Jan. 14 update: Diving deeper into the tunneling stoppage
When you're sick, you go to the doctor. Based on your symptoms, the doctor checks for obvious clues about what might be ailing you. If his initial search doesn't yield the answer, he takes additional steps. He might run more thorough tests or send you to a specialist who has the tools and knowledge to give you an accurate diagnosis and develop a treatment plan.
When crews operating Bertha, the SR 99 tunneling machine, experienced increasing resistance at the front of the machine, they stopped tunneling. They started looking for obvious things that could have slowed the machine's progress. As the weeks went on, the search intensified. They found clues, including part of an 8-inch-diameter steel pipe inside the machine's excavation chamber.
But obvious things, it turns out, aren't necessarily at the root of this issue. It's time to see the specialist. Or, to use tunneling lingo, it's time to go hyperbaric.
Don't worry. If you don't know what "going hyperbaric" means, you're not alone. Cryptic as it appears, the word "hyperbaric" actually does a decent job of explaining itself -- even if you can't pinpoint its meaning, you get the sense it involves something complicated. And you're right.
Simply put, hyperbaric refers to pressure that is greater than the atmosphere we live and breathe in every day. In the tunneling world, it means using air pressure to stabilize the ground in front of a tunneling machine so crews can safely work in areas that would otherwise be filled with soil and water.
To understand why this is so complicated, it helps to consider Bertha's anatomy. At the very front of the machine lies the giant green cutterhead that grinds its way through the earth. All that dirt (and whatever else the machine encounters underground) gets pushed into the 5-foot-wide excavation chamber. There, it is mixed with water and soil conditioners that make it easier to extract through the machine's screw conveyor. Since Bertha stopped, the excavation chamber has been filled to the brim with water, muck and more. Crew members have peeked through a hatch into the chamber, but they can't safely go inside and work unless they create hyperbaric conditions.
Building a bubble for Bertha
Our contractor, Seattle Tunnel Partners, spent much of December lowering the water pressure in and around the machine. The next step is to create an air bubble within the excavation chamber so crews can inspect Bertha more closely. The bubble is created by injecting bentonite, a soil conditioner with the consistency of thick chocolate milk, into the chamber. As the chamber fills, the mixture flows outside the cutterhead to fill the 1-foot-wide space between the ground and the front of the cutterhead.
The goal is to create a protective bentonite membrane around the front of the machine. Once the membrane is in place, crews will fill the chamber with compressed air. The air will push the bentonite mixture toward the dirt in front of Bertha as we turn her giant screw below to pull out tunnel muck and create more space for the expanding air bubble. Eventually, the bubble will fill up the upper half of Bertha’s excavation chamber. When crews are confident the ground and air pressure are stable, they'll go inside to perform their work.
Before they enter the chamber, crews will have to adjust their bodies to the pressure created by the compressed air. It's the same process scuba divers go through during the course of an underwater dive. Our workers don't need diving gear. They simply crawl inside specialized pressure chambers built into the machine for a few minutes until their bodies are ready for the conditions inside the chamber.
Five workers will work in the excavation chamber at a time. Each crew will spend three hours inspecting Bertha and working to get her moving again. On the way out of the excavation chamber, they'll spend another hour in the pressure chamber readjusting to atmospheric pressure before the end of their shift. Each crew member can only enter the pressurized excavation chamber once every 24 hours.
The goal of going hyperbaric is to learn, definitively, what caused the issues that slowed the machine. Once Seattle Tunnel Partners knows what's wrong, they can come up with a plan to address the issue and resume tunneling.
We recognize that this process isn't moving as quickly as some would like. The reason for this is that, like going hyperbaric, none of this is easy. On the contrary, tunneling is incredibly complicated, Bertha is incredibly complicated, the ground conditions where she's located are incredibly complicated – you get the idea. This is challenging work. The safety of our workers and the success of our work are paramount. A thoughtful approach is just what the doctor ordered.