Ordinary maintenance, extraordinary conditions

With Bertha, the SR 99 tunneling machine, having safely reached her planned maintenance stop, Seattle Tunnel Partners is preparing to complete up to one month of planned maintenance. While the maintenance is routine, some of the methods crews will use to complete it are anything but. That’s because they’ll be performing some work in hyperbaric conditions as they get Bertha ready to tunnel beneath the Alaskan Way Viaduct and downtown.
Hyperbaric conditions are those in which the air pressure is greater than the atmosphere we live and breathe in every day. Before crews can work in these conditions, they must first adjust their bodies to the pressure. It's the same process scuba divers go through during the course of an underwater dive. STP’s workers won't need diving gear. Instead they will spend several minutes inside specialized pressure chambers within the machine that help their bodies adapt to the conditions they’ll be working in. 
Soil vs. concrete
To perform work when the tunneling machines is below the water table, crews must pump pressurized air into the excavation chamber to stabilize the ground in front of the machine. Doing this creates an air bubble that allows crews to safely work in areas that would otherwise be filled by soil and water. The depth of the water table and the soil conditions surrounding the machine help crews determine the amount of pressure that is needed to stabilize environment in the chamber. The graphics below illustrate the process.
The advantage of performing hyperbaric work while the machine is embedded in concrete, rather than soil, is that concrete greatly reduces the need to pump in compressed air. This is because concrete, by its very nature, is already stable. As a result, crews can safely work in hyperbaric conditions for longer periods of time.
STP has a number of work activities that can only take place in hyperbaric conditions, including a number of tests and the replacement of cutting tools. Thanks to this protected underground area – which was built in 2013 – crews will be able to complete their work more quickly and get back to tunneling.