Crews undergo special preparations before working on Bertha’s cutterhead

Bertha, the giant boring machine digging a tunnel along the Seattle waterfront, is on a break at Spring Street.

Seven crews from Seattle Tunnel Partners and Ballard Marine Construction will work around the clock during the scheduled stop to perform maintenance on the space behind the giant machine’s cutterhead, according to the Washington State Department of Transportation. Before they can do the work, crews must inject a type of clay, known as bentonite, to stabilize the ground and create a seal to prevent water, soil and air from entering the space.

Crews will over-pressurize the space to “counteract the ground and water pressure at the front end of the machine,” WSDOT says. The conditions will be similar to those in an underwater dive.

Crew members must spend several minutes in a special chamber to prepare for the pressure change. WSDOT says the amounts of time they’re able to work in these conditions depends on the pressure of the work space. But in similar projects, crews were able to spend up to an hour in the space before decompressing and returning to the surface.

Crews will make their first “hyperbaric intervention” next week, according to WSDOT.

WSDOT’s new videos show the chambers that crews will use to adjust to the conditions and enter the space behind Bertha’s cutterhead.

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