High above the spot where Bertha, the SR 99 tunnel machine, sits idle during a scheduled maintenance stop, workers with Seattle Tunnel Partners and Ballard Marine Construction explained Tuesday what it takes to go deep underground and safely perform inspections and necessary repairs on the hulking machine.

Bertha is currently stopped 120 feet below Spring Street, about a third of the way through its 9,270-foot journey to cut a path for a new highway to replace the Alaskan Way Viaduct.

Crews sent down to perform cleaning and maintenance of Bertha’s cutterhead will encounter hyperbaric conditions, similar to those found in an underwater dive. This pressurized workspace is created when a type of clay, called bentonite, is injected in front of the machine to create a seal to keep out water and soil — and keep in breathable air. Compressed air is introduced to push against the bentonite and counteract ground and water pressure.

Standing in front of bucket of bentonite on Tuesday, just below the location where the elevated viaduct was cut off and redirected some time ago, STP project manager Chris Dixon said the number of “hyperbaric interventions” performed by workers will depend on the condition of the cutterhead.

A maintenance stop in March near Yesler Way lasted about six weeks and workers performed 125 hyperbaric interventions before Bertha eventually moved on and tunneled beneath the viaduct.

“We’re going to start the interventions after the Fourth of July weekend, so Ballard is here getting the equipment ready,” Dixon said at site near South King Street and Alaskan Way. “We don’t know how long this particular stop is going to take until we get in there and see what the extent of the maintenance work is.”

Seven crews of five workers each rotate on a 24-hour basis, accomplishing just a few hours of work each day. According to the Washington State Department of Transportation’s project website, the amount of time that crews can safely work in the conditions varies depending on the pressure. In previous interventions on the project, crews were able to spend up to an hour in the conditions before decompressing and returning to the surface.

The surface site set up by subcontractor Ballard Marine, experts in supporting large and complex tunnel projects, features stacked shipping containers that house the high-tech equipment necessary to create the hyperbaric conditions to which workers will be introduced.

The site mainly serves as a clinic where a worker experiencing symptoms of decompression sickness can be treated. The large medical lock can also sleep up to 10 workers in hyperbaric conditions for up to 28 days if maintenance work demanded more hours. A hyperbaric shuttle can transport them to the pressurized manlock that connects to Bertha and allows access to the excavation chamber.

“Because their bodies are acclimated to it when they go into the tunnel they can work up to six hours at a time,” Dixon said of workers coming from the surface chamber. “So if you have three crews doing that you can get 18 hours of work in a day. We haven’t needed to go to that step [on previous stops] because the work hasn’t been that extensive.”

Crews will be working in the upper third of the area occupied by Bertha’s cutterhead. Dixon said cutting tools are checked on both sides of the machine’s “spokes.” A 1/8 rotation is done eight times to access all of the spokes and replace what’s necessary. “Once we complete that whole cycle the maintenance is basically done.”

Justin Costello, a hyperbaric division manager with Ballard Marine, said workers can only enter the site once per day and need at least 18 hours between compressions. He said conditions underground in the work area are hot and humid, but similar to atmospheric pressure.

Workers will likely perform about a half an hour of work at this particular maintenance location. They then return to a manlock attached to the machine to decompress for about an hour and a half. They then leave the manlock and go to a clinic that’s also on the machine where they’re tested and undergo observation for an hour or so. They are free to go when medical evaluation determines they have returned to baseline values established before they entered the work site initially.

WSDOT released two videos on Tuesday which show what the working conditions are like. In one, workers welding a plate as part of a valve replacement last spring wear oxygen masks to protect against fumes.

A second video shows what it’s like to crawl through the hyperbaric chambers to access the workspace.

STP is still evaluating how long the remaining tunneling will take. As Bertha bores through Seattle’s underground, the machine places rings to form the tunnel’s outer wall. So far, 466 of 1,426 rings have been placed. At a pace of about five rings a day and 30 rings a week, there could be 8 months of tunneling ahead before the machine reaches the north portal, Dixon said.

Bertha has passed the lowest point of the tunnel and has completed a 4-percent downgrade from the south portal of the tunnel, Dixon said. The machine is now climbing at 1.6 percent as it proceeds to about Lenora Street. After that, Dixon said the uphill grade changes to 3.6 percent to about Wall Street, and from there to Thomas Street and the end, it will climb at a 4-percent upgrade.

Scheduled maintenance stops also affect the overall pace of the project. Media members assembled for Tuesday’s site tour pressed Dixon repeatedly for timetable estimates.

“It takes as long as it’s going to take,” Dixon said.

Correction: A previous version of this story misspelled the name of STP’s Chris Dixon.