Bertha, World's Largest Boring Machine, Ready to Dig

Jul 19, 2013

The State Route 99 tunnel boring machine, better known by her nickname “Bertha,” is poised and ready to begin the dig under downtown.  

To get a bird’s eye view, we climb three stories on temporary metal staircases near Pioneer Square. At the top, we’re standing on what remains of the lower deck of the Alaskan Way viaduct. 

Most of Bertha is hidden from view. Just a white curve of her back pokes above the launch pit. But underneath, she’s 57.5 feet in diameter—that’s 7 feet wider than any tunnel boring machine built yet. 

During the 14-month drive, the cylindrical machine will tunnel through mostly glacial soil, the dense hard-packed stuff Seattleites are used to encountering in the backyard. And 850,000 cubic yards of dirt will be moved out of the tunnel.   

“If you were to pile all that dirt up in the middle of Safeco field … it would exceed the height of the roof by quite a ways,” said Matt Preedy, the state Department of Transportation employee responsible for getting the tunnel dug.

In fact, the dirt would be more than 100 feet higher. Preedy says the dirt will be hauled out by an ever-extending conveyor belt. By the time it reaches the north end of downtown, it will stretch almost 2 miles long.

“That conveyor belt will take all that tunnel muck out of the tunnel, up out of the launch pit, over to the port of Seattle where it’ll be loaded onto barges. And those barges will then take the spoils over to a gravel pit reclamation site on the other side of Puget Sound,” he said.

There, the soil will be used to fill an old quarry. Bertha will start out slowly—just about 6.5 feet each day. When she hits full production and things are really humming, she’ll be eating dirt at a rate of 40 to 60 feet each day.