Crews begin installation of Big Bertha’s new seal system

Updated Jun 19, 2015
Bertha’s front end, which weighs 2,000 tons, is pulled to the surface. Credit: WSDOT FlickrBertha’s front end, which weighs 2,000 tons, is pulled to the surface. Credit: WSDOT Flickr

While the rest of her massive frame waits below ground, Big Bertha has undergone the most pivotal portion of her badly-needed brain surgery.

According to a report from the Puget Sound Business Journal, repair crews with Seattle Tunnel Partners, the lead contractor on the drilling of a new State Route 99 tunnel beneath Seattle, have begun the installation of a new seal system on the world’s largest tunnel-boring machine.

RELATED >> Damage to Big Bertha worse than previously thought

The new seal system arrived earlier this month and installation began June 5. Bertha was brought to halt in January 2014 when the machine’s seven rubber seals meant to protect the main bearing became clogged by sand and water.

To make the repairs, crews dug a 120-foot pit to reach the machine and hauled its front end to the surface. The new seal system should be easier to access if things go wrong again, the Journal reported.


The clogged seals were initially thought to be the extent of the machine’s damage. But in May, after the machine’s front end had been lifted to ground level, STP and the Washington State Department of Transportation learned the damage extended to the steel casings around the seals which, “broke apart, sending fragments into the drive gears.” In addition, the machine’s bull gear, which rotates the drill face, was cracked along with several pinion gears.

Bertha is two years behind schedule on drilling the tunnel which will replace the Alaskan Way viaduct. When it went down, the machine was only one-tenth of the way through digging the 1.7-mile tunnel.