Float-In Cofferdams-PDF

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The paper describes float-in cofferdams as an alternative to conventional bottom-founded cofferdams for constructing bridge pile caps below water but elevated above the river or seabed. The method is suited to foundations using large-diameter drilled shafts or piles, which can support pile caps at different heights in the water column.<br /><br />A thin pile-cap shell is prefabricated on shore or on a barge while foundation piles are installed. The shell is launched, towed to the site, positioned over the piles, and lowered onto them. After sealing and grouting or placing tremie concrete around the pile connections, the cofferdam is dewatered so the pile cap and pier can be completed in dry conditions. Reusable follower cofferdams may extend the shell walls and are removed after construction.<br /><br />The approach can shorten schedules by allowing pile installation and shell fabrication to proceed concurrently. It can also reduce sheet-pile use, eliminate or reduce conventional tremie seals and temporary falsework, improve concrete quality through off-site fabrication, and limit exposure to difficult weather and water conditions.<br /><br />Successful installation requires careful attention to pile-position tolerances, watertight joints, launch and transport stability, ballast control, draft limits, guidance and landing systems, and temporary support forces. The cofferdam must withstand changing loads as it is landed, sealed, dewatered, and filled with concrete.<br /><br />Applications on the Bath-Woolwich Bridge in Maine and the new Carquinez Bridge in California demonstrated the method’s effectiveness for deep-water bridge foundations. At Bath-Woolwich, concurrent work saved about four months; at Carquinez, the design reduced temporary works and enabled construction of large tower footings in deep, fast-moving water.

Keywords

float-in cofferdam

bridge pile caps

drilled shafts

prefabricated shell

dewatered cofferdam

bridge foundations

tremie concrete

temporary falsework

deep-water construction

Bath-Woolwich Bridge