April 1 project update: Inspections and maintenance continue
Seattle Tunnel Partners has now completed more than 60 shifts of hyperbaric work inside the space behind the cutterhead of the SR 99 tunneling machine. Inspections and maintenance have been ongoing since the machine reached a planned maintenance stop last month. Crews have spent most of their time cleaning muck from the cutterhead openings and spokes, measuring and assessing cutting tools, and replacing them where necessary.
As this previous post explains, hyperbaric conditions are those in which the pressure is greater than the atmosphere we live and breathe in every day. Crews must undergo a rigorous physical examination and training prior to working in hyperbaric conditions. Working in these conditions requires them to first slowly adjust their bodies to the pressure, similar to what scuba divers must do to safely complete an underwater dive. This adjustment takes place in a pressurization chamber located within the machine.
Crews spend up to one hour in the pressurization chamber (shown in the photo below) prior to going to work in the space behind the cutterhead. When the pressure in the chamber matches the pressure in the space behind the cutterhead where they’ll be working, crews can enter through small hatches.
Five crew members at a time work in the space behind the cutterhead. At the current working pressures, they can safely spend between 1 to 2 hours working before returning to a decompression chamber to readjust their bodies to normal pressure. Decompression takes up to 90 minutes. Crews can only complete one shift of hyperbaric work per day. It’s hard work in tight quarters, as the short video provided by STP illustrates.
When STP’s maintenance is complete, they will begin tunneling beneath the Alaskan Way Viaduct. WSDOT will close the viaduct for approximately two weeks during this section of the tunnel drive. We will provide the public with advance notice of the closure, but the start date isn’t yet known. It will depend on the amount of work that must be completed while the machine is in the maintenance stop. Check www.99closure.org for additional details as the closure approaches.