June 28 project update: New videos show crews at work inside the SR 99 tunneling machine

Posted on Jun 28 2016 5:47 PM
Over the next several weeks, specialized crews will complete routine cutterhead maintenance at the front end of Bertha, the SR 99 tunneling machine. To safely do this, crews must stabilize the ground in front of the machine. They do this by injecting a type of clay, known as bentonite, into the front end of the machine. This creates a seal that prevents water and soil from entering – and air from escaping – their work space. 
Next, crews over-pressurize the space by introducing compressed air, which pushes against the bentonite to counteract the ground and water pressure at the front end of the machine. This newly created "hyperbaric" work space has pressure levels that are higher than regular atmospheric pressure, similar to conditions found in an underwater dive. The graphic below illustrates the process.
New videos
Preparations for this work are underway now that Bertha has stopped for planned maintenance beneath Spring Street. Crews expect to make their first “hyperbaric intervention” next week. Seven, five-member crews will work around the clock to perform maintenance in the space behind the cutterhead. 
Each crew member must spend several minutes in a special chamber to prepare for the greater pressures they’ll experience while working in hyperbaric conditions. The amount of time that crews can safely work in these conditions varies depending on the pressure of the hyperbaric work space. In previous interventions on this project, crews were able to spend up to an hour in these conditions before decompressing and returning to the surface. 
The video below shows the chambers crews use to adjust to hyperbaric conditions and enter the space behind the cutterhead.
The video below was provided by Ballard Marine Construction, the firm responsible for completing this work on the tunnel project. It shows crews at work behind Bertha’s cutterhead during a planned maintenance stop earlier this spring.
The duration of the maintenance stop will depend on the extent of the work that’s needed. STP’s previous maintenance stop near Yesler Way lasted approximately six weeks. We’ll continue to provide updates as their work progresses.

Recent tunneling updates