Developing a Fundamental Understanding of k-sigma for High Confining Stresses Under Field Conditions

该项目解决了我们对高覆盖层压力（深地面标高）下液化触发和液化阻力基本行为理解的科学空白。液化触发是研究人员和实践者几十年来一直在努力解决的重要问题。影响大型堤坝和其他水库的液化设计和修复决策对公共安全至关重要，并且可能涉及巨大的费用。饱和砂和其他无黏性土液化触发评价的现行规范（SoP）主要使用基于现场贯入试验的图表，如标准贯入试验（SPT）和锥贯入试验（CPT）。用于校准这些图表的现场地震液化案例历史与浅层地面高程相对应。对于涉及土坝、尾矿坝和其他水库的工程项目，SoP使用K-sigma修正系数从这些图表中推断出现场经验。对于同一类型的土壤，目前K-sigma系数的SoP范围可以在0.45到0.85之间，其值范围相差几乎两倍。此外，在NSF EAGER研究资助下，pi的初步研究结果揭示了深地面高度土壤液化地震反应的新情况，对目前的假设提出了重大质疑。离心试验表明，在现场条件下，系数K-sigma值大于1.0，比目前使用的至少高50%。数值模拟结果也证实了这一结论。这种对K-sigma系数的低估可能会导致不必要的补救措施，由于个别大坝的液化危害，补救措施的成本有时高达1亿美元以上。在离心分离机中，地面高度为1 atm处的土壤液化响应与实验室中不排水的土壤液化响应相当，而在地面高度为6 atm处的土壤液化响应差异很大，在地面高度较深的地方，砂土在震动过程中更容易部分排水，从而减缓了孔隙压力的积累，使系数K-sigma值增大。结果表明，目前使用不排水测试来推断从1atm到更高的覆盖层压力的行为的SoP（没有验证外推的历史案例）似乎大大低估了更深海拔高度的液化土壤阻力。一个综合的研究计划，使用离心机和小样本测试以及1D和2D数值模拟的并行程序，远远超出了我们在EAGER项目中对一种沙子的有限离心机测试。在低围压和高围压条件下，对两种洁净砂和一种粉质砂进行了广泛的试验，离心机模型提供了各种排水边界，数值模拟通过参数研究和检验土坝下水平排水的影响，扩展了离心机结果。本研究计划旨在为实践人员和研究人员提供适当的设计工具，以评估高覆盖层压力下现场土壤液化触发和液化阻力。此外，使用计算机程序Dmod2000、FLAC和/或OpenSees的实验工作和数值分析的预期重大贡献将提供给工程社区。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The project addresses a scientific gap in our understanding of the fundamental behavior of liquefaction triggering and liquefaction resistance at high overburden pressure (deep ground elevations). Liquefaction triggering is an important issue which researchers and practitioners have been struggling with for decades. Liquefaction design and remediation decisions affecting large embankment dams and other impoundments are critical to public safety and they may involve great expense. The current State of Practice (SoP) of liquefaction triggering evaluation of saturated sand and other cohesionless soils uses mainly charts based on field penetration tests such as SPT (Standard Penetration Test), and CPT (Cone Penetration Test). Field earthquake liquefaction case histories used for calibration of these charts correspond to shallow ground elevations. For engineering projects involving earth dams, tailings dams and other impoundments, the SoP extrapolates the field experience from these charts using a correction coefficient K-sigma. The current SoP for the coefficient K-sigma can range between 0.45 and 0.85 for same type of soil, a range of values differing by a factor of almost two. Furthermore, preliminary findings by the PIs under an NSF EAGER research grant have shed new light on soil liquefaction earthquake response at deep ground elevations, throwing significant doubt on the current assumption. Centrifuge tests indicated that under the field conditions the values of coefficient K-sigma were above 1.0 and at least 50 percent higher than used in current practice. The results were also confirmed by numerical simulations. Such underestimation coefficient K-sigma could lead to unnecessary remediations, with the cost of remediation due to liquefaction hazard under individual dams sometimes reaching more than hundred million dollars.While the soil liquefaction response at shallow ground elevation (1 atm) in the centrifuge was comparable to that of undrained soil in the laboratory, it was very different at deep ground elevations (6 atm), with the sand being more partially drained during shaking at deep elevations, thus slowing down the pore pressure buildup and increasing the values of coefficient K-sigma. The results suggest that the current SoP of using undrained testing to extrapolate behavior from 1 atm to much higher overburden pressure - with no case histories validating the extrapolation, seem to underestimate significantly the liquefaction soil resistance deeper elevations. A comprehensive research plan using parallel programs of centrifuge and small-sample tests as well as 1D and 2D numerical simulations that go well beyond our limited centrifuge testing of one sand in the EAGER project. Two clean sands and one silty sand in both loose and dense condition - will be extensively tested at low and high confining pressures, with the centrifuge models providing a variety of drainage boundaries and the numerical simulations extending the centrifuge results through parametric studies and examination of the effect of the horizontal drainage under an earth dam. The research plan aims at providing practitioners and researchers with the proper design tools to evaluate triggering of soil liquefaction and liquefaction resistance in the field at high overburden pressure. Furthermore, the anticipated significant contribution of the experimental work and numerical analyses using computer programs Dmod2000, FLAC and/or OpenSees, will be made available to the engineering community.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.