Special Reports

Bridge 'can opener' back in the drawer

Builders of the new Tacoma Narrows bridge will not use their experimental "can opener" to remove the steel bottoms of the bridge foundation on the Tacoma side of the channel.

The 17-ton tool, developed by engineers at Tacoma Narrows Constructors to make the job go faster, was used to cut out about half of the bottoms on the Gig Harbor foundation.

The technique worked, said Dennis Engel, the project engineer for the state Department of Transportation. But the can opener did not make clean cuts, leaving it to a powerful crane equipped with a toothed clamshell bucket to wrench out the crumpled remains.

"It was too hard on the equipment," Engel said. "The crane and the bucket were both taking a beating."

The foundations, or caissons, are made of concrete and as tall as 15-story buildings. Eventually, they will support the bridge's two towers.

Inside, a grid of concrete walls divides them into 15 individual shafts. The shafts have steel bottoms that must be removed so workers can dredge beneath them, thereby sinking the massive structures to a secure footing in the seabed.

Divers armed with underwater torches will remove the 15 steel bottoms on the Tacoma caisson, Engel said. Divers and the can opener crew worked side by side on the Gig Harbor caisson.

The 68-foot-long device worked like a giant spear. A crane lowered it into the dredge wells in the caisson and repeatedly dropped it around the perimeter, using its weight to slice through the half-inch steel bottoms.

"The divers are a sure thing and their productivity is high," said Claudia Cornish, a DOT spokeswoman. "The feeling was that the other method was just beating things up too much."

The divers will work double shifts through this week and next, Cornish said. At the rate they've been cutting, she said, divers should be able to remove one bottom per day.

Bridge crews are using a crane and bucket to remove gravel from beneath the Gig Harbor caisson, slowly sinking it to its eventual position 57 feet beneath the floor of the Narrows.

When the bottoms of the Tacoma caisson have been removed, the dredging operation will shift back and forth across the Narrows, Engel said.

While the dredging is taking place on one side, crews will set up forms and extend the tops of the concrete walls on the other, Engel said.

TNC had hoped to have both caissons finished by June but now is running at least a month behind schedule.

In the context of the five-year project, that delay is insignificant, Cornish said.

Rob Carson: 253-597-8693

rob.carson@mail.tribnet.com

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