Seismic Evaluation of Pile-Founded Highway Bridge on Saturated Peat

Abstract

Interstate 90 crosses Mercer Slough near Bellevue, Washington. Mercer Slough is filled with a very soft, thick peat deposit. Very little is currently known about the dynamic behavior of such soils or the seismic response of bridges supported on peaty soils. In this study, the seismic response of the westbound structure of the I-90 bridges crossing Mercer Slough was investigated. Both linear spectral and nonlinear dynamic analyses were performed. For the spectral analysis, two different column and three different foundation stiffness values were used, along with two different earthquake spectra. For the nonlinear analyses, soil-structure interaction and inelastic column behavior were included. Equivalent Winkler-type pile foundation models were used with two different conditions of fixity at the pile cap. In addition, earthquake acceleration time-histories that included the effects of the soil motion were used. The response of the bridge from the nonlinear analysis was found to be similar to that from the spectral analyses, but the response was found to depend significantly on the foundation assumptions.