Performance-Based Earthquake Engineering Design Evaluation Procedure for Bridge Foundations Undergoing Liquefaction-Induced Lateral Ground Displacement

摘要

Liquefaction-induced lateral ground displacement has caused significant damage to pile foundations during past earthquakes. Ground displacements due to liquefaction can impose large forces on the overlying structure and large bending moments in the laterally displaced piles. Pile foundations, however, can be designed to withstand the displacement and forces induced by lateral ground displacement. Piles may actually pin the upper layer of soil that would normally spread atop the liquefied layer below it into the stronger soils below the liquefiable soil layer. This phenomenon is known as the pile-pinning effect. Piles have been designed as pins across liquefiable layers in a number of projects, and this design methodology was standardized in the U.S. bridge design guidance document MCEER/ATC-49-1. A number of simplifying assumptions were made in developing this design procedure, and several of these assumptions warrant re-evaluation. In this report, some of the key assumptions involved in evaluating the pile-pinning effect are critiqued, and a simplified probabilistic design framework is proposed for evaluating the effects of liquefaction-induced displacement on pile foundations of bridge structures. Primary sources of uncertainty are incorporated in the proposed procedure so that it is compatible with the Pacific Earthquake Engineering Research (PEER) Center performancebased earthquake engineering (PBEE) framework. A detailed description of the problem and of the current approach to evaluating liquefaction-induced bridge damage is first presented. The PEER-PBEE methodology is described, with emphasis on the components of the method that are more pertinent to the problem under study. Several preliminary evaluations are performed before the proposed simplified procedure is applied. These preliminary assessments include the estimation of the seismic hazard at the site, a liquefaction triggering assessment, and an evaluation of the liquefaction-induced flow failure potential at the site.