Progress made in search for tunnel blockage

Posted on Jan 3 2014 1:47 PM

It was a busy holiday season for crews working to get Bertha, the SR 99 tunneling machine, digging again. While we still can’t say for sure what halted the machine on Dec. 6, we’re much closer to finding an answer than we were when all of this started.

Here’s a recap of what crews have been up to since the blockage occurred:

  • On Dec. 7, our contractor, Seattle Tunnel Partners (STP), began prepping the machine and surrounding environment so crews could inspect the cutterhead and excavation chamber safely.
  • On Dec. 17, a brief visual inspection of a small section of the excavation chamber and cutterhead found dirt, sand and cobbles – nothing unusual.
  • Over the holidays, STP crews were on-site to drill dewatering wells, maintain the machine and switch out cutting tools on the machine’s face. 
  • Earlier this week, STP drilled 17 small-diameter, exploratory holes near the front of the machine to see if they could identify an obstruction. They encountered obstructions in four of the holes.
  • On Jan. 2, the water pressure was low enough and enough soil was removed from the excavation chamber to inspect the top 15 feet of the chamber.
  • This inspection showed an 8-inch-diameter steel pipe protruding through one of the many openings in the cutterhead. We believe the steel pipe is a well casing installed by WSDOT in 2002 after the 2001 Nisqually earthquake to better understand how groundwater flows through this area. The location of this pipe was included in reference materials in the contract.
  • We also believe at least some of the obstructions found by the exploratory holes are pieces of the 2002 steel pipe, which could be a contributing factor in the delay of boring.
  • Other potential factors include changing soil conditions that may have caused excessive wear of cutting tools, potential objects in front of the cutterhead or objects in the lower portion of the excavation chamber that still aren’t visible.

 

Next steps

STP is considering several options to remove the steel pipe and identify other potential obstructions.

While Bertha is stopped, other work is taking place. This includes:

  • Extensive construction on the north and south ends of the tunnel.
  • Preparatory work to dismantle the push frame and temporary tunnel rings in the launch pit.
  • Preparatory work to reconfigure the launch pitand build the roadway within the tunnel.

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It’s too early to speculate about the cost or long-term schedule implications of this issue. We’ll continue to work with STP to determine ways to make up time lost during the blockage. Our focus is now on addressing this issue safely and in a timely manner so we can resume tunneling as soon as possible.
 

Catch up on Bertha's story

Dec. 20, 2013 update -- We won't know more about Bertha's blockage until 2014

Dec. 13, 2013 update -- Tunnel crews lowering groundwater to get a closer look at what's blocking Bertha

Dec. 5, 2013 update -- Seattle Tunnel Partners: Almost the proud owners of the world's largest tunneling machine

Nov. 25, 2013 update -- Bertha has much to be thankful for as she nears South Jackson Street

Oct. 30, 2013 update -- Bertha begins her week of scheduled maintenance