Tacoma Narrows Project Reaches First Milestone
The new bridge is definitely needed. Right now, traffic is funneled across a 52-year-old bridge that carries 85,000 vehicles each day-25,000 more than the maximum for which it is intended. The new bridge, paralleling the existing one, will ease the congestion by handling eastbound traffic.
For just a few hours on July 21, however, the project had the opposite effect. Cars slowed down, pedestrians on the bridge walkway gawked, and pleasure boats filled the water below as tugboats brought the 12,700-tonne, concrete and steel caisson from the Port of Tacoma and maneuvered it into position.
If it looked like an enormous undertaking, it's because it was. Bechtel and Kiewit engineers had been planning the day for two months, determining the best strategy for bringing big structure-essentially a seven-story high box-18 kilometers along a stretch of water noted for large tidal swings, fast currents, and pesky winds.
In theory, the plan was simple: Pick a time when tidal exchanges would be low, creating slack tides and slow currents. Then use the ingoing tide to bring the Caisson in from the port, and the outgoing tide for final maneuvering. Nothing is simple, however, when the structure you are towing is 40 meters long, 24 meters wide, 24 meters tall, and almost 15 meters of it is submerged, making it easy prey to changing currents.
As it turned out, the narrows cooperated, no unforeseen problems developed, and the move turned out to be easier than anticipated. The three tugboats towed the caisson out of port shortly after 3 a.m., maintaining a leisurely pace of just under 3 kilometers per hour. At one point, they cut that to less than 2 kilometer per hour to take advantage of the slackening tide. They delivered the caisson to its new home adjacent to the Tacoma-side caisson of the existing bridge at about 1 p.m.
The days following, divers secured the caisson to prepositioned anchors on the floor of the narrows. Over the next few months, the structure-which is mostly hollow and floats on 15 steel domes-will sink slowly under the weight of concrete and steel during construction of the tower above it, which eventually will rise some 50 meters above the water. The second caisson-at a shallower spot-will be brought to the site in August. By December, both will be embedded firmly on the seabed, a solid foundation for the future of Tacoma Narrows.
Bechtel is a trusted engineering, construction and project management partner to industry and government. Differentiated by the quality of our people and our relentless drive to deliver the most successful outcomes, we align our capabilities to our customers’ objectives to create a lasting positive impact. Since 1898, we have helped customers complete more than 25,000 projects in 160 countries on all seven continents that have created jobs, grown economies, improved the resiliency of the world's infrastructure, increased access to energy, resources, and vital services, and made the world a safer, cleaner place.
Bechtel serves the Energy; Infrastructure; Manufacturing & Technology; Mining & Metals; and Nuclear, Security & Environmental markets. Our services span from initial planning and investment, through start-up and operations. www.bechtel.com