The troubled but refurbished Highway 99 tunnel machine Bertha resumed drilling in a sandy pit early Tuesday, continuing its long journey to South Lake Union.

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Workers at the Seattle waterfront switched on the power to the world’s largest boring machine and revved it forward early Tuesday, bringing hope to the Highway 99 tunnel project after a two-year delay.

The drill known as Bertha advanced 1½ feet, the Washington State Department of Transportation announced in an update.

Hitachi Zosen, builders of the $80 million device, have spent millions more to reinforce and redesign the front end so profoundly that in some ways, it’s a new drill.

Bertha restarts

The Highway 99 tunnel machine cut through sand Tuesday after a two-year delay. Other key dates, presuming no more major problems:

July 30, 2013: Bertha starts grinding its way out of the launch pit in Sodo.

Dec. 6, 2013: Machine stalls, as grit penetrates the outer bearing seals, at Pioneer Square.

Dec. 22, 2015: Bertha advances, the first step in a new, 450-foot phase of the dig lasting about four weeks. The machine will then stop for inspections.

March 2016: Digging scheduled to resume under Alaskan Way Viaduct, requiring about a two-week traffic closure.

January 2017: Tunnel boring scheduled to be finished at South Lake Union.

April 2018: Estimated completion date for tunnel, followed by a few weeks to connect the entrance ramps, before traffic enters. The viaduct is to be demolished later.

However, there exists only one way to learn whether the rejuvenated Bertha has the right stuff to complete its gritty voyage to South Lake Union.

And that is to push forward.

Government agencies lack a Plan B, so failure is not an option.

Tuesday morning’s push of 1½ feet provided Seattle Tunnel Partners (STP) enough space behind Bertha’s drive motors to fasten the next concrete ring at the 1,085-foot mark of the planned 9,270-foot tube.

Chris Dixon, STP project manager, is calling this a testing phase. The team is measuring how Bertha responds while rotating through heavy loads of compacted sand. Last week, a fixed steel arm in the front end broke and needed A one-day repair.

Before stopping sometime Wednesday for a workers’ holiday, another few feet of drilling is planned inside Bertha’s repair-access vault.

This week’s two-day push would leave the nose of the drill just short of the north edge of the concrete vault, dug in 2014 so STP could reach and lift the 4 million-pound front end for repairs.

The winning bid from STP called for the tunnel to be completed this month.

When 2016 begins, the machine is scheduled to push through the concrete north wall of the vault, then spend four weeks moving through a mix of clay and sandy soil for another 450 feet. If all goes well, it would stop in late January for a few weeks’ inspection before diving under the Alaskan Way Viaduct, which is predicted for March. The viaduct will be closed for about two weeks as a precaution, in case Bertha causes excessive underground vibrations.

At that point, the risks in the $2 billion tunnel project intensify. A serious mistake in soil excavation or measurement could damage the viaduct. If the machine fails beneath downtown buildings, it becomes more difficult to repair.

Bertha needs a year to reach South Lake Union, where the north portal of the four-lane Highway 99 tunnel is almost completed.

Over the weekend, workers pumped in a layer of concrete, mixed with more sand over the buried machine. STP chose concrete instead of the original soil because hard rains would have made it difficult to control the weight and density of soil, says the state Department of Transportation (WSDOT). Soil will be added later to top off the vault.

Bertha was expected to restart Monday, but the concrete layer needed extra time to set up because of heavy rain, said WSDOT spokeswoman Laura Newborn.

Tuesday morning, surveying was also taking place at the vault, normally done to ensure the ground doesn’t settle. STP sent more workers to the south operations building, near CenturyLink Field, to install new shipments of panels.

On Dec. 6, 2013, Bertha overheated after rubberized seals failed around the main drive bearing, so grit leaked in. Small metal pieces broke and chipped some gear teeth, contractors have said. Bertha had hit a steel pipe three days earlier, and STP blames the impact for triggering other problems, a theory the state disputes. STP’s own insurers have asserted that the boring machine’s design was inadequate.

Hitachi Zosen performed the multimillion-dollar repairs this year, deploying about 35 engineers, designers and repair specialists to Seattle as of November, the company says. Steel plates were added to stiffen the front end, along with more cutting teeth, and stronger rubber seals to protect the bearing.

The main partners, Dragados USA of Spain and Tutor-Perini of California, have said in court documents that overall costs for repair, including excavation of the giant repair-access vault, are expected to exceed $143 million. It could take years for STP, Hitachi Zosen, their insurers, and Washington state to sue or negotiate who pays for repairs and delays, beyond the basic $1.35 billion STP contract.

If Bertha encounters no other major problems, the four-lane tube could open to traffic in the spring of 2018.