Verrazano-Narrow Bridge: Construction of Tower Foundations and Anchorages

The tower foundations for the world's longest suspension span are open-dredged concrete caissons, constructed by the sand island method. The caissons are 229 ft by 129 ft in plan. They are founded at a depth of 105 ft below sea level on a sand-gravel stratum and at a depth of 270 ft on a compact granular stratum, respectively. Concrete was supplied by a floating batch plant and placed in 10-ft lifts. Excavation was performed through 17-ft-diameter dredging wells by crane and bucket. The base was sealed by tremie concrete. The caissons are topped by a 4-ft-thick distribution slab that supports the two granite-faced tower pedestals. The two anchorages are composed of several massive concrete blocks that resist the pull of the bridge cables by their weight and the friction at the bottom of their foundations. Both anchorages rest on shallow spread footings. Excavation was done primarily by scrapers. Concrete batching and mixing plants at the site of each anchorage were equipped to control concrete placing temperatures by addition of flaked ice. Triangulation across the Narrows was checked by Tellurometer.