Drilled Shaft Foundations for the Float-In Dam on the Monongahela River at Braddock, PA-PDF

PDF

Pdf Summary

The Braddock Dam on Pennsylvania’s Monongahela River was the U.S. Army Corps of Engineers’ first navigation project built using “float-in” or “in-the-wet” construction. Instead of enclosing and dewatering large sections of the river, crews installed foundations underwater and floated two prefabricated concrete dam segments into position. This approach reduced disruption to navigation and river flow, construction time, cost, and environmental impacts.<br /><br />The 600-foot dam was assembled from two hollow concrete segments, cast downstream, towed to the site, and lowered onto prepared foundations. Its foundation system included upstream and downstream sheet-pile cutoff walls, H-piles supporting the tailrace, and 89 drilled shafts supporting the piers and gate sills. The shafts were installed through alluvial deposits into rock, with 72-inch rock sockets drilled through 78-inch permanent steel casings. Most were reinforced and filled with tremie concrete; their tops incorporated steel tension-anchor and shear-pin assemblies to connect them to the completed dam. Twelve set-down shafts initially supported and leveled the floating segments, while 77 shafts provided long-term support.<br /><br />After the segments were lowered onto the set-down shafts, hydraulic flat jacks leveled them. Crews grouted the space beneath each segment, filled its compartments with concrete, and grouted the shaft connections to lock the structure to its foundations. Piers, tailrace panels, and gates were then completed.<br /><br />The successful project demonstrated that large navigation structures could be built without conventional, extensive cofferdams and riverbed dewatering. The authors concluded that the method could offer a cost-effective alternative for future inland-waterway projects.

Keywords

Braddock Dam

Monongahela River

U.S. Army Corps of Engineers

float-in construction

in-the-wet construction

navigation project

prefabricated concrete segments

drilled shafts

hydraulic flat jacks

inland-waterway projects