While stressing public safety as a top priority, Washington State Dept. of Transportation officials remain locked in an investigation to determine the “amount and extent of settlement that recently occurred on and near the Alaskan Way Viaduct.”
That viaduct has already been deemed at risk for a seismic event and a massive undertaking to replace the elevated roadway with a bored tunnel is underway. Of course, we all know that the tunnel project has now sat largely idle for roughly a full year, as Bertha, North America’s largest tunnel-boring machine, overheated and stopped tunneling early in the process in December 2013.
As crews spent months determining the cause of the machine malfunction—still without definite word on why it occurred—they determined Bertha needed new seals and a new $5 million main bearing. To get that bearing into Bertha, crews have been digging an access pit in front of Bertha, where she sits, directly adjacent to a portion of the still-open Alaskan Way Viaduct.
As part of the larger tunneling project, a settlement monitoring system was installed at points along the route and recently that system detected approximately one inch of ground settlement near the pit that contractor Seattle Tunnel Partners is building to access and repair the tunneling machine. Crews have seen the same amount of settlement on the Alaskan Way Viaduct.
Some settlement was expected during tunnel construction and during the tunneling machine repair work. This settlement appears to have come in the last month.
Additional survey work the morning of Sunday, Dec. 7, came back inconclusive and crews are still working to figure out the exact amount of settlement occurring near the access pit.
A small amount of differential settlement—when the ground settles unevenly over an area—was found, a more dangerous discovery than uniform settling. While the additional survey work did not find that the differential settlement has caused any new damage to the viaduct or any building or utilities in the surrounding area, on-the-ground surveys will continue this week by historic architects and structural engineers.
Seattle Tunnel Partners is working with its geostructural designer to stop the dewatering in a deliberate manner in order to ensure the structural integrity of the access pit and surrounding structures.
And, of course, continued analysis of the area and monitoring will continue throughout the access pit construction.
While Seattle Tunnel Partners looks to fix Bertha and have the tunnel boring again this spring, about 15 months from the original shut down, new challenges won’t help the process.
Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He also writes for Popular Mechanics, Sports Illustrated and more. You can follow him on Twitter at @tdnewcomb or visit his website here.