Special Reports

Gigantic caisson lands within inches of target

Crews steered the first massive foundation for the new Tacoma Narrows bridge to the Gig Harbor side of the Narrows floor on Monday, coming within inches of their target.

In an intricately choreographed maneuver, engineers lowered the giant caisson, as big as a 12-story building, the final 8 feet to touchdown by releasing air from 15 closed chambers in its bottom and tugging on anchor lines to nudge it into position at 4:20 p.m.

"It was basically uneventful, which is good news for us," said Erin Babbo, spokeswoman for Tacoma Narrows Constructors. "We placed it within our tolerances, and it's plumb, so everybody's pretty happy here."

The touchdown was a significant milestone in the construction of the $849 million suspension bridge, according to project engineers.

The maneuver was difficult, said caisson manager Tom Sherman, not only because of the huge size of the 33,000-ton structure, but also because of the rapid currents and large tidal exchange in the mile-wide channel.

Monday was chosen for touchdown attempt, Sherman said, because of a low slack tide at 4:15 p.m., followed by a relatively low high tide.

Now that the caisson is on the bottom, engineers are pumping water into it from the top to reduce its buoyancy.

Today, if all goes well, crews will add concrete to the caisson's interior and exterior walls to more firmly plant it to the bottom.

"Things went so well, we're proceeding with our plans to pour more concrete on Tuesday," Babbo said.

Eventually, the caisson and its twin on the Tacoma side of the channel will support the two 510-foot towers of the new suspension bridge.

The new bridge is the longest suspension bridge built in the United States since 1964. When finished, it will span 2,800 feet between towers and 5,400 feet in all.

The Tacoma-side caisson, which is being constructed in deeper water, is lagging about a month behind the Gig Harbor caisson.

Once the caissons are on the bottom, they will be flooded with seawater, and scuba divers will cut out the false bottoms in each of 15 honeycombed wells.

Cranes equipped with clamshell diggers then will dredge soil out of the wells, as more concrete is added to the caissons' tops, pushing them deep into the sea floor.

Caisson construction is to be completed next summer. Building the towers will take another year. The new bridge is to open in spring 2007.

Rob Carson: 253-597-8693

rob.carson@mail.tribnet.com

Note: The caisson on the Tacoma side is a month behind the one on the Gig Harbor side.

Timeline

Monday: Caisson in place

Today: Concrete added to walls

Next summer: Caissons complete

Spring 2007: Bridge complete

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