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The Highway 99 tunnel machine could be sidelined for several more weeks, to fix damaged seals that lubricate and protect the drive system that spins the giant cutter.

The revelation comes two months after the drill failed to grind ahead and operators shut it down. The machine known as Bertha tweeted in December that she was doing fine, just facing an obstruction. An 11-day inspection in January found no big obstacles, turning engineers’ attention inward.

Officials on Friday announced the seal assembly is damaged, and that probably contributed to heat spikes within the main bearing Dec. 7 and during a restart attempt Jan. 28-29.

“Contractors are not entirely sure what’s happening to the seals. They’re letting sand in, which is not good,” said Matt Preedy, deputy Highway 99 administrator for the state Department of Transportation (DOT). “Either you’ve got gaps somewhere, or you’ve got cracks in the seals.”

In an announcement Friday, officials compared the part to the bearings of a car axle that are designed to keep road grime out.

Preedy said the ongoing investigation — which includes an influx of staff from drill maker Hitachi-Zosen — could take up to two weeks before the Seattle Tunnel Partners (STP) contracting team issues a strategy to repair and restart the machine.

Hitachi-Zosen could wind up replacing some or all of the 25-foot-diameter rubberized seal. The company has fixed them before and has several methods, Preedy said.

It’s unlikely the $5 million bearing must be fully replaced, and STP hasn’t reported bearing damage, Preedy said.

DOT also announced Friday the December blockage happened mainly because soil clogged the openings in the cutterhead — as opposed to a pipe or some other obstacle. During inspections in January, workers climbed onto the giant 57-foot-diameter disc, under hyperbaric pressure, and broke up the clogs.

Bertha has been idle since Dec. 7, when contractors tried in vain to push forward.

The state said in a Dec. 10 news release that STP “proactively” stopped drilling Dec. 7 due to increasing resistance — but a document released this week says the machine actually shut itself down that day due to high temperatures.

Overheating can result from damaged seals, but there can be other causes, Preedy said.

Troubles with heat and grease appeared while operators tried to drill forward Dec. 7, according to a routine DOT quality-control inspection report, released this week following public-records requests from KIRO-TV and The Seattle Times.

After moving at more than 30 feet a day, Bertha slowed to only 4.4 feet on Dec. 6. On Dec. 7, subcontractors began to drill probes down from ground level, looking for an obstruction.

They stopped because of a decision to try to get Bertha moving again. That afternoon just before 1 p.m., operators were “trying to move forward with little success,” while several managers from Dragados USA were in the control room, said the quality-control report.

Temperatures were above normal in the drive system, and workers that day were using extra grease to try to flush out the main bearing because sand was found in a grease sample, the report says.

“Machine shuts down several times due to high temperature at the main bearing,” the inspector wrote.

Managers at DOT said a week ago that temperatures reached 140 degrees, about 1½ times normal, in early December and during the tests last week.

Bertha has traveled 1,023 feet of the 9,270-foot route from Sodo to South Lake Union, advancing on only 36 days since startup July 30.

Preedy said STP is still investigating the type and extent of seal damage.

“We have to give our contractors and Hitachi the time they need to do complex adjustments on this piece of machinery,” he said, to ensure it’s fully prepared to finish the dig. “If you try to get an answer too fast, you’re going to get a wrong answer.”

The rubberized seal assembly is made from a series of discs that are joined to form a cylinder — and that cylinder lubricates the interior of the slightly wider drive ring, he explained.

Previously there were issues with the seals during late-2012 tests in Osaka, Japan, said Preedy. After they were adjusted, the machine ran fine before it was disassembled and shipped to Seattle, he said. But tunnel officials planned to take another look at the Osaka tests, he said.

The $80 million drill, stalled near Pioneer Square, remains under warranty from Hitachi-Zosen until it reaches 1,300 feet.

DOT tunnel administrator Todd Trepanier and STP project director Chris Dixon have said previously the machine didn’t show major damage and is capable of finishing the long bore from Sodo to South Lake Union.

The first inkling of seal damage came during last week’s test, Preedy said. STP gave a more-definitive description to DOT on Thursday, confirming seals were damaged, he said.

The main dilemma now is how to remove and replace parts near the front of a machine that’s longer than a football field. Many sections, especially the trailing gear in back, can be readily disassembled.

But the front end moves only forward because of the 149 smaller-diameter concrete rings that have already been fastened in the rear, forming the future highway tube. And the closer that workers get to the front, the less space there is for them to move amid the machinery.

Preedy told reporters he’s confident that parts to fix the seals can be delivered from the rear. If that’s not possible, STP would face a more daunting mission — to dig and descend 60 feet through soil and groundwater to the front end.

Mike Lindblom: 206-515-5631 or Twitter @mikelindblom