Sound Transit now considering a tunnel for light-rail near Northgate

Sound Transit is working on a design change that would move about a half-mile of the future Northgate light-rail extension into a bored tunnel.

A few years ago, the best route along Interstate 5 was thought to be one that carves into hillsides, or in a trench slightly below the surface. But there are conflicts with three freeway overpasses — the north-south bridge at Fifth Avenue Northeast; and east-west bridges for Northeast 80th and 85th streets.

Engineers now believe the agency would be better off avoiding the overpasses entirely, by tunneling below them, said Ron Endlich, a senior manager on the project.

Bored tunnels tend to be extremely expensive, but Endlich said this stretch would cost $5 million to $10 million less than the 2008 plan, when voters approved $18 billion for new lines to Lynnwood, Overlake and Federal Way. Keeping that route would have required short, complicated cut-and-cover type tunnels at the three overpasses, Endlich said. A cut-and-cover tunnel is a concrete trench, with a lid.

Sound Transit built twin bored tunnels through Beacon Hill, and is now digging them through Capitol Hill to Husky Stadium. Another bored tunnel to Roosevelt was to emerge near Northeast 75th Street, in the old plan.

But with the new plan, the bored tunnel would instead emerge between Northeast 85th Street and Northeast 92nd Street.

Taking that route could reduce construction noise in the nearby west Maple Leaf neighborhood, Endlich said. Also, trucks could pick up the tunnel dirt next to I-5, then enter the northbound freeway using a temporary lane.

Transit-board members will decide this fall whether to approve a bored tunnel. The cost estimate on the ballot was about $1.4 billion for the 4.3 miles from Husky Stadium to Northgate, to open by 2020.

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