Highway 99 tunnel-project updates: More viaduct settlement; repair update

(Added 9:27 pm: New image from camera over ‘access pit’ built to retrieve part of tunnel machine for repairs)

WSDOT has published another update on the Highway 99 tunnel project. You can read the entire update here; we’re excerpting two parts of it, starting with word of more Alaskan Way Viaduct settlement:

Survey crews have confirmed that the Alaskan Way Viaduct between South Main Street and Railroad Way South has settled up to ¼ inch in the past month. Our bridge experts are confident this minor, uniform settlement does not pose any new safety risks to the public. The viaduct remains vulnerable to earthquakes, but it is still safe for everyday use. If we had any reason to believe the structure was unsafe, we would not hesitate to close it.

These latest measurements only apply to this section of the viaduct. We have not measured any similar trends elsewhere on the viaduct, in nearby buildings or the ground surface. We expect the viaduct will continue to experience minor settlement until it is removed. Viaduct settlement has made headlines in recent months, but it’s important to remember that this isn’t a new issue. The viaduct has been settling for years, as you can see in our inspection log. Hundreds of monitors have been installed in the ground and on the viaduct to help us monitor settlement, and we will continue to keep a close eye on the structure’s condition as construction continues.

WSDOT also says the operation to move the tunnel machine into the repair pit could start within days:

STP is making final preparations for Bertha’s move into the 120-foot-deep pit that will allow crews to hoist the machine to the surface for repairs. STP has spent the past few days injecting grout into the ground and installing new drains alongside the machine. These measures will help crews control groundwater as Bertha slowly moves forward through the wall of the pit, an effort that STP plans to begin sometime in the next week.

STP may begin preparing the shaft wall for Bertha’s breakthrough even before the machine starts mining. Starting as soon as this weekend, crews plan to use heavy equipment to chip away the portion of the pit’s southern wall where Bertha will eventually emerge. This process is similar to scoring a surface before you cut into it, and will allow the concrete to break away in a controlled manner when Bertha finally moves into the pit.

Approximately 20 feet of unreinforced concrete stands between Bertha and daylight. The duration of Bertha’s dig will depend on her ability to mine through and digest concrete while operating with a damaged seal system. STP expects Bertha to overheat, as she has during their most recent attempts at mining. They will likely mine until the machine becomes too hot, then take a break. To speed up the move, crews may choose to continue chipping their way toward Bertha from within the pit during times when the machine is cooling off.

Moving Bertha into the pit is just the beginning of the repair process, which this narrated video (links to YouTube) explains in detail. The machine must be partially disassembled before it can be lifted to the surface, a process that will likely take significant time and effort. We will provide regular updates on Bertha’s progress as soon as she starts moving.

13 Replies to "Highway 99 tunnel-project updates: More viaduct settlement; repair update"

  • Nw mama February 13, 2015 (11:31 pm)

    I hope they are planning to use this work they’ve done to dig this massive hole to integrate as a rescue method for when the “big one” hits Seattle / magnitude 7-9 earthquake. Thousands stuck in the tunnel – need a method for mass evacuation…

  • MAS February 14, 2015 (8:20 am)

    Is it just me, or does that last corner northbound as you climb up to the top deck of the viaduct feel like it’s now canted outward from the curve centroid?

    As drivers, we’re used to mild banking that tends to equalize the sideways forces of a curve like that with downward force. In the case of that corner, it feels like it’s either been flattened, or is actually canting outward at typical speeds. Given the subsidence totals for that area of the roadway, and the possibility that both sides of the roadway are not sinking at the same rate (I’ve never seen specific observations on that, but the topological view I have seen does support this), a few inches of difference on one side of the roadway could provide this unsettling sensation.

    Anyone else feel that as you crest that climb and the slow curve to the north, just as you hit the top deck?

  • MAS February 14, 2015 (8:23 am)

    Oh, and “unsettling sensation” was an entirely accidental pun. If I’d thought of it beforehand, I’d claim it – but I didn’t.

  • John February 14, 2015 (8:35 am)

    Nw mama,
    The tunnel is designed for such events.
    Now, lets talk about the viaduct and hundreds of bridges and freeway overpasses that are not designed or upgraded for the “big one.”

  • miws February 14, 2015 (8:54 am)

    +1 for honesty, MAS. ;-)



  • Rick February 14, 2015 (9:08 am)

    Good comment mama. Might take ’em a month or so to get to ya. But at least you didn’t fall off that nasty old viaduct. If only the tunnel had an emergency vehicle lane. Oh,wait..

  • Ethan February 14, 2015 (10:12 am)

    I’m curious: Where do you get these images of the pit from? Is there a webcam URL or something?

  • dbf February 14, 2015 (11:18 am)

    From the images I”ve seen, only stairs between upper and lower portion of tunnel. Nothing to the surface except entrance and exit. Might have to practice jogging on top of parked cars.

  • wetone February 14, 2015 (1:27 pm)

    1/4 ” in a month is a lot of movement and if it continues at that rate I see the viaduct going way past the safety point of staying open quickly. The “original” amount of sinking allowed was less than 3″ per STP and WSDOT, but then they just re-adjusting things to fit their play book. Sure can’t be good for area infrastructure and buildings in area. Will be very costly to tax payers for rebuild of area. If someone’s home was sinking a 1/4″ per month in this city they would condemn it for being unsafe pretty quickly ;)

  • max February 14, 2015 (7:54 pm)

    Oh jeez. No one is saying that it is sinking “1/4″ per month” … and yeah … I sure as hell would rather be in this tunnel that up on a viaduct when that big earthquake does hit.

    Enough with the hysteria.

  • dbf February 14, 2015 (10:25 pm)

    @max, YOU choose your hell, I’ll choose mine. Would rather be above surface in catastrophe, than couple hundred feet below. But hey, we need names for the memorial…

  • coleman February 15, 2015 (8:19 am)

    I cant wait till they turn into a mussem. Maybe they can add a Starbucks and an Ivars too make it feel more like Seattle. Sounds fun.

  • Bertha DeBlues (@BerthaDeBlues) February 15, 2015 (1:46 pm)

    I’m just doing isometric exercises, getting ready for next week!

Sorry, comment time is over.