A peek inside Beacon Hill transit tunnel

When future light-rail trains stop at Beacon Hill, the commuters there won't find tracks on the surface. Instead, they'll ride an elevator to meet the trains deep underground.

A Sound Transit contractor, Obayashi Corp., recently finished excavating the new station site, 165 feet beneath the hilltop, as part of a $2.7 billion, 16-mile route from downtown to Seattle-Tacoma International Airport, to open in late 2009.

"It is probably the most challenging construction project along the whole Link light rail line," said transit-board member Larry Phillips, a Metropolitan King County Council member from Seattle, said during a tour Sunday morning. "The methods used to mine out this station have never before been used at this depth. We are standing here in an engineering and construction marvel that will be known throughout the world."

Beacon Hill's watery soils, deposited by ancient glaciers, are prone to caving in.

So workers had to dig the giant, arch-shaped station holes just 4 or 5 feet at a time, install steel braces against the dirt (and sometimes, add a wire mesh lining), then spray concrete to shore up the curved wall, said Sound Transit resident engineer Rick Capka. Finally, a stronger concrete wall and waterproof lining will be added.

Portland's light-rail system has a deeper station at Washington Park, west of downtown, but it was carved from rock, an easier job.

Next month, Sound Transit will begin to bore the northbound tunnel through the hill, after finishing the twin southbound tube this spring. The one-mile southbound segment, which includes an aerial station near Franklin High School, cost $297 million, and one man has died on the job.

Phillips said the tunnel proves Sound Transit has the know-how to deliver 50 more miles of light rail, if voters approve it this fall. But the mining is tricky enough that officials canceled a deep train platform they once promised at busy First Hill.

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