The day after the Highway 99 tunnel-boring machine hit a buried pipe in early December, the crew stopped mining for 70 minutes to remove fragments of steel and rock that had made their way inside.
The material had reached the conveyor screw, designed to pull stones and mud to a conveyor belt in the rear of the machine.
After the stoppage, late Dec. 4, the giant drill known as Bertha went back to work Dec. 5, moving 32 feet north. The next day, the machine quit grabbing dirt and traveled only four feet in three hours, prompting operators to shut it down.
The tunnel contractors’ production reports, issued daily to the state, were released to The Seattle Times through a public-records request. They provide new details about what led up to a nearly two-month shutdown.
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Officials have said that steel damaged some cutting tools on the rotating drill, but they haven’t mentioned before that the pipe was intrusive enough to lead Bertha’s operators to open and clean the enclosed conveyor screw.
Seattle Tunnel Partners (STP) has argued the pipe was the main reason the machine eventually stopped moving forward.
The state Department of Transportation (DOT) waited a month to disclose that a pipe hit occurred, and said the steel doesn’t fully explain Bertha’s problems.
Then in mid-January, Transportation Secretary Lynn Peterson told state lawmakers she had broader concerns over how STP was running the machine since boring began July 30.
The state’s tunnel managers haven’t said what they think is keeping Bertha from moving. DOT has called an expert panel to review the situation. The machine moved two feet on Tuesday, as an experiment, another two feet Wednesday, then stopped for maintenance.
A concrete chunk and at least one pipe segment were found during 11 days of recent inspections, DOT said, but test probes found no huge obstacles, such as an oversized boulder.
The state and its contractors are likely to negotiate or litigate, with millions of dollars at stake, who will pay for the delay.
The pipe strike happened just as Bertha was hitting her stride, the reports show.
On Monday, Dec. 2 the drill advanced 26 feet, then on Dec. 3 another 59 feet — its best day ever. Officials said the pipe strike happened sometime Dec. 3, knocking half of the 119-foot, eight-inch diameter well casing to the surface. The pipe had been used by DOT for groundwater tests in 2002 and 2010.
Bertha got a late start on Dec. 4, to clean concrete-grouting equipment, but traveled through sandy silt 32 feet, close to its fall average, that day’s report says. before the 70-minute stoppage to “clear the screw of steel projectiles and boulders.”
Matt Preedy, deputy project director for DOT, said Friday that the tunnel crew opened the rear gate of the conveyor-screw chamber to remove loose metal pieces — there wasn’t anything wedged or stuck, he said.
“The reason they stopped was a safety issue,” said Preedy. Tunnel workers didn’t want metal to ride on the long conveyor belt from Sodo to Terminal 46, where a barge or trucks remove the excavated materials, he said.
The pace slowed Dec. 5, to about 24 millimeters per turn of the 57-foot-diameter cutterhead, which spins about once per minute. By day’s end, the boring rate decreased to 10 millimeters per turn, though the report doesn’t state reasons. Still, the machine went 32½ feet.
But on Dec. 6, after creeping only four feet in three hours, operators turned the machine off.
DOT said later that some steel and rock were pushed beyond the screw and out the back of the machine that week.
STP project director Chris Dixon, asked last month why he didn’t stop Bertha right away after the pipe strike, said he thought the pipe was shallower, and had been knocked out of the way.
In that brief chat, he didn’t mention that steel had been plucked out of the conveyor screw. The red screw is a vulnerable piece, as shown earlier in the project when it was jammed by bunches of fiberglass.
Mike Lindblom: 206-515-5631 or email@example.com. On Twitter @mikelindblom