EAGER: Exploratory Research on Seismic Liquefaction of Sand at Low and High Confining Pressures

地震引起的水饱和砂土液化已经给港口、桥梁、高速公路、地下管道、建筑物基础和土坝造成了数十亿美元的损失。液化是一种复杂的现象，人们对其了解甚少，在工程项目中实际的液化评估是利用以往地震观测结果校准的经验图表来完成的。虽然保守，但在可液化砂层垂直有效围压为1个大气压数量级的大多数情况下，该方法都能很好地工作。然而，这些图表不能直接外推到更高的压力，比如在高土坝下存在的压力，那里的压力可能达到8或10个大气压。实验室测试表明，直接使用这些图表将是危险的不保守的，而且在那些高压下，图表预测的液化阻力应该减少多达60%。减少的确切价值具有重大的经济影响，目前是专家之间争论的主题。部分问题在于为解决这一问题而进行的研究的实证性质。在过去的几年里，pi已经发展出一种新的、更理性的方法，这显示出很大的希望。这个早期概念探索性研究（EAGER）项目将通过对一种松散砂进行一系列创新的循环加载和离心机实验室测试，探索这种更合理方法的应用，找到这种松散砂在高压下减少液化阻力的确切规律，并为进一步研究其他松散和致密砂以及工程实践中发现的一系列情况制定模板。目前对饱和砂层的地震液化触发评价使用基于简化程序的经验图表，并校准了有效垂直覆盖层压力约为1个大气压。在像高土坝这样的工程中，垂直压力可能达到8-10个大气压。循环应力控制实验室测试表明，液化图中的循环阻力比（CRR）必须降低。减少是通过K-sigma因子来衡量的。不同的研究人员提出，在垂直压力为8个大气压时，K-sigma的值在0.45和0.85之间差别很大。本探索性研究采用了一种不同的基于现场测量剪切波速的循环应变方法，以提供更合理的框架并降低K-sigma的不确定性。将利用松散砂和少量应变控制循环三轴试验和离心机模型试验，初步试验确定触发液化所需的循环剪切应变随压力的变化，以及在1个大气压至8个大气压之间的CRR。

Liquefaction of water-saturated sand soils due to earthquake shaking has caused billions of dollars of damage in ports, bridges, highways, buried pipelines, foundations of buildings, and earth dams. Liquefaction is a complex phenomenon which is poorly understood, and practical liquefaction evaluation in engineering projects is done using empirical charts calibrated by observed performance in past earthquakes. While conservative, this procedure works well in most cases in which the liquefiable sand layer is under a vertical effective confining pressure of the order of 1 atmosphere. However, the charts cannot be directly extrapolated to much higher pressures, like those existing under a tall earth dam, where the pressure may reach 8 or 10 atmospheres. Laboratory testing has shown that using the charts directly would be dangerously unconservative, and that the liquefaction resistance predicted by the charts should be reduced by as much as 60 percent at those high pressures. The exact value of the reduction has significant economic implications and is currently a subject of controversy among experts. Part of the problem is the empirical nature of the research being conducted to settle the issue. In the last few years, the PIs have developed a fresh and more rational approach, which shows great promise. This EArly-concept Grant for Exploratory Research (EAGER) project will explore the application of this more rational approach through a small series of innovative cyclic loading and centrifuge laboratory tests on one loose sand, finding the exact law of reduction of liquefaction resistance at high pressure for that loose sand, and developing a template for further research for other loose and dense sands as well as for the range of circumstances found in engineering practice.Current seismic liquefaction triggering evaluation of saturated sand layers uses empirical charts based on the Simplified Procedure and calibrated for an effective vertical overburden pressure of about 1 atmosphere. In projects like tall earth dams, the vertical pressure may reach 8-10 atmospheres. Cyclic stress-controlled laboratory testing suggests that the Cyclic Resistance Ratio, CRR, from the liquefaction charts must be decreased. The reduction is measured by the factor K-sigma. Various researchers have proposed very different values of K-sigma between 0.45 and 0.85 at a vertical pressure of 8 atmospheres. This exploratory research adopts a different, cyclic strain approach based on the field-measured shear wave velocity, to provide a more rational framework and decrease the uncertainty in K-sigma. A preliminary experimental determination will be conducted of the variation with pressure of the cyclic shear strain needed to trigger liquefaction, as well as of CRR, for pressures between 1 atmosphere and 8 atmospheres, using a loose sand and a small number of strain-controlled cyclic triaxial tests and centrifuge model tests.