Contractors develop 60-foot spear to sink caisson

What does a contractor do with a 60-foot I-beam and a leftover anchor plate? Tacoma Narrows Construction of Tacoma, Wash., recently bolted the two together to form a super-sized spear that they call “The can opener.”

TNC is building the caissons for the new $800 million Tacoma Narrows Bridge, which is the first long-span suspension bridge to be built in the United States since 1964. Since construction began in September 2002 the project has stayed on schedule, until the excavating stage ran into some difficulty with the caissons. The company is now a month behind, and TNC is being pressured to have the caissons completed by June so that construction on the rest of the bridge can begin.

The caissons are kept afloat by a watertight seal and are supported by 20-foot steel domes. To sink the caisson the original plans called for divers to cut the steel with torches, but because of the depth–each caisson is 14 stories tall–the divers have to deal with extreme pressure. Each diver can only work 50 to 70 minutes at a time, and before rising to the surface have to rest for about 15 minutes on the way up. Once out of the water, the divers then have to spend more than an hour in a decompression chamber. Even if a crew of divers work two 10-hour shifts a day, the divers would take at least eight to 10 days to finish each caisson.

In order to finish on schedule, TNC has been researching other ways to cut the steel supporting the caisson. While the original plan would work, the contractors need a speedier method.

TNC engineers came up with the “super spear” method by joining together a leftover anchor plate and a 60-foot I-beam. According to their plan, a crane will hoist the spear and dangle it inside one of the shafts, pointed downward. The spear will then be released, and the weight of the device will drive its wedge-shaped head into the steel domes. By raising and lowering the spear around the perimeter of the dome, engineers hope to cut the top out like a can-opener.

Although nothing like the super-spear has been developed before, TNC mangers expect that it will be twice as fast as the divers and cheaper. However, if the spear gets caught in the caisson, then divers will be needed, and the project will again be delayed.

According to Kent Werle, construction superintendent of the Gig Harbor caisson, it might seem faster in the beginning, but end up taking longer in the end. “We’ll just see what happens,” Werle told the News Tribune.

TNC expects to begin the spear process this week. The project is scheduled to be finished in early 2008.