Underground road tunnel nears completion in Seattle

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

Once complete, two miles of Highway 99 will be rerouted to run beneath the city of 650,000 people (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, two miles of Highway 99, an elevated roadway along a densely populated waterfront, will be rerouted to run beneath the city of 650,000 people.

A sinkhole, a two-year delay and a $480m (£384m) claim by contractors have challenged the $3.1bn project since it began in June 2013.

The underground highway, the tunnel portion of which was budgeted at $2bn, was initially expected to be completed by the end of 2015 and has been widely compared to Boston’s 16-year “Big Dig” tunnelling 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. “There is still more work ahead but this moment is one worth celebrating.”

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

The 57ft wide borer made by Japan’s Hitachi Zosen Corp cost $80m and was the largest in the world when tunnelling started in 2013.

As engineers make plans to break down the 8,000 tonne 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 $3.1bn price tag is comparatively small – the Big Dig cost an estimated $22bn – progress was complicated by plans to dig beneath some of the most tightly packed neighbourhoods in downtown Seattle.

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

Reuters