Massive 'launch pit' dug for Seattle tunnel boring machine


by KING 5 News

Posted on August 23, 2012 at 8:38 AM

SEATTLE - It's an operation that is so massive, so complex, it's hard to get your brain around it.

At the south end of the Alaska Way viaduct crews are preparing the site where they will start digging the tunnel next summer.

"We have the world's largest tunnel boring machine being built in Japan as we speak," said Matt Preedy, the Alaskan Way Viaduct deputy program administrator for WSDOT. "In order for it to start its journey under downtown building the 99 tunnel, it has to be launched out of the launch pit."

The pit will be 400 feet long, and 80 feet deep. On Wednesday, they were installing massive pilings, that will be part of the reinforced walls. A crew of about 300 are working around the clock to make sure it finishes in time.

"This hole is big enough with just enough room to work around it, to drop the tunnel boring machine into," said Preedy.

The tunnel boring machine, or TBM, will be as big as the largest Washington state ferry.

In March 2013, the manufacturer will deliver it from Japan in more than 40 pieces to terminal 46 along the waterfront. Those pieces will be dropped into the launch pit, where the TBM will be assembled and put to work.

Boring starts next summer. It will take just over a year for it to make its way under downtown, and will come out at 6th and Thomas where construction is already underway.

With all the earth being moved around, WSDOT has an archaeological team on site sifting through all the dirt. They have found some interesting artifacts from turn-of-the-century Seattle.

"We're finding brick bottles ceramics bones from people's meals," says Kevin Bartoy, WSDOT archaeologist.

Some of these artifacts and the model of the boring machine is on display at a storefront called "Milepost 31" at 211 1st Avenue in Seattle's Pioneer Square.

The tunnel is suppose to open for traffic in December of 2015, just 3 and half years from now.