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

由于地震引起的水饱和砂土的液化已经在港口、桥梁、高速公路、埋地管道、建筑物地基和土坝中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，工程项目中的实际液化评估是使用经验图表进行的，这些图表是根据过去地震中观察到的性能进行校准的。 虽然是保守的，但在大多数情况下，当可液化砂层处于1个大气压量级的垂直有效围压下时，该程序工作良好。 然而，这些图表不能直接外推到更高的压力，比如那些存在于高大土坝下的压力，那里的压力可能达到8或10个大气压。 实验室测试表明，直接使用图表将是危险的不保守的，并且在这些高压下，图表预测的抗液化性应减少多达60%。 减少的确切价值具有重大的经济影响，目前专家之间存在争议。 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开发了一种新的和更合理的方法，这显示了很大的希望。 EARLY概念探索性研究资助（EAGER）项目将通过对一种松散砂土进行一系列创新的循环荷载和离心实验室试验，探索这种更合理的方法的应用，找到该松散砂土在高压下抗液化强度降低的确切规律，并为进一步研究其他松散和密实砂土以及工程实践中发现的各种情况开发了一个模板。砂层使用基于简化程序的经验图表，并针对约1个大气压的有效垂直上覆压力进行校准。 在高土坝这样的工程中，垂直压力可能达到8-10个大气压。循环应力控制的实验室试验表明，液化图中的循环阻力比CRR必须降低。 减少量由因子K-sigma测量。不同的研究人员提出了在8个大气压的垂直压力下，K-sigma在0.45和0.85之间的非常不同的值。 本探索性研究采用了一种不同的，基于现场实测剪切波速度的循环应变方法，以提供一个更合理的框架，并减少K-西格玛的不确定性。 将使用松散砂和少量应变控制的循环三轴试验和离心模型试验，对1个大气压和8个大气压之间的压力下，触发液化所需的循环剪切应变随压力的变化以及CRR进行初步实验测定。

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.