Crews building the new Tacoma Narrows bridge are poised to pass another milestone today.
About 4:45 this afternoon, Tacoma Narrows Constructors workers will lower the 39,000-ton foundation for the east tower of the new bridge to the Narrows floor.
The massive concrete structure, as big as a 15-story building, is floating about 7 feet from the bottom at low tide. It must be lowered to within inches of its target, making the endeavor a demanding technological challenge.
Today's touchdown effort will be somewhat easier, engineers said, because it will be almost identical to the process used on the Gig Harbor-side caisson, which was lowered into position Dec. 15.
The Tacoma side of the channel is deeper, however, and its tidal currents are stronger.
"Caissons are not built in this kind of environment very often," said Tom Sherman, TNC's caisson construction manager.
"In most places, crews are building caissons in lakes or rivers where water currents flow in one direction and there are only small, if any, tide fluctuations," he said. "In the Narrows, we have tides that change water levels up to 18 feet and have strong currents that flow in both directions."
The Tacoma caisson touchdown will be a daylong event, starting about 8:30 a.m. Engineers will gently steer the structure to the seabed using seawater, air and computer-monitored anchor cables as controls.
They'll control the caisson's buoyancy by alternately pumping water into its 15 dredge wells and by releasing air from watertight chambers on its bottom.
As the caisson drops, its alignment will be monitored by electronic distance meters, Global Positioning System units and laser beams directed at it from the shore.
If crews determine the caisson is not at the ideal location for touchdown, they will pump air into the structure to increase its buoyancy, lift it off the seabed and try again, said Claudia Cornish, spokeswoman for the state Department of Transportation.
Immediately after touchdown, crews will add more weight to the caisson to prevent high tides from lifting it off the seabed. This will be done with water ballast today, followed by adding more concrete to the caisson's walls Friday.
Rob Carson 253-597-8693