Woodrow Wilson Bridge Cofferdams-PDF

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The Woodrow Wilson Bridge replacement spans approximately 1.1 miles across the Potomac River between Virginia and Maryland. Its foundations include fixed-span piers in shallow water and two bascule-span piers in a deeper, 36-foot-deep navigation channel. Construction required cast-in-place concrete pile caps to be built below the waterline.<br /><br />The riverbed presented major geotechnical challenges: up to 50 feet of very soft, scour-sensitive alluvial clay overlies denser terrace deposits and hard Potomac Group soils. Foundations used large open-ended steel pipe piles, with caps extending below the river bottom at some locations.<br /><br />For shallow-water piers, contractors used conventional interlocking steel sheet-pile cofferdams, internally braced and sealed with tremie concrete to permit dewatering. The tremie slab spanned between piles and resisted uplift; shear rings on the piles helped develop the required bond. Although soft sediments raised concerns about tremie placement and support, the concrete was ultimately placed directly on the excavated subgrade.<br /><br />Conventional deep cofferdams were unsuitable for the bascule piers because they could constrict the narrow navigation channel and worsen contraction scour. Instead, “Potomac River” cofferdams were supported by the foundation piles, limiting their depth and obstruction of river flow. The contractor’s modified design used hanger beams on pile extensions, allowing much of the cofferdam to be assembled above water and then lowered into place. Sheet-pile sidewalls and a formed floor enclosed a thick tremie-concrete seal, which resisted uplift and supported the wet pile-cap concrete.<br /><br />The project demonstrates that cofferdam design must account for water and environmental loads, soil stability during dewatering, construction sequencing and equipment, and effects on the permanent foundations. Minimizing channel obstruction was especially important to controlling scour at the bascule-span foundations.

Keywords

Woodrow Wilson Bridge

Potomac River

cofferdam design

bascule span

pile caps

tremie concrete

open-ended steel pipe piles

scour sensitivity

geotechnical challenges

navigation channel