Special Reports

An excavating challenge

When engineers designed the caissons for the new Tacoma Narrows bridge, they wanted the airtight chambers on the bottoms to be heavy duty.

They used plates of structural steel, a full half-inch thick and welded securely into place.

The air chambers performed flawlessly, maintaining their watertight seal all the way to the bottom of the Narrows and keeping the massive caissons buoyant so crews could steer them into position.

But now the chambers have to be opened so crews can excavate beneath them, and the steel is looking heavier than ever.

Since the beginning, the plan has been to send divers down to cut out the steel with underwater torches.

But with caisson construction running about four weeks behind schedule, Tacoma Narrows Constructors boss Manuel Rondon challenged his top engineers to come up with a better can opener.

The engineers believe they might have done exactly that. What they've arrived at could be the world's largest spear - 68 feet long and 17 tons.

Monday, they'll try it out on the Gig Harbor caisson.

The diving plan has not been abandoned. The divers will start cutting next week, too, working in a chamber on the opposite side of the caisson.

"The good thing about torching them out is that it's a definite, finite process," said Kent Werle, construction superintendent of the Gig Harbor caisson. "There are fewer unknowns."

But the task the divers face is daunting.

The caissons are the height of 14-story buildings, divided by a grid of concrete walls into 15 shafts that run from top to bottom.

The steel domes that must be removed are at the bottom of the shafts, under about 140 feet of water. Each dome is approximately 20 feet square, shaped like a quonset hut and rock solid.

Cutting torches work well under water. But at that depth, progress will be slow. Divers will have just 50 to 70 minutes of bottom time because of the extreme pressure. To avoid the bends, they'll need to pause for at least 15 minutes on their way up, then spend more than an hour in a decompression chamber on the surface.

Even working two 10-hour shifts a day, the divers would take at least eight to 10 days to finish each caisson.

TNC desperately wants to reduce that time.

Unlike other aspects of the bridge project, the caissons are considered part of the "critical path" of construction, meaning that, until they're finished, the towers can't go up, and until the towers go up, the cables can't be attached to them.

If the caissons aren't finished on schedule in June, it will set construction back, complicating subcontractors' schedules and causing cost overruns.

With the challenge to remove the domes faster in front of them, TNC engineers tried to be as open-minded as possible, Werle said.

The idea that seemed to hold the most promise was a device that would slice through the steel.

After calculating how much force it would take to penetrate the steel, TNC engineers came up with the super spear, fashioned out of a leftover anchor plate and a 60-foot I-beam, 14 inches across.

According to their plan, a crane will hoist the spear and dangle it inside one of the shafts, point down. Theoretically, the weight of the device will drive its wedge-shaped head into the steel. The crane operator then will lift the wedge out, leaving a 5-foot slit.

By raising and lowering the spear as the crane moves it around the perimeter of the dome, engineers say, they'll be able to cut the top out like a jackknife opening a can of beans.

When the top is loose, the crane operator will lower a clamshell bucket, clamp onto it and lift it out the top.

On official drawings, the device is called a "Dredgewell Opener." Around the TNC offices, it's more often referred to as "the can opener."

TNC managers recoil from words like "punch" or "strike," when it comes to describing the spear's function.

"There is no striking involved," said Andy Hoff, one of TNC's top caisson engineers. "It all has to do with gravity. We have a large amount of weight, so it will take zero to minimal velocity to cause penetration."

If gravity alone doesn't work, Hoff said, the crane will lift the spear a foot and drop it. If that doesn't work, he said, they'll try dropping it from 2 feet.

If the can opener works as well as its supporters hope, it will be at least twice as fast as the divers.

It would be cheaper, too. Relative to the bridge's $849 million cost, it will cost almost nothing.

TNC had the leftover caisson anchor in storage and simply used cutting torches to shape it into a wedge. Aside from that, all it took was a 60-foot length of I-beam and a few bolts.

The can opener would be safer, too.

If a diver were to be seriously hurt or killed, the tragedy would haunt TNC and the new bridge forever.

"So far nobody has even stubbed a toe out there," Werle said. "We want to keep it that way."

As crews prepare the can opener for its test run, Werle is keeping an open mind, but is regarding it as an experiment, not a panacea.

If the spear leaves even a small amount of steel between cuts, he notes, it will be next to impossible to pull free with the crane.

"A 2-inch piece would hold back the whole crane," Werle said.

Divers could go down and clean up the cuts, he said, but by then they would be working in a more difficult environment, with crumpled, torn metal and no staging in place.

"Sometimes what seems to work faster in the beginning ends up taking longer in the end," he said. "We'll just see what happens."

Rob Carson: 253-597-8693

rob.carson@mail.tribnet.com

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