Not all routine maintenance is considered equal. While Bertha, the SR 99 tunneling machine, will be at a planned maintenance stop for the next several weeks, there are some unique challenges to that work.
Bertha has tunneled more than 1,500 feet since its last pit stop, and the front end of the cutterhead is now about 120 feet beneath Spring Street near Post Avenue in downtown Seattle. While performing regular maintenance is a crucial part of keeping the machine in good working order, working on a machine that deep underground is no easy task.
|The steps needed for crews to do pressurized work on Bertha deep under downtown Seattle.|
First, crews need to stabilize the ground in front of the machine 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, they'll 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 creates a "hyperbaric" work space with pressure levels that are higher than regular atmospheric pressure. This is basically like conditions during an underwater dive.
The length of the maintenance stop depends on how much work is needed. Seattle Tunnel Partners' previous routine stop near Yesler Way lasted about six weeks. You can stay updated on the progress on the Alaskan Way Viaduct Replacement website.