June 20 project update: Tunnel crews nearing planned maintenance stop

Posted on Jun 20 2016 5:47 PM
After another week of good tunneling progress, Seattle Tunnel Partners is preparing Bertha, the SR 99 tunneling machine, for its next planned maintenance stop.
 
STP crews have now mined a total of 2,971 feet and installed 449 concrete tunnel rings. The top of the tunneling machine is located approximately 120 feet below Post Avenue, between Madison and Spring streets.
 
Crews have mined nearly 1,500 feet since Bertha’s previous planned maintenance stop near Yesler Way. STP plans to stop soon because the soils in the area are dense and stable, providing more suitable conditions for maintenance. The stop could begin this week and is expected to last several weeks, depending on the extent of maintenance needs. STP’s previous maintenance stop lasted six weeks.
 
Over the course of the first week, STP will perform a thorough inspection of various systems throughout the machine. This inspection will help STP verify how much maintenance work is needed. Crews will then prepare to inspect the cutting tools that cut the ground in front of the machine, and replace these tools as needed. These tools wear down over time and must be replaced multiple times during the course of the tunnel drive. 
 
Hyperbaric work
 
Before crews can inspect and replace cutting tools, they must first stabilize the ground in front of the tunneling machine. They do this by using compressed air and a type of clay, known as bentonite, to create an air bubble. This air bubble allows them to safely work in the area behind the cutterhead, which would otherwise be filled by soil and water. 
 
Before crews can safely work in this environment, they must first adjust their bodies to air pressure that is greater than the atmosphere we live and breathe in every day. It's the same process scuba divers go through during the course of an underwater dive, but STP’s workers won't need diving gear. Instead they will spend approximately an hour inside specialized pressure chambers within the machine that help their bodies adapt to these ‘hyperbaric’ conditions. The graphics below illustrate the process.
 
(Click the image above for a larger view)
 
When all necessary maintenance is complete, crews will resume tunneling toward First Avenue. Our next regular progress update is slated for Thursday, but we’ll post something sooner if crews reach the maintenance stop before then.
 

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