Tacoma Narrows Bridge - Tacoma, Wash.
This project consisted of constructing a second 5,200 foot long suspension bridge parallel and adjacent to the existing bridge.
General Construction Company, part of the Tacoma Narrows Constructors joint venture, was responsible for construction of the two foundation caissons that support the suspension towers of the new Tacoma Narrows Bridge. Each finished caisson is 80 feet wide, 130 feet long and up to 234 feet tall and located in water depths of about 140 feet, where they penetrate over 60 feet into the sea floor and extend to the surface above the highest tide elevations. Construction of each caisson started with the fabrication of an 18 foot high steel cutting edge with a temporary steel seal on the bottom. Initial construction of the floating caissons took place pier-side at the Port of Tacoma, after which the partially constructed caissons were floated into position and tethered in place by a system of 32 anchors, designed to stabilize the caissons against tidal currents that can reach seven knots. Caisson construction then proceeded in the Narrows, forming caisson walls and inner stiffeners above the cutting edge and lowering it closer to the bottom with each 10 foot lift of concrete, while keeping the forming operation just above the surface. The floating caissons were touched down on the floor of the Narrows, well within tolerance for these massive structures. Once the cutting edges touched down, divers removed the steel seals from the bottom. As additional lifts were poured at the surface, sediment was dredged from the caisson interiors with a clamshell, allowing the caisson to drive into the sea floor. When each caisson reached its design penetration, a 25 foot thick tremie concrete seal was placed in the bottom of the cutting edge. A 15 foot thick cap was built on each caisson, followed by the bridge tower construction.