Seattle tunnel, among world’s biggest, takes step toward completion

Slideshow preview imageSEATTLE (Reuters) – Bertha, one of the world’s largest tunnel boring machines, emerged north of downtown Seattle on Tuesday shrouded in clouds of dust and chunks of falling concrete, completing the most difficult phase of a plan to build a highway under the heart of the city.

The breakthrough of the cylindrical drilling machine marks a major step in one of the most ambitious American municipal projects in recent years. Once complete, 2 miles of Highway 99, an elevated roadway along a densely populated waterfront, will be rerouted to run beneath the city of 650,000 people.

Seattle’s underground highway

A sinkhole, a two-year delay and a $480 million claim by contractors have challenged the $3.1 billion project since it began in June 2013.

The underground highway, which had initially been slated to cost $2 billion and be completed by the end of 2015, has been widely compared with Boston’s 16-year “Big Dig” tunneling project, which suffered through cost overruns, design flaws, worker fatalities and other problems.

“This is a historic moment in our state’s transportation history,” Washington state Governor Jay Inslee said in a statement after the drill’s breakthrough.

After emerging into a large open-air pit a few blocks from Seattle’s Queen Anne neighborhood, Bertha will be cut into pieces and hauled away over several months.

The 57-foot wide borer made by Japan’s Hitachi Zosen Corp cost $80 million and was the largest in the world when tunneling started in 2013.

As engineers make plans to break down the 6,700-ton machine, work to fill the tunnel with a double-decker roadway has already begun. State planners hope to have the first car travel through the tunnel in early 2019.

While the project’s $3.1 billion price tag is comparatively small – the Big Dig cost an estimated $22 billion – progress was complicated by plans to dig beneath some of the most tightly packed neighborhoods in downtown Seattle.

Bertha overheated and stalled partway through the project in December 2013, putting completion into doubt. Tunneling was delayed two years as engineers dug a 120-foot access pit to make repairs.

(Editing by Lisa Von Ahn and Bill Rigby)