Coastal Plain (SCCP). This research has four broad implications. First, the results will likely be used to amend the United States Geologic Survey seismic hazard maps with site-specific data from SCCP. Second, the results will allow for an accurate assessment of liquefaction potential in SCCP, as well as other areas of the world with aged soils. Third, the results will provide a detailed database for studying the effect of aging soils on their geotechnical properties, specifically, the effect of age of soils on lowering their susceptibility to liquefaction. Fourth, the results will provide methods that may decrease the uncertainty of procedures for seismic hazard assessment in other intraplate areas of the world with few historic earthquakes and little, or no, strong motion recordings. In addition, the six investigation sites will be developed for possible future large-scale field shaker tests, blast-induced liquefaction studies, and full-scale pile load tests. The George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) large-scale mobile shakers developed at the University of Texas (NEES@UTexas) provide a new and promising approach to measuring in situ the liquefaction resistance and dynamic behavior of soils at shallow depths. This research will also provide opportunities for undergraduate and graduate students, including students from underrepresented groups and a predominately undergraduate institution, to participate in geotechnical field and laboratory studies.