Light-rail breakthrough: 375-ton drill tunnels out of Beacon Hill

The teeth of a 300-foot-long drill busted through the east slope of Beacon Hill on Tuesday, marking the halfway point in Sound Transit's project to build twin light-rail tunnels.

The machine, which entered the hill in January 2006, emerged within an inch of its target after burrowing more than 4,300 feet. As the drill, 21 feet in diameter, crept forward, robotic arms behind the machine set hundreds of pre-cast pieces of the southbound tunnel wall in place to prevent cave-ins.

"With the soil conditions it was nice, and the machines ran nice. We had a good, competent crew," said the drill operator, Pat Gould.

Workers behind the drill head began rinsing away the muck so the machine can be dismantled. Next month, the parts will be trucked back around the hill to the Sodo area, to be reassembled before the machine is launched again in July to dig the northbound tunnel.

Digging must commence from Sodo because thousands of dump trucks take away the soil coming out the back end of the drill — and industrial Sodo is a better place for truck traffic than the residential east side of Beacon Hill. The second train tube will be done by the end of this year, transit spokesman Bruce Gray said.

The $295 million segment, being built by Japan-based Obayashi, includes a deep underground station with an elevator to the surface near Beacon Avenue South, plus an elevated station near Franklin High School. The pace of construction has been slowed by unstable soil in the vertical shafts, and a February supply-train crash that killed a worker.

Better familiarity with the soil and fewer scheduling conflicts with excavations of the shafts are expected to aid the pace of the second tunnel's construction.

Transit officials say the overall 16-mile, $2.7 billion route from Westlake Center to Seattle-Tacoma International Airport should open by late 2009, as scheduled.

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