Seattle Tunnel Partners is making a backup plan in case tunnel-boring machine Bertha is too weak to grind through the concrete repair-pit wall.

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For the past year, Highway 99 tunnel builders have said their ailing drill, Bertha, had just enough strength to grind forward about 20 more feet — to reach open air and be rescued by crane.

But as that moment nears, they’re preparing in case the machine fails.

The Washington State Department of Transportation is reviewing a draft plan by Seattle Tunnel Partners to chip a circular hole from within the concrete access vault, to clear a path in front of the machine.

That kind of Plan B could add more delay to a project already trending two years behind schedule.

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A clue about Bertha’s health emerged when crews drove it forward three feet last fall, to reach the outer edge of the pit wall.

Temperatures within the machine’s front end reached 194 degrees, nearly the boiling point of water — says a project risk log The Seattle Times received through a public-records request.

The tunnel-boring machine reached 140 degrees after advancing only four feet Dec. 6, 2013, causing engineers to turn it off in the first place.

An official with tunnel partner Dragados USA wrote in the log that they will review the problem with Hitachi-Zosen, which built the $80 million machine, and proceed accordingly. It suggested an option of mining backward from inside the rescue pit, rather than making Bertha do all the work.

Contractors still are expressing confidence Bertha can burrow into the vault, Matt Preedy, deputy Highway 99 tunnel administrator for the Washington State Department of Transportation (WSDOT), said last week.

“It might make it through the 20 feet of concrete, it might not. The contractor has contingency plans if it does not make it all the way through,” he told a dozen interested people at the Milepost 31 interpretive center.

Machine operators might forge ahead for a while, stop for cooling, and run it again, in cycles, he said in an interview.

But that method might take long enough that contractors decide to halt Bertha and resort to carving a hole from the inside, he said. While doing so, they’d have to scoop concrete fragments off the floor of the vault.

“It would take quite a bit more time than if they were able to successfully mine through,” Preedy said.

During a similar rescue operation in 1994, a tunnel machine in Sarnia, Ontario, was repaired successfully after breaking into a concrete pit half the size of the one in Seattle.

STP didn’t respond to an interview request. The companies’ project manager, Chris Dixon, has been too busy lately, said WSDOT spokeswoman Laura Newborn.

The tunnel’s purpose is to replace the Alaskan Way Viaduct before an earthquake knocks it down, in the process creating space for parks and redevelopment.

The state initially said it would open the $2 billion, four-lane, tolled tunnel from Sodo to South Lake Union at the end of 2015, but contractors are now aiming for late 2017.

State Transportation Secretary Lynn Peterson has quit making schedule predictions, until the machine’s 4 million-pound front end can be lifted to the surface and repaired.

Hitachi-Zosen plans to replace Bertha’s circular main bearing, add stiffer seals to resist grit, reinforce the front end with steel plates and rods, and widen gaps in the rotating cutterhead, so dirt tumbles past more easily.

First, Bertha needs to break free.

Contractors reached a milestone last weekend by pouring a concrete cradle where the cylindrical drill can rest securely. Steel rail beams were installed, for Bertha to slowly slide forward on them, nudged by its hydraulic thrusters in the rear.

If crews dig toward Bertha, they must avoid cracking the wall of the repair pit. Another challenge is to keep groundwater — at four times atmospheric pressure — from seeping through. To finesse this problem, STP also built concrete walls around Bertha, so soils can be pumped almost dry.

At a briefing Monday, City Councilmember Kshama Sawant asked state managers, “What are the cost overruns going to be? You must be developing some sort of estimates, and what are taxpayers supposed to be thinking at this moment?”

Such questions will take years to untangle. For now, the evidence of the tunnel’s viability isn’t on paper but in the ground, where contractors haven’t quit.

“They’re putting out a huge amount of effort right now, with no guarantee they’ll be reimbursed for any of the extra effort,” Preedy said. “But they’re doing it anyway.”