Design of the Drilled Shaft Foundations for the Cooper River Bridge-PDF

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The paper describes the design of drilled-shaft foundations for the Cooper River Bridge in Charleston, South Carolina. Because the bridge faces substantial seismic, hurricane, and vessel-impact loads, designers selected large-diameter shafts to provide high axial and lateral capacity while limiting the number of substructure units.<br /><br />The shafts extend through soft coastal clays and sands into Cooper Marl, a thick, stiff-to-hard, cemented calcareous soil layer. Site investigations included 148 borings, 63 cone penetration tests, and full-scale axial and lateral load testing. Test results informed the soil-resistance models and design parameters. Main tower foundations use eleven 10-foot-diameter shafts each, protected by rock islands; other main-span piers use groups of four to eight shafts, while approach piers generally use one shaft per column.<br /><br />Axial design relied on side friction and end bearing in the marl. The design also considered settlement, downdrag from settling rockfill islands, and different safety factors for routine, extreme, and seismic loading. Lateral resistance was assessed using load-test-derived p-y curves and nonlinear soil–structure interaction analyses.<br /><br />Seismic design addressed two earthquake levels: a 500-year Functional Evaluation Earthquake and a 2,500-year Safety Evaluation Earthquake, with the latter governing. Analyses incorporated inertial forces, ground movement, and spatially varying earthquake motions. On the Mount Pleasant side, designers also evaluated liquefaction and lateral spreading. Large shafts substantially limited predicted movement, avoiding the need for additional liquefaction mitigation.<br /><br />Overall, the project combined extensive geotechnical investigation, field testing, and advanced structural analyses to develop foundations capable of supporting the bridge while meeting demanding seismic and vessel-impact performance requirements.

Keywords

drilled-shaft foundations

Cooper River Bridge

geotechnical investigation

axial capacity

lateral capacity

Cooper Marl

seismic design

load testing

p-y curves

liquefaction mitigation