Special Reports

Like building a skyscraper under water

Once the partially completed caisson is secured at the bridge site, Tacoma Narrows Construction crews will extend its walls upward.

They'll add layers of steel and concrete to its top, 10 feet at a time, maintaining the honeycomb pattern as they go.

Each time workers add a 10-foot section, they'll lower the caisson 10 feet, adjusting the anchor cables as they go to keep the structure in position.

The process will be similar to building a skyscraper, engineers say, except that instead of rising into the air, the structure will sink progressively into the water.

At a depth of 154 feet, the foot of the Tacoma-side caisson will touch the sea floor.

At that point, divers with underwater cutting tools will descend to the bottom of each of the 15 wells in the caisson. They'll cut out the steel floors in the wells, opening access to the seabed.

Then cranes with "clam shells" on long cables will pull sand and gravel out of the wells, reducing the resistance of the earth and allowing the structure to cut through the sea floor as more layers are added at the top.

By carefully choosing which wells to remove dirt from, engineers will steer the caisson's descent into the earth, keeping it level and plumb.

At 62 feet, engineers say, the caisson will be established firmly enough to support the bridge towers. At that point, they'll cap the top and start building the tower.

Construction of the caisson on the Gig Harbor side of the bridge will lag behind the Tacoma caisson by about a month.

Because the water is not as deep on that side, the caisson won't have to be as tall. It will descend through 133 feet of water and sink 54 feet into the seabed.