The holiday season provided some rest for Bertha, the tunnel-boring machine roughly three-quarters of the way through its route to bore a new State Route 99 tunnel under downtown Seattle, but that doesn’t mean crews have had it so easy.

Following a holiday break, Seattle Tunnel Partners, the general contractor boring the tunnel for owners Washington State Dept. of Transportation, performed routine maintenance and inspections on the machine, which included inspection of the machine’s cutterhead tools under normal atmospheric pressure. And that’s when the contractor decided to begin a hyperbaric maintenance stop to inspect the other cutterhead tools on the machine.

Bertha’s rotating cutterhead has more than 700 tools that scrape away the earth in front of the machine and not all of these tools were created the same. Some can be accessed and replaced during routine weekend maintenance, while others require workers to reach them only via hyperbaric conditions.

The 16 cutterhead spokes are all hollow, allowing crews to get to them through normal maintenance, just as they can reach the centerplate cutting tool. The pre-cutting bits that run along the spokes of Bertha’s cutterhead can also get replaced under normal conditions. Some of these bits weigh up to 600 pounds and can take an entire day to switch out.

But the tool that shows up on Bertha’s cutterhead more than any other, the scrapers, do require hyperbaric intervention to replace. These tools line the sides of the spokes and are responsible for finishing the job started by the pre-cutting bits. They scrape away the loose soil and send it through the openings in the cutterhead toward the machine’s mixing chamber.

Since the maintenance period began on Jan. 5, crews have replaced 250 cutterhead tools over the course of 184 hours of hyperbaric shifts, a work environment similar to performing an underwater dive. The last time Bertha stopped for hyperbaric maintenance was in October and crews worked under the conditions for one week.

With the top of Bertha’s cutterhead located about 160 ft below Third Avenue, about halfway between Blanchard and Bell streets in downtown Seattle, crews are less than 2,500 ft from the receiving pit near Seattle Center and the Space Needle. Seattle Tunnel Partners calls this hyperbaric maintenance “critical” for the machine as it prepares for the final 25 percent of the tunnel drive.

Follow Tim Newcomb on Twitter at @tdnewcomb.