All that stands in the way of Bertha, the country’s largest-diameter tunnel-boring machine, and the soil under downtown Seattle is the reassembling and testing of the 7,000-ton Japanese-made machine.
Her launch pit stands at the ready.
As part of the plan to replace the Alaskan Way Viaduct with an underground bored tunnel, crews needed to build an 80-ft-deep launch pit to send the 57.7-ft-dia machine through for the 1.7 miles it plans to dig.
The 326-ft-long machine was built in Japan and underwent a 5,000-mile voyage to the Port of Seattle earlier this year. After 10 days of unloading, all 41 pieces of the machine sit near the pit to the west of Seattle’s stadiums, ready for reassembly and testing before embarking on the boring later this summer.
The pit itself took one year to construct. It included removing 86,000 cu yds or soil. Before excavation even began, crews drilled more than 200 piles as many as 100 ft deep to form the pit’s walls. The perimeter of the pit is 80 ft wide and 400 ft long.
Assembly of the machine has already started at the south end of the pit and now that the front end of the pit is complete, crews have started building the body of Bertha near the spot where it will first push into Seattle’s soil.
“If Bertha is the star of the project, the launch pit is her stage,” says Linea Laird, Washington State Dept. of Transportation administrator for the project. “Completing the launch pit means we’re that much closer to the start of tunneling.”
Crews are also preparing the surrounding area for tunneling, including strengthening the soil and building protected work areas along the initial section of the tunnel route so they can perform scheduled inspections of the machine before it starts digging beneath downtown, obviously the trickiest part of the process.
At the same time, work continues near the north end of Battery Street Tunnel to prep for Bertha’s emergence at the end of the new tunnel.
Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He has also written for TIME, Popular Mechanics, Popular Science and more. You can follow him on Twitter at @tdnewcomb or visit his website here.