Verrazano-Narrow Bridge: Design of Tower Foundations and Anchorages

The world's largest suspended span rests on large concrete caissons, 229 ft by 129 ft in plan. These 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. The outer walls are 5 ft thick, the interior walls are 3 ft and 4 ft thick. The bottom of the walls is heavily armored with structural steel to form cutting edges. The caisson is topped by a 4-ft thick distribution slab. Two granite-faced pedestals support the steel tower. The anchorages are large blocks of concrete resting on shallow spread footings. A judiciously selected construction sequence balanced anchorage loads with cable pull to control soil pressures. Extensive soils explorations provided basic information on stratification and character of underlying soils. Laboratory soils tests permitted evaluation of expected settlements of tower foundations and anchorages and of a sliding friction factor for design of the anchorages.