| Liquefaction-induced lateral spreading has caused significant damage to pile foundations during past earthquakes due mainly to the large forces imposed by ground displacements on the piles and overlying structure. Pile foundations, however, can be designed to withstand the displacements and forces induced by lateral spreading. Piles may actually "pin" the upper layer of soil that would normally spread atop the liquefied layer into the stronger soils below the liquefiable soil layer. The incorporation in bridge design of this "pile-pinning" effect was standardized in the MCEER/ATC-49-1 document.; In this study, some of the current assumptions involved in evaluating the pile-pinning effect are critiqued (e.g., geometry and rigidity of the sliding mass), and a simplified probabilistic design procedure is developed for evaluating the effects of liquefaction-induced lateral spreading on pile foundations of bridge structures. Primary sources of uncertainty are incorporated in the proposed procedure so that it fits within a performance-based earthquake engineering (PBEE) framework. The proposed procedure is validated through its application to three important "case" histories: Landing Road Bridge (1987 Edgecumbe Earthquake), Showa Bridge (1964 Niigata Earthquake), and a centrifuge model test performed at U.C. Davis. Its use and the insights it can offer are also illustrated through a realistic bridge example.; From the results of this study, several key findings are made. First, soil liquefaction and the "pile-pinning" effect have a critical effect on the structural response and resulting performance of a bridge that is founded on piles that pass through a liquefiable soil layer with firm soils above and below it. Second, the residual longitudinal displacement of the abutments is a good index of the overall seismic bridge performance, and PBEE provides a sound methodology for evaluating this performance. Third, in the PBEE approach, the most influential steps are the characterization of the ground motion, characterization of the residual undrained shear strength of the liquefied soil, of the uncertainty in estimating seismic displacement, and of the relation between seismic displacement, resulting damage, and repair actions. Finally, the combination of engineering judgment and straightforward analytical models can lead to reasonable estimates of the seismic performance of bridge systems undergoing liquefaction-induced lateral spreading. |
