After four years, the biggest tunnel boring machine finally finishes digging under Seattle
Shortly after my son was born, a 325-foot-long boring machine started carving out a tunnel under the city of Seattle. The plan was to carve a tunnel 57 feet in diameter under the damaged Alaskan Way Viaduct, replacing that roadway with a new subterranean one. However, before my son’s first Christmas in 2013, the tunnel boring machine ground to a halt, pushing back it’s completion date to five days ago.
The massive machine was named Bertha, at the suggestion of grade school students, after Seattle’s only female mayor, Bertha Knight Landes. Weighing nearly 8,000 tons, it is the largest tunnel boring machine in the world with a sticker price of $80 million. These superlatives have been overshadowed by the machine’s first year of operation though, since after 1,000 feet of digging, engineers noticed that bearings were overheating, and the digging was halted. The biggest tunnel boring machine in the world was then left inoperative for two years.
Damaged while digging
There’s some controversy about the exact cause of the damage, but it seems that Bertha hit a pipe buried underground. While the 57-foot cutting blades could tear through soil and concrete, they were never designed to tear though metal, and the collision required replacement of the main bearings. However, Bertha was still underground at that point, and at 325-feet-long, wasn’t going to back out or turn around gracefully. To repair it, crews dug a hole from the surface, 120 feet down to Bertha’s damaged cutting head, then removed and hoisted it to the surface. Once those were finished, the machine was rebuilt and started digging again in December of 2015.
On April 4, 2017, Bertha finally finished the 9,270-foot tunnel, averaging just under seven feet of digging a day in the end. The roadway will still require a couple of years of prep before anyone can drive through the tunnel, but Bertha’s days of digging, and existence, are over. The hulking machine can’t be transported as is, and so for the next few months, workers will take it apart into 20-ton pieces that can be trucked away. Those pieces will then be sorted for reuse or recycling. It may seem inglorious for this record-setting piece of engineering, but then again, Bertha outlived her expected lifespan by at least two years already, and most folks in Seattle won’t be sad to hear of her passing.
My four-year-old asked: Does it move on rails?
While rails would help distribute the weight somewhat, Bertha doesn’t even have wheels to speak of. In addition to drilling through rock and soil, the machine also helps build the tunnel as it goes, laying in 40,000-pound ring segments before pushing forward. Part of that push involves pushing hydraulic pistons off the mounted ring segments nudging things onward without the need for train rails, tires or treads.
Source: After 4 Years, Seattle’s Giant Tunneling Machine Finally Breaks Through by Jack Stewart, Wired