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

Rensselaer Polytechnic Institute, Troy NY

Investigators

Abstract

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.