Beacon Hill tunneling a breeze

Three months into the job, you might say that Sound Transit's new Beacon Hill Tunnel has been simply boring.

Drill operators have yet to face any nasty surprises in the first 400 feet, as the gigantic Emerald Mole worms its way eastward through the hill, officials say.

The transit agency offered the news media its first look at the site Wednesday afternoon, during a lull in construction. Drilling has halted while a long conveyor belt is being attached to carry out the dirt, making it safe to walk inside.

At the tunnel mouth under Interstate 5, the scent of fresh bread drifted on the wind, from nearby Gai's Bakery. Inside, the air smells muddier, circulated by noisy pumping machines.

Tunneling costs and challenges have hindered Sound Transit for a decade, ever since voters approved a regional rail-and-bus plan in 1996. A proposed tunnel to the University District was postponed in 2001 and is still not funded.

The one-mile Beacon Hill segment alone is costing $292 million — a $72 million increase from what was estimated three years ago. More than half that amount, however, is actually because a second station was added to the tunnel contract. Millions more were spent to dig a test shaft and study the unstable soils. Grout had to be injected into the hill, so water and loose sand don't seep into the underground station, where an elevator shaft for future passengers is being mined 16 stories down from the hilltop.

The project includes a second, elevated station near Franklin High School, where the tunnel emerges from the hill's eastern slope.

Fortunately for the Emerald Mole crew, working deep below the hill, the soil has been easy silt and glacial till.

"It's really nice material. It's not abrasive. It also holds its profile well," said Pat Gould, who operates the drill's computerized controls for lead contractor Obayashi.

Like a rotary razor, the blades on a 21-foot drill head churn at a typical speed of 1 ½ revolutions per minute. Dirt drops toward a rotating screw that pushes it out the back end.

To prevent cave-ins, Mitsubishi of Japan built the drill so it can also install the concrete tunnel wall, right away. At the back end, a robotic arm snatches a concrete, circular section of the wall — like picking up a six-pack of beer with two fingers, one tunnel manager said — and sets them in place. Workers bolt the curved pieces together, to complete a solid ring.

This tunnel eventually will carry southbound tracks to Seattle-Tacoma International Airport.

Once the machine breaks through to Rainier Valley, it will be dismantled, brought back to Sodo, then rebuilt so it can dig a twin tunnel for the tracks heading back from the airport to downtown. The drill must be taken apart because it is too heavy to truck around the hill in one piece.

As tunnels go, there have been few problems or fights to date, said Obayashi's project director, Paul Zick. "This has been a pretty good one."

Light-rail opponent John Niles calls the project "very much a practice tunnel" for future lines. He thinks difficulties on Beacon Hill played a role in Sound Transit's recent decision to give up on building a deep underground station on First Hill.

He said the agency should prove it can tackle Beacon Hill, before local politicians on the transit board proceed with any more tunnels. The transit board today is expected to approve a route and finance a plan to dig from downtown to Husky Stadium. And a partly tunneled line to Bellevue might reach the ballot next year.

The Beacon Hill tunnel is part of an initial $2.7 billion, 16-mile line from Westlake Center to the airport, to open in 2009.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com