Deep inside big dig for stalled Brightwater sewage plant

Three hundred and 20 feet below an elementary school in Bothell, in a spot that can be reached only through a 1 ½-mile tunnel, sits a giant tunnel-boring machine that's going nowhere.

Inside the 35-foot machine, workers are laboring around the clock to get it back up and running so it can finish digging the tunnel that will carry treated wastewater from the new Brightwater treatment plant to Puget Sound.

The machine, originally intended to finish its job last February, has been idled since May while awaiting critical repairs. King County officials haven't said what caused the damage, and it isn't known how much the county, contractor or manufacturer will end up paying. They acknowledge the problems will cost "tens of millions" of dollars.

The $1.8 billion Brightwater project, scheduled to open next year, is designed to handle growth in northern King and southern Snohomish counties. The plant will clean wastewater to a higher degree than existing treatment plants in Seattle and Renton.

To reach the boring machine — which is a bit wider than a Boeing 737 but has all the charm of an industrial-engine room — workers must climb down a tall metal-scaffolding stairway and then catch a little white train that rumbles through the 14 ½-foot-wide tunnel.

Things seemed quiet inside the machine midday Wednesday until three "divers" got the green light to work on its damaged front end. Because of the great depth, they must pass through an airlock.

Sometime this week the five diving teams — they're called that even though they don't work under water but rather in a pressurized air pocket in front of the machine — will begin welding new pieces onto the broken steel rim of the digger's snout.

King County wastewater officials expect the machine to start "mining" again in six to eight weeks. Fixing that machine won't be the end of the problem. A second machine at about the same depth under Lake Forest Park was shut down in June because of similar front-end damage.

It won't reach its Shoreline destination until 2011, and the tunnel isn't likely to be ready to carry treated sewage until 2012, as much as two years behind schedule.

The two idled machines are essentially stuck in the central portion of the 13-mile tunnel that will take treated wastewater from the Brightwater plant being built on Highway 9 in Snohomish County north of Woodinville to its outfall in Puget Sound off Point Wells.

The outfall and the eastern tunnel segment are finished, and the western tunnel segment is expected to be completed in February.

Working on solution

It's taken seven months since the boring machine in Bothell was shut down — and most workers laid off — to figure out what to do and then to remove groundwater at the front of the machine. Less water means less compressed air is being pumped into the divers' work space in front of the machine's cutting edge to counteract the water pressure.

Six wells drilled from the surface and another 10 wells through the sides of the tunnel reduced the pressure so it's now roughly equal to a dive in 40 feet of water, said resident engineer Derek Dugan.

Because of high groundwater pressure and soft soils in the deep tunnel route, officials decided to use a new kind of machine rarely used in the United States. The machine pumps a liquid slurry in front of its cutting blades and then sends a mix of slurry and dirt through pipes to the surface.

Dugan describes the tunnel route as "a mishmash" of silt, sand, gravel and clay. "This is as complex as I've ever seen," he said. If tunneling technology hadn't improved in the last decade, he said he doubts King County would have opted to tunnel so deep underground.

Officials at Sound Transit, which is preparing to dig light-rail tunnels through Capitol Hill between downtown Seattle and the University of Washington, and the state Department of Transportation (DOT), which is planning a deep-bore tunnel to replace the Alaskan Way Viaduct, are closely watching King County's Brightwater experience.

Groundbreaking

The viaduct replacement, like Brightwater, would break new ground: DOT wants to keep its tunnel diameter under 54 feet, said program Administrator Ron Paananen, "primarily because the largest tunnel built to date previously was about 51."

The DOT once rejected such a large tunnel, but other countries have since shown it can be done.

"Five years ago people would have said, 'You've got to be kidding me,' " former state Transportation Secretary Doug MacDonald said. "Today, they say if they're doing it in Madrid, we should think about doing one that's a foot larger here."

Keith Ervin: 206-464-2105 or kervin@seattletimes.com