June 10, 2015
More than 10 years after an earthquake damaged the Alaska Way Viaduct in Seattle, Washington, J. Harper Contractors removed the structure with some massive technology.
In February 2001 Seattle was shaken by one of the largest earthquakes in the state of Washington’s history. The 45-second, 6.8-magnitude event damaged the Alaska Way Viaduct on State Route 99, 80 kilometers from the quake’s epicenter in Nisqually, Washington. The double-deck bridge was repaired soon after, but it continued to be monitored because of safety concerns. In early 2009 the state decided to replace the viaduct.
Demolition of concrete structures is a specialty of J. Harper Contractors, which began removing part of the Alaskan Way Viaduct in October 2011. The initial assignment from general contractor Skanska USA was to demolish and remove 10 000 metric tons of concrete in a 400-meter span of the structure. This would make way for crews to begin building a diversion to relieve traffic during construction of the bridge’s underground replacement, a three-kiolmeter-long, four-lane tunnel.
J. Harper Vice President Jeff Slotta says he knew what he wanted for this job right away: an Atlas Copco Combi Cutter CC 6000 U. Slotta was familiar with the CC 6000’s use in Europe; it is the largest Combi Cutter available, with a service weight of more than 6 metric tons. (J. Harper’s other Atlas Copco equipment includes three HB 3000 heavy duty breakers, a BP 2900 R bulk pulverizer and a smaller Combi Cutter, the CC 3300.)
“Our company has worked on larger projects and probably could have accomplished this task with the equipment we already had,” Slotta says. “But the heavy structural concrete of the bridge would have taken much longer without a larger cutter.” The job was finished in just 30 working days.
“ It just squished them, munching down on those columns, slicing through rebar all day long. ”
The Combi Cutter has an option that features steel-shearing blades in the throat of two concrete cracker jaws. The jaws are driven by separate pistons and operate, using Slotta’s description, “like alligator jaws, with teeth in front to pulverize, crushing and swallowing concrete down its throat to get to the No. 18 rebar.” J. Harper recovered more than 1 000 metric tons of the 5.7-centimeter-diameter rebar during the demolition phase.
HB 3000 breakers were used to demolish both concrete decks of the viaduct. The CC 3300 and pulverizer worked on the lower deck and beams, while the CC 6000 tore down the upper deck, towers, bent supports and deep concrete beams. The steel-reinforced, 18-meter-high towers were 1.2 by 1.2 meters wide, and the concrete bents – the supporting system that elevated and supported the roadway – had concrete beams that were almost a meter thick.
“The CC 6000 didn’t even hesitate,” says Slotta. “It just squished them, munching down on those columns, slicing through rebar all day long.”
When Slotta first made his request, he didn’t realize that the CC 6000 would come all the way from Germany; this would be the first one used in the US. Fortunately, J. Harper had allowed three months of lead time before using the cutter in the project. The CC 6000 requires a larger carrier than the 300- and 400-series excavators that J. Harper usually uses in its demolition fleet, so Seattle-based Modern Machinery mounted the new cutter to a Komatsu 800-series excavator.
Slotta notes that “Additional modifications on the Komatsu were pretty much limited to installing extra hydraulics to run the Combi Cutter and adding extra guards to protect the operators.” J. Harper places a premium on safety, and having the right equipment for the job helps the company live up to its brand promise: Providing customers with a “drama-free demolition experience.”
“I’d rate this as one of the smoothest wrecking jobs we’ve ever done,” says Slotta. “We’ve had absolutely no issues with either the tool or the excavator.” The project also included transporting concrete rubble to the company’s portable crusher at a staging area about a mile offsite. While completing the first phase of demolition, J. Harper was awarded additional portions of what is anticipated to be at least a six-year project, including crushing 30 000 metric tons of concrete and demolition of other sections of the viaduct.
Written by Joe Bradfield