Repairs to Big Bertha will require a 12-story pit and removing the tunnel-boring machine’s cutterhead
Wayne Grayson | February 28, 2014
Big Bertha cutterhead closeup

Big Bertha’s cutterhead being lowered into place last year for the start of her dig beneath Seattle.

Officials from the Washington State Department of Transportation have announced that in order to repair the world’s largest tunnel-boring machine, her head will need to come off.

The announcement, which came during a presentation to city council members in Seattle, comes nearly three weeks after WSDOT officials announced that Big Bertha would be out of commission for months due to the complexity of needed repairs.

Several of the machine’s cutterhead openings became clogged with dirt in late January and damage was done to seals around the machine’s main bearing, causing the machine to overheat.

What Big Bertha leaves behind. Two workers walk through the first rings of tunnel dug by Big Bertha in October.

What Big Bertha leaves behind. Two workers walk through the first rings of tunnel dug by Big Bertha in October.

According to Seattle radio station KUOW, to repair the machine, workers will dig a pit 12-stories deep directly in front of Bertha to gain access to the cutterhead. They’ll then remove the head and lift it from the pit and lay it on its side on the surface. It’ll be no easy task. The cutterhead is five stories tall and weighs 700 tons.

Matt Preedy, a deputy administator with WSDOT, told Seattle KUOW that Bertha’s current position is very fortunate as it’s “almost the perfect place” to perform such repairs. “If you had to pick a place somewhere along the alignment that you had to do this sort of thing—take the cutter head off—this is the best place to do that,” Preedy said.

However, the Seattle city council wanted reassurances that the machine wouldn’t break down again—the next time would the third time—when under the heart of the city where digging another pit would be a near impossibility. WSDOT officials were not able to provide such reassurances but are coming up with a plan for that should it happen.

And because of current estimates that Bertha’s downtime is costing Seattle Tunnel Partners, the contractor behind the dig, $150,000 a day, the council wanted an idea of just how many months the repairs would take.

Bertha has completed one-tenth of the 1.7-mile State Route 99 tunnel below Seattle, which will carry a double-deck highway. The new tunnel will replace the SR 99 Alaskan Way Viaduct.

The machine has encountered two major setbacks so far. After nearly two months of downtime caused by the machine hitting a steel pipe, she advanced four feet near the end of January only to be brought to a halt due to a warning light being triggered in the control room that signals overheating.

The tunnel was originally scheduled to be completely drilled by the fall of 2014 and open to traffic by the end of 2015.


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