Seismic Design Ground Motions and Downdrag of Pile Foundations for a Multiple-Span Bridge in Liquefiable Soils

This paper presents practical approaches for developing site-specific design seismic parameters and acceleration response spectral (ARS) curve for a bridge foundation design in liquefiable soils. A 15-span bridge, cast-in-place, prestressed box girder structure supported on Abutment 1, Bents 2 through 15, and Abutment 16 is proposed to cross a creek where alluvial soils are present to a deep depth that are susceptible to liquefaction subject to the design earthquakes. Both conventional liquefaction analysis using the industrial standard approach and a nonlinear effective-stress analysis with pore pressure generation were performed to determine liquefaction depth. A neutral plane is estimated above which the pile is subject to negative skin friction (downdrag force) and below which the pile has positive skin friction. After liquefaction depth and downdrag forces are determined, the pile lengths, axial and lateral capacities were analyzed using the industrial standard design approaches.