The Alaskan Way Viaduct Replacement Tunnel is a bored road tunnel that is under construction in the city of Seattle in the U.S. state of Washington. It is scheduled to open in late 2015. The 2-mile (3.2 km) tunnel will carry State Route 99 under Downtown Seattle from the SoDo neighborhood to South Lake Union in the north.
Since 2001, the proposed replacement of the Alaskan Way Viaduct has been the source of much political consternation demonstrating the Seattle process. Options for the structure, which carries 110,000 vehicles per day, included either replacing it with a cut-and-cover tunnel, replacing it with another elevated highway, or eliminating it while improving other surface streets and public transportation. The current plan emerged in 2009 when government officials agreed to a deep-bore tunnel.
Source: Washington State Department of Transportation
The $80 million tunnel boring machine (TBM) Bertha was created for this project by Hitachi Zosen Corporation near Osaka, Japan. The 326 ft (99 m), 6,700-short-ton (6,100 t) tunnel boring machine was disassembled into 40 pieces and shipped to Seattle where it was reassembled in the launch pit near the south end of the future tunnel. From there, the record-breaking 57.5-foot (17.5 m) diameter borer moves in 6.5 ft (2.0 m) increments toward the north end.
Washington State Department of Transportation (WSDOT) nicknamed the tunnel boring machine "Bertha" after Seattle's first female mayor, Bertha Knight Landes. This name was chosen from names submitted by kindergarten through 12th grade students for a naming competition.
The initial phase of demolition and removal of the viaduct began on October 21, 2011. Only a southern portion of the viaduct was removed at that time; the viaduct along the central waterfront will remain open for traffic until the tunnel is complete.
On July 30, 2013, "Bertha" began boring out the tunnel, starting digging through the first of ten phases of the tunnel. The boring is expected to take 14 months. Boring works was delayed in 6th December when Bertha was stopped by struck by a mysterious object - steel pipe, installed as a well casing for an exploratory well drilled as part of the planning phases of the project.
The long pipe was an 8-inch diameter, 115-foot-long “well casing,” used to measure groundwater during studies in 2002 on the Alaskan Way Viaduct replacement project.
The department installed the pipe in 2002, shortly after a 6.8 magnitude earthquake in 2001, as a well casing to better monitor groundwater flows in the area. In a news release Friday, officials said the location of the pipe was included in materials provided to the contractor on the $1.4 billion tunnel project, Seattle Tunnel Partners. But the contractor reported that it didn't know the pipe was there.
The boring machine, billed as the largest in the world, can grind through rock, dirt and timber — but not thick metal.
Crews worked extensively over the holidays to try to identify the mystery blockage. They examined Bertha's excavation chamber, just behind its cutter-head; drilled wells to relieve water pressure around the machine; and earlier this week drilled 17 exploratory holes near the front of the machine to find the obstruction.
Finally, on Thursday, enough soil had been removed and the water pressure had dropped enough that crews could inspect the top 15 feet of the excavation chamber, the department said. That's when they saw the pipe protruding through one of the many openings in the cutter-head.
Experts believe some of the obstructions found in the exploratory holes are pieces of the pipe, but the DOT also says it's not sure the pipe is the only thing blocking Bertha. There could be other obstructions that aren't yet visible, or changing soil conditions might have caused excessive wear of the machine's cutting tools.
Seattle Tunnel Partners is considering several options for removing the pipe and identifying other potential obstructions. It's too soon to know how much the delay will cost, or how it will affect the long-term schedule of the 1.7-mile tunnel project, officials said.