Inside Seattle’s Waterfront Tunnel Project

The largest-diameter tunnel-boring machine in the world arrives in Seattle this spring.

By Karen Taylor Quinn February 20, 2013 Published in the March 2013 issue of Seattle Met

This spring, Seattle welcomes the world’s largest-diameter tunnel-boring machine, nicknamed Bertha, delivered straight from Japan in 41 pieces and custom designed to begin carving out the city’s new deep-bore tunnel this summer. Here’s the dirt on the massive earth mover. 


Digging In
Bertha will likely carve through eight types of soil. One of the shallowest, called fill, is made up of loose dirt, buried man-made objects, and water. Bertha will also encounter sandier soil, followed by rocks and boulders, then, roughly 200 feet down (but possibly before that, too), sticky, hard clay. Though it’s quite firm, clay is easy to bore through because it won’t cave in or flood.  

Where will it go?
A quarry in Port Ludlow, Washington

How much will there be?
900,000 cubic yards

How much dirt is that?
If you piled it all onto the turf at Century-Link Field, it would create a mountain more than 100 feet taller than the stadium’s roof. 


Driving Miss Bertha

1. An operator unlocks the machine and activates a computer that runs hundreds of tests before the machine fires up its engines, which produce 25,000 horsepower. It takes 25 engineers to operate Bertha, using GPS that transmits new geologic data to the engineers every three seconds. 


 2. The machine’s mouth, called the cutter head, slowly chips away at the earth, advancing about 35 feet per day. It operates like a worm, “swallowing” the dirt and passing it back to a conveyer belt that leads out of the tunnel and onto a waiting barge in Elliott Bay.  

 3. As the “worm” wiggles its way through the earth, 206 feet below downtown at the deepest, it sloughs off concrete rings that line the tunnel. The tunnel will ultimately consist of 1,427 such rings made from more than 244,000 tons of concrete.



Automobile Tunnels, a Comparison

Longest in the World Norway’s Lærdal, 15.2 mi

Longest in the U.S. Boston’s Big Dig, 3.5 mi

Seattle’s DBT 1.7 mi



Early Achievements in Tunneling
25 BCE Romans dig out a mile-long burial vault from Naples to Pozzuoli by heating the volcanic rock with bonfires, then cracking it with basins of cold water. 

1818 Inspired by a worm he’d seen boring through a wooden ship, French engineer Marc Brunel invents the first tunneling machine. 

1872 The Swiss begin digging a nine-mile railroad tunnel through the Alps using dynamite; about 200 men will die during construction.

1875 After nearly 20 years of construction, America’s first TBM-created tunnel is completed in Western Massachusetts.

Serious Spin
By the project’s completion, the cutter head will have rotated the equivalent of 2,300 miles, or roughly the distance from… 


Published: March 2013

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