Project Summary

A commonly used tool for designing deep foundations subjected to lateral loads is the so-called "beam-on-nonlinear-Winkler" foundation (BNWF) method, also known as the "p-y method". The BNWF method can be extended to cases of ground failure such as lateral spreading by imposing a profile of ground displacement on the free ends of the p-y springs as shown below. This conveniently allows foundation designers to model the complex loading conditions that occur during lateral spreading as a single equivalent-static model which can be analyzed using software such as LPILE or the free, open-source program OpenSees. Results of these analyses enable structural design by predicting shear and moment demands as well as providing estimates of displacement and rotation for performance evaluation.

PEER recently published a synthesis of research and design guidelines for pile foundations in laterally spreading ground, available here. In order to determine if the equivalent-static BNWF method can provide reliable results, it is necessary to validate the procedure against case studies of actual bridge performance.

The 2010 M 7.2 El Mayor-Cucapah Earthquake in northern Baja California, Mexico, caused liquefaction and lateral spreading that damaged a pair of parallel bridges, both of which are supported on deep foundations. A span of one of the bridges collapsed as a result of significant translation of one of the bents, whereas the adjacent bridge only suffered minor flexural cracks without yielding at the same bent location. This provided our research team with an excellent opportunity to validate the equivalent-static BNWF analysis method since the full range of performance occurred at a single site, from collapse to minor damage. By creating numerical models of each bridge with the same soil profiles and geotechnical parameters, we demonstrated that the method correctly predicted the actual behavior of each bridge without bias towards predicting poor or good behavior. This demonstrates that the equivalent-static BNWF analysis method for lateral spreading is a valid tool for foundation design. For more details, see the publications related to this project.

                                                             Photos after GEER (2010)

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