Seismic Considerations for Bridge Foundations-Paper

Paper

Pdf Summary

The paper examines seismic risks to foundations of an 11-span bridge crossing the Missouri River near St. Louis, an area influenced by the New Madrid Seismic Zone. While western foundations bear on shallow bedrock, eastern piers pass through more than 30 m of alluvial deposits containing silts and sands susceptible to liquefaction. Bents 10 and 11, located in the flood plain, were considered particularly vulnerable.<br /><br />A site-specific probabilistic seismic hazard analysis established design spectral accelerations for a 1,000-year return period. A record from the 2002 Denali earthquake was selected and scaled to match the target spectrum. The researchers then used FLAC two-dimensional numerical modeling to assess foundation behavior under static and earthquake loading, including soil–pier interaction, liquefaction, lateral ground movement, and forces on the piers.<br /><br />The analyses predicted liquefaction in a silty-sand layer during the design earthquake, with zones developing as shaking progressed. Significant riverside slope movement occurred, partly relieving pore-water pressure. At Bent 10, modeled lateral displacements were approximately 2–3 cm at selected points; empirical estimates ranged from about 1 cm to a conservative 5 cm. The modeling also estimated earthquake-induced pier forces and deformations.<br /><br />The study concludes that numerical modeling can provide useful order-of-magnitude estimates of seismic foundation demands and help guide structural design adjustments. It presents a case history showing how site-specific hazard assessment and analysis of liquefaction and lateral spreading can inform bridge-foundation design.

Keywords

seismic risk

bridge foundations

liquefaction

New Madrid Seismic Zone

probabilistic seismic hazard analysis

FLAC modeling

lateral spreading

alluvial deposits

ground motion scaling

pier forces