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