Tuesday, September 15, 2009

A Boring Machine

WSDOT takes another step in designing and building the SR 99 bored tunnel project.

By guest blogger Eric Balliet

As the proposed replacement for the Alaskan Way Viaduct, the SR 99 bored tunnel promises to be a challenging undertaking. It would be one of the largest diameter bored tunnels in the world, with an outside diameter of approximately 54 feet. At almost two miles long, it would also be one of the longest highway tunnels in the United States.

How would such a large structure be built?
A bored tunnel is constructed using – you guessed it – a tunnel boring machine. Advances in technology and decades of tunneling experience have produced a machine that essentially chews through the ground and simultaneously constructs the outer shell of a tunnel in its wake. To help understand this process, we have posted an animation of how a tunnel boring machine works. This is just an example, however; the details of the machine used for our project will be determined by the contractor.

A bored tunnel machine

Those of you without YouTube access can view a Windows Media version.

Not so boring anymore
In the years since the viaduct replacement program began, tunnel boring machines have been developing at a rapid rate, with a major increase in diameter, better ground control and improved reliability. They can now safely excavate under almost any type of soil, rock or groundwater conditions.

In anticipation of building the tunnel, crews working for WSDOT have been gathering soil samples along the tunnel’s route. When testing is finished in October, we will have samples every 100 to 400 feet, to depths of 100 to 300 feet below the surface. This information will help in the design of the boring machine, so it can handle the soil conditions we expect to encounter during construction.

Visit the Alaskan Way Viaduct program Web site at http://www.alaskanwayviaduct.org/to learn more about the proposed SR 99 bored tunnel and other improvements that are part of the viaduct’s replacement.


Dan Cronin said...

Looks awesome, but I think a question needs to be addressed that everyone is going to ask and that is: What about traffic/congestion on the waterfront?

duaneu said...

For comparison, what is the diameter of the I-90 Mt. Baker tunnel that carries the westbound mainline and express lanes?

Eldon said...

Here is the answer to the question by duaneu about the diameter of the I-90 Mt. Baker tunnel:

From Wikipedia: "At 63 feet (19 meters) in diameter, it is the world's largest diameter soft earth tunnel, having been bored through clay."

I happen to know that a large tunnel boring machine was not used in this pretty unique I-90 tunnel project. Instead, smaller tunnels were built one at a time all around the outside of the tunnel. Each small tunnel was filled with concrete, and then the next small one was built next to it. When all the small tunnels were finished, the interior of the tunnel was removed using regular construction equipment (like front-end loaders). The small tunnels around the sides of the big tunnel provided the structural support.

Anonymous said...

I see precast concrete sections traveling up I-5 from somewhere near Gig Harbor that greatly resemble the size needed to build such a tunnel. Are these for this project or are there other tunnels being built currently? Maybe the new sound transit light rail tunnel?

Anonymous said...

Those pieces you see could be going to the discharge tunnel being built for the Brightwater waste treatment facility. It runs from near the 522/SR9 interchange Woodinville and ends offshore near Shoreline. TBMs are being used to make that tunnel as well though its much smaller in diameter then this project will be.

Anonymous said...

This tunnel does nothing for Seattle traffic other than to make it worse than it already is. This section currently provides excellent access to the Qwest Field, Safeco Field, International district, SODO and industrial area it abuts, the south end of downtown, Pioneer square, pike place market and the north end of downtown, Belltown, Lower Queen Anne, Seattle Center, Magnolia, and Ballard for those who do business, live in, and are visiting the city.

John said...

I applaud WSDOT to finally address this growing concern. Many of my friends, family and coworkers travel over this daily and I think of would could hapen to them and myself in the event of a significant seismic event every time I cross the Viaduct. Here are some inetesting facts regarding usage :

Alaskan Way Viaduct

Opened: 1954

Length: 2.2 miles

Capacity: Carries nearly a third of Seattle's north-south traffic. During peak traffic, more than 9,000 vehicles per hour cross the downtown segment of the viaduct. State engineers estimate that as many as 820 people could be on the viaduct at any given time during peak rush hour.

Other uses: Also carries a major portion of the city's utilities, including electric, telephone, gas, water and sewer.


gillian said...

Looking forward to see and use the tunnel. Hope they can finish it asap. It will be a great solution for traffic because for now, it's the cause of traffic. Used Forklifts

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