Rolling with it: How the SR 99 tunnel is designed to withstand earthquakes

Posted on Jan 7 2019 2:49 PM

You’ve heard it before but it bears repeating: the primary purpose of the Alaskan Way Viaduct Replacement Program is safety. The viaduct is an aging and seismically vulnerable structure, and retiring it from our highway network will make us all safer.

It’s not just addition by subtraction. We are replacing the viaduct with a modern tunnel, built with sophisticated systems that work together to keep vehicles moving and drivers safe. Learn more about how the tunnel’s systems work on our new Tunnel Safety page.

The viaduct’s vulnerability to earthquakes was the biggest motivation for its replacement, and here is another way the new tunnel shines. As it happens, tunnels are a rather safe place to be in an earthquake. If you find this counterintuitive, we’ve produced a video in conjunction with seismic and structural experts to help us explain:

Engineers in our earthquake-prone region designed the tunnel to withstand a strong earthquake – roughly one that happens every 2,500 years. This would include a magnitude 9 earthquake off the coast of Washington, where the Juan de Fuca plate of the earth’s crust forms the Cascadia Subduction Zone. The tunnel design also takes into account earthquakes that might occur along the Seattle Fault.

There are many design elements that work together to create a safe tunnel:

  • Structure: The SR 99 tunnel is built with more than 1,400 strong concrete and steel rings, each 6.5 feet wide. These rings are bolted together to form the tunnel, and while very sturdy, they have some flexibility to account for ground movement. This means they can move and return to their round shape. The roads inside are also designed to be flexible, allowing them to move with earthquake waves and remain functional.
  • Shape: The round tunnel can withstand lots of pressure from the outside – much like a submarine underwater keeps its round shape and withstands oceanic pressure.
  • Location: Tunnels that are deep underground experience less movement from the energy waves of earthquakes. Those energy waves increase in size as they approach the surface, so a tunnel will not experience the same degree of movement as an above-ground structure like a viaduct.

The inherent advantages of a tunnel, combined with state-of-the-art seismic engineering, means the new SR 99 tunnel is designed to stand up to future earthquakes.

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