Job ahead for 'Bertha' the tunnel boring machine

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by JAKE WHITTENBERG / KING 5 News

Bio | Email | Follow: @jakewhittenberg

KING5.com

Posted on July 19, 2013 at 7:39 AM

With digging set to begin on the new State Route 99 tunnel under downtown Seattle, the public is invited to dedicate the machine in charge of the digging.

The Washington State Department of Transportation invites you to attend a 'Pre-Bore Dedication' at the dig site on Saturday July 20th, at 11:30 a.m. Crews will celebrate the start of the dig and the public can have a chance to sign your name on a tunnel liner segment.

For more information on the project and ceremony visit the WSDOT website.

You can also follow Bertha the boring machine on Twitter at @BerthadigsSR99.

So how does the Bertha actually dig the tunnel? Here are a few answers to frequently asked questions.

When does the digging start?

There is no exact start time for the dig, but the Washington Department of Transportation expects Bertha to fire up by the end of July.

How big is the tunnel boring machine?

Bertha is 57.5 feet tall and weighs 7,000 tons. The cutter head is designed to rip through giant boulders and engineers expect Bertha to encounter 8 different soil types on its journey under downtown Seattle. Its expected to pull 850,000 cubic yards of dirt from the ground.

The machine has its own electricity supply on site, and the equipment that follows the cutter head during the dig stretches several hundred feet. 20-30 crew members will be on the machine at all times as it digs 24 hours a day. Weekends are set aside for maintenance.
  
How long is the tunnel?

The tunnel is approximately 2 miles long. As Bertha digs, crews will install 1250 rings or segments behind it that make up the tunnel. The deepest point is about 200 feet underground.

How long does the digging last?

About 14 months.

Where does the tunnel end?

The tunnel will surface at about 6th and Thomas near KING 5!

Are buildings downtown at risk?

WSDOT says engineers have installed 1200 pieces of equipment in downtown Seattle to monitor any movement at street level. Lasers have been placed on several buildings and point at reflectors mounted on area buildings. Engineers will be monitoring those reflectors, and if the lasers move even a fraction of an inch, they will know to stop digging and assess whether vibrations have caused any shifting on the surface.

Is the tunnel earthquake proof?

The tunnel is designed to withstand a 2500 year earthquake. That equates to about 9.0 earthquake measured on the Richter Scale.

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