Bertha Rising: The ailing cutterhead assembly breaks the surface
By David Kroman
By David Kroman
Update 5:00 PM Tuesday: At 2:30 PM, Bertha's cutterhead was successfully placed down on the surface, marking the end of what turned out to be a very successful lift. Crews will now begin disassembling the cutterhead and drive unit for repairs.
Update: 8:00 AM Tuesday: According to the WSDOT website, Seattle Tunnel Partners decided to wait until Tuesday morning to finish the lift. Operations to set the cutterhead down on the surface will resume shortly.
Original Story: On Monday, Seattle Tunnel Partners and Dutch crane operator Mammoet began lifting the fourth and final piece of the tunnel-boring machine Bertha to the surface. Removal of the 2,000-ton drive unit is arguably the most significant milestone since Bertha broke down in December of 2013, because it will allow crews to fully diagnose what, exactly, went wrong and how to fix it.
Last month Chris Dixon of Seattle Tunnel Partners (STP) laid out what he called an “aggressive schedule” for repairing Bertha. Assuming the lift continues to go well into Tuesday morning, STP will just meet its first deadline: getting all four pieces (the cutterhead's drive unit and the three-piece shield) of the tunnel boring machine to the surface by the end of March. Earlier this month, crews successfully removed the lighter shield pieces that make up the protective layer behind Bertha's cutterhead.
Dixon was clearly pleased about hitting STP’s schedule, a rare occasion for the Alaskan Way Replacement Project. Originally, the tunnel was scheduled to be completed by the end of 2015. But the rubber seals meant to reduce friction on the rotating cutterhead malfunctioned, causing Bertha to overheat and maybe damaging the machine’s main bearing as well. The rescue project itself has become a massive undertaking, pushing back the tunnel’s completion date (optimistically) to the end of 2017.
Throughout March, as a smaller crane pulled the first three Bertha pieces from the access shaft near Main Street and Alaskan Way, workers could be seen preparing Mammoet’s gantry crane — "super crane" as WSDOT calls it — to hoist the 57-foot-tall face of Bertha.
This assembly includes not only the cutterhead but also sixteen 8,000-pound motors and the main bearing behind the head. When asked what makes lifting Bertha's face so difficult, University of Washington Construction Engineering Professor Joe Mahoney said with a laugh, “it’s heavy.”
So heavy, in fact, that the super crane, which resembles an AT-AT walker from Star Wars, was custom built for the job. At 105 feet tall, the crane has towered over the viaduct for months now, its six legs currently straddling the access pit.
The crane rests on two underground concrete walls that were originally built by STP to control ground settlement. When concerns that Seattle’s east/west winds could shift a disproportional amount of weight to one side of the crane or the other, threatening the foundation, crews added hydraulic supports to distribute the weight equally between all of the crane’s legs.
“It’s not what I call complicated engineering," said Mahoney. "It’s just heavy. You need beefy stuff.”
Mammoet has performed similar lifts before, including excavating a Russian submarine and moving pieces on the Chernobyl project. A big lift like this is rare, said Mahoney, but not unheard of. “Some of the components of power plants can weigh up to 1,000 tons,” he said. Still, he added, “Ain’t nothing like Bertha.”
Mahoney, Matt Preedy of WSDOT and STP’s Dixon all agreed that the key was moving slowly. Six cylinders, all turning at an imperceptibly slow rate, each reeled in 54 ¾-inch cables at a careful, glacial 4 inches a minute. According to Dixon, crews began testing the lift early Monday morning, adding weight to the crane in 20 percent increments. “We went 20 percent, 40, 60, 80," said Dixon. "We couldn’t test the lift in advance because nothing else weighs 2,000 tons.”
Originally, STP planned to lift the cutterhead section from inside the access shift, turn it horizontally, reverse the super crane and set down the piece. But as the drive unit rose from the pit — dirty, rusted, and larger than any words or picture can convey — it was already turned at an angle. WSDOT’s Communication Director, Laura Newborn, said STP realized they had enough room to turn the piece inside the shaft, which meant crews wouldn’t have to lift the drive unit as high to clear the surface.
In addition to the rubber seals, STP will replace the main bearing behind the cutterhead, whether it’s broken or not. Both WSDOT and STP have maintained they will not know what else may have malfunctioned until the drive unit is removed. WSDOT’s Preedy said, at first glance, there are no surprises, but it will take all of April to disassemble the pieces and determine what else might need to be fixed.
If all goes according to schedule, the repairs will take place in May, the four repaired pieces will be lowered back into the pit in June and reassembled in July, and Bertha will resume tunneling in August.
Frustration with the tunnel project has been building, both in Seattle and statewide. Twice, state representatives from Eastern Washington tried to kill the project. The relationship between WSDOT and the City Council has been strained. Costs associated with the delay remain a question mark.
But Mahoney, who is not directly involved with the tunnel project, defends Bertha. “People anticipate perfection,” he said. “But big, one-off projects are so hard to do perfectly.”
And when problems inevitably arise, said Mahoney, “You fix them and you move on.”