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FILE PHOTO -- Workers make preparations to lift the cutting head from Bertha, the world's largest tunnel-boring machine, and lift it out for repairs in Seattle, Washington March 9, 2015. REUTERS/Jason Redmond/File Photo

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By Tom James

SEATTLE (Reuters) - Bertha, one of the world’s largest boring machines, will cut through a thick concrete wall at midday on Tuesday if all goes according to plan, completing the most difficult phase of building a highway under the heart of downtown Seattle.

Reaching open air through 5 feet (1.5 m) of concrete is a major step in one of the most ambitious American municipal projects in recent years. Once complete, 2 miles (3 km) 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 $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" tunnelling project, which suffered through cost overruns, design flaws, worker fatalities and other problems.

“It’s like shoving a five-story building through the ground under downtown Seattle,” project director Chris Dixon said at a Monday news conference.

Seattle’s tunnel will be among the world's largest, about seven times the size of one in a typical subway, Dixon said.

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 57-foot (17 m) wide borer made by Japan's Hitachi Zosen Corp cost $80 million and was the largest in the world when tunnelling started in 2013.

As engineers make plans to break down the 6,700-ton (6,100-metric 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's is estimated at $22 billion - 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-foot (37 m) access pit to make repairs.

(Editing by Lisa Von Ahn)

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