Clouds of white dust marked the start of tunnel-boring machine Bertha’s 1.7-mile voyage under downtown Seattle.
The grinding began at 3:45 p.m. Tuesday and will continue for about 14 months. As the drill moves from Sodo to South Lake Union, workers behind it will install the decks of a stacked, four-lane highway tunnel.
For the first few days, the machine is working through a slab of concrete, 15 to 18 feet thick, before it gets to soft fill soil that early settlers and industries dumped along the waterfront. Tunnel contractors have tried to reinforce Bertha’s path using grout, as well as concrete pilings on both sides of the future tunnel.
The digging will begin slowly.
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The initial pace is 6 feet per day, increasing to 35 feet per day later in the project.
Tuesday night’s shift was scheduled to last until 2 a.m. Wednesday — a normal day will be two 10-hour shifts.
Just a half-hour into Tuesday’s grind, the rotary cutter paused for about 10 minutes. Some of the 56 hydraulic thrusters, which are arranged in a circle to propel Bertha forward, were out of sync and needed adjustment. “It’s going to be one of those hit-or-miss things,” said Greg Hauser, deputy project manager.
Not everyone appreciates the tunnel, especially Seattle environmentalists who oppose spending $2 billion — or $3.1 billion if the interchanges and feeder streets are counted — to serve automobiles.
Public transit, which now carries more than 24,000 weekday riders on the old Alaskan Way Viaduct, will be slowed by having to navigate surface streets into downtown in 2016.
Mayor Mike McGinn, who skipped the July 20 opening ceremonies, said later that despite his longtime opposition, he is hoping the work goes smoothly, given that voters endorsed the project in a 2011 advisory ballot, and ground was about to turn.
The tunneling machine’s thrusters are pushing north off temporary concrete rings, which are backstopped by a steel brace, visible from outside the pit.
The machine will have to push off about 13 temporary rings before the front end has advanced 85 feet and goes entirely underground.
Then the first of 1,450 permanent rings will be set, forming the tube to South Lake Union.
Bertha will advance 6 feet at a time, shoving off the rim of each ring, until the next ring is installed.
At 57 feet, 4 inches in diameter, this is to be the widest single-bore tunnel ever drilled.
Hauser predicted that sometime around October the machine will go under the old Alaskan Way Viaduct. This is considered the most sensitive part of the path, running a mere 30 to 40 feet below some viaduct pilings, and past some historic Pioneer Square buildings.
Hundreds of monitoring devices will check for ground movements of a fraction of an inch, throughout downtown.
From a viewpoint on the old viaduct, Hauser didn’t venture an opinion about whether Tuesday was a successful day.
“I’m never satisfied. It’s never enough,” he said. “It has to be right. We have no business being underground if the equipment is not set up.”
He was hoping to advance 3 or 4 feet by 2 a.m.
Mike Lindblom: 206-515-5631 or email@example.com. On Twitter @mikelindblom