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.