A seismic retrofit design was carried out for an existing 110-year old railway bridge crossing a wide river. The existing foundations that have performed acceptably under static conditions, are founded on potentially liquefiable soils. A retrofit strategy was developed using large diameter driven pipe piles to transfer the bridge loads to non-liquefiable strata. The retrofit design is required to withstand a 475-year return period earthquake. A suite of 15 spectrally matched ground motions were developed based on the contribution to the uniform hazard response spectra (UHRS). The equivalent linear ground response analysis and simplified liquefaction assessment showed that the sand layer liquefies during the earthquake and the foundation soils undergo lateral spreading. The preliminary design of the retrofit piles was carried out based on the soil-pile interaction to lateral loads using a beam configuration with nonlinear lateral load transfer curves in the software LPile. Nonlinear seismic deformation analyses were performed using the finite difference program FLAC and effective stress based constitutive soil model UBCSAND during the detailed design. The extent of liquefaction estimated using the simplified liquefaction assessment was validated with the FLAC deformation analysis. Based on the FLAC simulation, the overall riverbed flattens during the seismic event, and the retrofit pile heads undergo a horizontal displacement of up to 0.5 m. The results of the FLAC analysis verified the preliminary LPile predictions.
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