Bertha reached her next planned stop smoothly and safely this past Saturday. According to Seattle Tunnel Partners, the machine functioned within required operating parameters, tunneling 300 feet in three weeks. STP has now mined a total of 1,560 feet.
Now, Bertha is scheduled for planned maintenance to inspect the cutterhead, main strive and screw conveyor, replace cutting tools, and extend the conveyor belt and high-voltage cable. Because 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, they built a protected underground area they've named Safe Haven 3.
Safe Haven 3 is a solid concrete block built underground. Since Bertha is tunneling 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. Pulling over to the specially designed concrete area gives STP crews a more stable environment to work in hyperbaric conditions.
Hyperbaric conditions are those in which the air pressure is greater than the normal atmospheric we live in every day. Before Bertha's crews can work in these conditions, they have to adjust their bodies to the pressure, similar to what 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.
Performing hyperbaric work while Bertha is embedded in concrete, rather than soil, greatly reduces the need to pump in compressed air. This is because concrete is already stable, so crews can safely work in hyperbaric conditions for longer periods of time.
Maintenance could take about a month, and no restart date has been set, WSDOT said. Seattle Tunnel Partners (STP) intends to drive Bertha out of the box in April, and drill below the foundations of the viaduct, where Yesler Way meets the waterfront.