SEATTLE — The world's largest tunneling machine is about to start burrowing under downtown Seattle.
The machine the state Transportation Department calls Bertha should begin boring by the end of the month and spend the next 14 months carving out a new route for Highway 99.
The $80 million machine is part of the $3.1 billion project to replace the 60-year-old Alaskan Way Viaduct, the double deck highway along the downtown waterfront.
Bertha was built in Japan and arrived by ship in April in 41 pieces. It has been reassembled in a pit near the CenturyLink Field and Safeco Field stadiums.
The Transportation Department and the contractor, Seattle Tunnel Partners, are holding a dedication ceremony on Saturday to give the public on last look before Bertha digs in.
Bertha is 326 feet long and weighs 7,000 tons. It will leave a tunnel nearly 58 feet in diameter.
Bertha is named for Berth Knight Landes, the first and so far only, woman mayor of Seattle. The city says she became the first woman mayor of a major U.S. city when she was elected in 1926.
Bertha is projected to complete the two-mile tunnel and punch through to the surface near south Lake Union by October 2014. Traffic is expected to start using the four-lane toll tunnel by the start of 2016.
The tunnel will allow the Transportation Department to finish demolishing the viaduct. Built in 1953 it has carried 110,000 vehicles a day. Officials said the aging structure had to be replaced because it could collapse in an earthquake. Its removal is part of a project to renovate the downtown Seattle waterfront, rebuilding the seawall, improving surface streets and adding new vistas.