This paper and its companion (Johnson et al. 2015) summarize the design assumptions, processes, and analytical methods that were employed in the seismic retrofit of an historic bridge. The North Torrey Pines Road Bridge in Del Mar, California, was built in 1933 and is eligible for listing in the National Register of Historic Places. The bridge had been classified as structurally and seismically deficient and functionally obsolete. A seismic retrofit project was performed to improve the seismic performance of the bridge while preserving its aesthetic and historic character. This paper describes the geotechnical and seismic hazard aspects of the retrofit project, which included deterministic and probabilistic seismic hazard analysis, earthquake time-history development, site-response analyses, evaluation of liquefaction triggering and its consequences, seismic slope displacement evaluations, and soil-structure interaction analyses. Geotechnical retrofit measures, including ground improvement and new abutment foundations, were developed and tailored to improving seismic performance to levels that were compatible with the seismic design criteria. The information presented in these two papers provides a case history that can serve as a point of reference for practicing engineers facing similar challenges.

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