Caisson Foundations for Mississippi River Bridges in the New Orleans Area-PDF

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The paper reviews open-dredged concrete caisson foundations used for four Mississippi River bridges in the New Orleans area: the Huey P. Long Bridge (1935), Greater New Orleans Bridge No. 1 (1958), Hale Boggs Bridge (1983), and Greater New Orleans Bridge No. 2 (1988). The caissons are founded either in dense sand or in hard, preconsolidated Pleistocene clay, at depths of roughly 150–185 feet.<br /><br />The Huey P. Long Bridge’s main piers rest on caissons founded in dense sand. Despite a planned roadway widening that will more than double some loads, engineers determined the existing foundations could remain in service. The projected added pressure is within the original design limit, and estimated additional settlement is about 1.5 inches. Over decades, the piers have also moved vertically by about seven-eighths of an inch as river levels change, apparently because underlying clay responds elastically to changing water loads.<br /><br />Greater New Orleans Bridge No. 1’s caissons were successfully sunk in deep water, with a willow mattress providing bottom stabilization and scour protection. The foundations have settled little, but the main piers have gradually moved apart—about 6–7 inches by 1975—requiring periodic adjustments to expansion joints and bearings.<br /><br />At the Hale Boggs Bridge, one caisson shifted 13 feet riverward while sinking through a sloping riverbank. Although it remained nearly plumb and stable after reaching its founding depth, the movement required a revised span arrangement.<br /><br />Greater New Orleans Bridge No. 2 has the largest caisson described. Its installation did not measurably affect the neighboring bridge pier. Both bridges’ piers show small vertical movements linked to river stage.<br /><br />The paper concludes that large open-dredged caissons provide stable foundations in New Orleans’ soft alluvial soils, can resist major ship impacts, and can be built in deep, fast-flowing water without prolonged underwater work.

Keywords

open-dredged caissons

bridge foundations

Mississippi River bridges

New Orleans

dense sand

Pleistocene clay

foundation settlement

scour protection

Hale Boggs Bridge

alluvial soils