3D Numerical Analysis for Seismic Retrofit of a Cellular Bulkhead

3D dynamic numerical analyses were performed to evaluate seismic retrofit alternatives for a cellular bulkhead constructed in the 1950's. Previous studies had found that the main contributing factor to the seismic instability of the bulkhead was the liquefiable loose hydraulic fills within and behind the bulkhead cells. Thus, the most promising retrofit alternative was improvement of these loose soils rather than a structural upgrade of the bulkhead. Ground improvement within the bulkhead cells was considered impractical due to the presence of the wharf deck, crane rails and supporting piles, as well as concerns about damaging the interlocking sheet piles of the bulkhead cells. Instead, various schemes for improving the subsurface materials directly behind the bulkhead were considered. Options analyzed included stone columns and deep soil mix panels reinforced with vertical I-beams. 3D nonlinear, dynamic analyses of representative segments along the wharf were performed with the program FLAC3D. Analyses for existing conditions showed up to 3 feet of lateral displacement of the cells at the pierhead line for the 475-year earthquake. With ground improvement, the displacements could be limited to less than one foot, differential settlements of the crane rail were within acceptable limits, and piles supporting the backland silos were undamaged.