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Construction crews are hard at work today at both ends of the SR 99 tunnel.
At the tunnel north portal, crews finished forming and placing rebar for the on- and off-ramps to Denny Way and also poured the roadway barrier. You can see construction progress on our SR 99: Tunnel Connections North camera. If you look closely, you can see the white tarps covering that freshly poured concrete.
At the tunnel south portal, crews are placing rebar on the northbound off-ramp that will take drivers to the waterfront or into downtown and other southern neighborhoods. Over the next few days they have several planned concrete pours as they build sections of the ramp’s road deck.
If you’re going to any of this weekend’s events, the City of Seattle pulled together some information to help you get around.
Look here for daily updates on traffic and construction progress throughout the #Realign99 closure. Track construction on our time-lapse cameras, and be sure to follow us on Twitter for updates, photos and videos.— more —
The new SR 99 tunnel’s south portal sits just west of Pioneer Square and the stadiums, on the southwest corner of downtown. One big piece of work to be accomplished during the #Realign99 closure is building a new street connecting the south portal’s on- and off-ramps to First Avenue South.
That one-block street will be called South Dearborn Street, as labeled in the rendering below:
Building South Dearborn Street requires removing part of the ramp structure (see it on Google Maps) that today carries northbound SR 99 from the construction detour up onto the viaduct. Crews will demolish the ramp this Saturday and Sunday, crunching the concrete in daytime hours while working on the future intersection’s traffic lights at night. The work will close Railroad Way South for the weekend; our Construction Notices and Detours page has more information.
Yesterday Rhine Demolition, the subcontractor doing the demolition, moved equipment into the work site:
While this is technically demolition work, removing this section of ramp is not the start of true viaduct demolition. This short span of ramp is the only part of the structure that will be taken down before the new tunnel opens. The full-fledged viaduct demolition is scheduled to begin in early-mid February.
With the ramp down, crews can pave the new South Dearborn Street beneath the (closed) southbound SR 99 ramp structure and build the new intersection with First Avenue South. Our new videos offer more detail on how South Dearborn Street works for northbound drivers getting off SR 99 right before the tunnel, or southbound drivers getting onto southbound SR 99 right after the tunnel.— more —
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.— more —