To get Bertha moving again, state officials announced Wednesday that they are sending in human reinforcements – in a giant, pressurized bubble.
The tunnel-boring machine has been stuck in place for more than a month after officials said it ran into an eight-inch wide steel pipe, but the Washington State Department of Transportation and its contractor, Seattle Tunnel Partners, want to find out if something else might have also forced Bertha to stop.
Tunnel workers will head underground into hyperbaric chambers that let their bodies adjust to the high air pressure down below. Then, with flashlights, they'll step out of the chambers and into a void behind Bertha's 57-foot-wide cutter head.
The pressure bubble is necessary to keep soil and water from filling up the space the workers will be inspecting for damage. A thick slurry of synthetic polymers and a clay known as bentonite will help the bubble hold back the soil and water.
Project officials said the work has to proceed slowly and carefully to make sure the pressure bubble doesn't spring a leak while workers are in it.
"Those workers would have to quickly get into those chambers,” said Todd Trepanier, who heads the project for the state transportation department. “If they were to stay in a quickly decompressioned atmosphere, it would not be healthy to them."
Each team of five workers has one member whose job is to watch the clock to make sure no one stays under pressure longer than is safe. That person's job also includes keeping an eye on the condition of the bubble, according to Seattle Tunnel Partners project manager Chris Dixon.
"Everything should be stable there," Dixon said. "You shouldn't see any water squirting in. You shouldn't see any of the bentonite breaking away. You shouldn't see any soil moving – if we're holding pressure and we have a good bentonite seal."
Because of the elevated pressure, the workers can only spend four hours at a time inside Bertha. Then they have to spend more than an hour in a decompression chamber before returning to the ordinary pressures of daily life.
WSDOT officials declined to estimate how much the high-pressure work could add to the cost of the $3.1 billion megaproject to replace the Alaskan Way Viaduct. On Wednesday, state Transportation Secretary Lynn Peterson told state lawmakers that the state has had concerns about Bertha’s operations since its launch in July. Peterson said the state transportation department is assembling a panel to review the tunnel work before it continues burrowing through downtown Seattle.
The contractors plan to empty out Bertha's top half only. After teams of workers inspect the top half of the cutter head, then they'll rotate the head 180 degrees to expose the other half. If that technique doesn't work, Trepanier said they'll have to make the bubble twice as deep. That would subject workers to higher pressures and risk.
"It's like going down an additional 30 feet scuba diving and then twice the square surface that you're trying to hold with that bentonite," Trepanier said.
While Bertha is North America's largest tunneling machine, the pressures the workers are expected to face are not as great as those on other tunneling projects. On some deeper projects, air pressure has been so great that workers could only work 45-minute shifts before heading for a decompression chamber.
Produced for the Web by Kara McDermott.