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Smooth sailing for bridge caisson

The most anticipated hunk of concrete and steel to travel to Tacoma in a long while arrived fashionably late Monday afternoon, greeted by crowds of construction workers and curious onlookers gathered to watch it come to rest under the Tacoma Narrows Bridge.

The 14,000-ton, 130-foot-long piece is the first of two caissons that will be lowered nearly 70 feet into the bottom of the Narrows to serve as the foundations for two towers that will support the new bridge.

Made up of enough concrete to fill a football field three feet deep and 3,500 tons of steel, the caisson will soon have 24 anchor cables attached to it. The caisson will then be slowly lowered into the seabed and gradually filled with concrete to create the tower.

Pulled by two Foss tug boats and escorted by a Pierce County Sheriff boat, the caisson traveled 10 nautical miles from the Port of Tacoma, where it was built over the last four months.

Tacoma Narrows Constructors Project Manager Manuel Rondon said the caisson’s journey this week was one-of-a-kind for many reasons.

“There hasn’t been a caisson this large moved in decades,” he said, explaining that the caissons underneath the current bridge were put in place nearly 70 years ago. “And the Narrows isn’t like any other body of water on earth,” so it offers some unique obstacles.

Rondon said unlike a river, whose water flows in only one direction, the water in the Narrows changes direction in accordance with the tides. Also, given the channel’s narrow span, the currents are strong — usually nearly 7 knots — and the depth can vary from 10-15 feet.

To provide the smoothest trip possible, Rondon said crews chose this week to move the caisson because the currents were expected to be less than half their typical speed — less than three knots — for five consecutive days.

To prepare for any possible mishap, such as tilting or swaying, Rondon said crews performed a battery of tests both in the laboratory and on a scale model of the Narrows.

Once the caisson is in position, crews will begin building the tower from the top down by adding layers of concrete 10-feet at a time. While the concrete is added, the caisson will slowly descend through 154 of water, then another 62 feet into the seabed.

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