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

该项目解决了我们在理解高上覆压力（深地面高程）下液化触发和抗液化的基本行为方面的科学空白。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蓄水池的液化设计和补救决策对公共安全至关重要，并且可能涉及大量费用。饱和砂土和其他无粘性土液化触发评估的现行实践状态（SoP）主要使用基于现场贯入试验（如SPT（标准贯入试验）和CPT（圆锥贯入试验））的图表。用于校准这些图表的现场地震液化案例历史对应于浅层地面高程。对于涉及土坝、尾矿坝和其他蓄水池的工程项目，SoP使用校正系数K-sigma从这些图表中推断现场经验。对于相同类型的土壤，系数K-sigma的当前SoP范围可以在0.45和0.85之间，值的范围相差几乎两倍。此外，在NSF EAGER研究资助下，PI的初步研究结果揭示了深层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。试验表明，在现场条件下，K-σ系数值大于1.0，比目前使用的K-σ系数值高50%以上。数值模拟结果也证实了这一点。这种低估系数K-sigma可能会导致不必要的补救措施，由于个别大坝下的液化危害，补救费用有时会达到数亿美元以上。虽然离心机中浅层地面标高（1 atm）的土壤液化反应与实验室中不排水土壤的液化反应相当，但在深层地面标高（6 atm）时则有很大不同，在深海拔处的震动过程中，砂被更多地部分排出，从而减缓孔隙压力的建立并增加系数K-σ的值。结果表明，目前使用不排水试验从1 atm到更高的上覆压力外推行为的SoP-没有验证外推的案例历史，似乎大大低估了更深海拔的液化土壤抵抗力。一个全面的研究计划，使用离心和小样本测试以及1D和2D数值模拟的并行程序，远远超出了我们在EAGER项目中对一种沙子进行的有限离心测试。两个干净的沙子和一个粉砂在松散和密集的条件下-将在低和高围压下进行广泛的测试，离心模型提供了各种排水边界和数值模拟扩展离心结果通过参数研究和检查土坝下的水平排水效果。该研究计划旨在为从业人员和研究人员提供适当的设计工具，以评估高上覆压力下土壤液化的触发和抗液化性。此外，预期的重大贡献的实验工作和数值分析，使用计算机程序Dmod 2000，FLAC和/或OpenSees，将提供给工程界。这个奖项反映了NSF的法定使命，并已被认为是值得的支持，通过评估使用基金会的智力价值和更广泛的影响审查标准。

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.