Tunnel-boring machine Bertha’s restart date will be delayed until Dec. 23, a month beyond the latest projection.

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The reassembly of tunnel-boring machine Bertha is taking longer than planned, so Seattle Tunnel Partners (STP) is predicting it will drill again Dec. 23, a month later than the goal announced this summer.

The Washington State Department of Transportation (WSDOT) posted the revised schedule Thursday in an online update.

The four-lane, tolled Highway 99 tunnel from Sodo to South Lake Union is now scheduled to be ready in April 2018, more than two years past the original December 2015 opening the contractors touted in 2010, when they won the $1.35 billion tunnel contract.

State tunnel officials discussed the timeline Thursday in Olympia with the Legislature’s Joint Transportation Committee. Chris Dixon, project manager of STP, explained how machine supplier Hitachi Zosen has reinforced the huge machine.

He discussed the road ahead, in which Bertha might drill under the old Alaskan Way Viaduct in late February. Several days of road closures are likely.

Lawmakers took the latest news with a congenial attitude.

They threw no tantrums. They voiced no queasiness about staying the course. They refrained from chastising project managers.

“We only pay STP for work they’ve completed,” Todd Trepanier, WSDOT’s Highway 99 administrator, reassured the committee.

The state has paid the tunnel contractors just over $1 billion. The committee invited Trepanier to explain how that’s possible, given that Bertha has drilled only 11 percent of its passage.

He answered: “Even though the tunnel is stopped, they only have $300 million worth of work yet to do.”

This is because the north and south portals are mostly completed, as are all the arc-shaped concrete segments that will form the tunnel walls, and the tunnel’s Sodo-area road decks.

“We are quite a ways down the road here,” Sen. Curtis King, R-Yakima, said afterward. “From my standpoint they have shown they can drill the hole, and do what they want to do. Am I happy that we’re two years late, am I content that we are two years late? No, I’m not. But we are.”

Why is the restart effort so slow?

It’s been difficult to maintain the precise space between rotating parts and fixed parts of 14 to 18 millimeters, within the 4 million-pound front end, Dixon explained.

“The movement of these gaps has been a real critical activity,” he said. “We’ve welded it in other places, and switched jacks around, to monitor these gaps.”

Clues emerged last week that the Nov. 23 restart goal might slip, as open-air testing of rotary parts due in mid-October wasn’t under way yet. A giant red lift tower remains on standby along the viaduct, until Bertha is proven fit to drill.

Sen. Tim Sheldon, D-Potlatch, thanked tunnel officials for giving him two tours inside Bertha, and praised the state’s plan to reconnect the street grid that’s now divided by Aurora Avenue North.

Linea Laird, WSDOT chief engineer, reminded lawmakers that as many as 76 options were considered in the 2000s, and how the state decided in 2009 to build the tunnel underground.

Eminent tunnel-industry executives persuaded WSDOT and Gov. Chris Gregoire in late 2008 that technology had advanced enough to make a huge tunnel cheaper than the more common method of building smaller twin tunnels.

That theory lost credibility when Bertha, the world’s largest tunnel-boring machine at 57 feet, 4 inches diameter, overheated and stalled on Dec. 6, 2013.

STP has sent insurance companies an estimate of $143 million for the direct cost of repairs, while the state predicts it will spend $78 million separately on WSDOT’s own overhead costs and inflation, based on a two-year delay. Contractors are expected to seek more than $300 million in extra pay, and the state intends to resist most of that.

Lawmakers disbanded an expert-review panel this spring and chose to assume oversight duties themselves.

Yet they posed only one uncomfortable question to Dixon.

Sheldon asked whether the rotary cutter’s cracked center pipe, built in Japan, had been damaged before the machine even reached Seattle. Dixon replied the ring was concealed for months underground until the repair operation exposed it, so he can’t give a sure answer.

Trepanier reported that $116 million in contingency funds remain, in the overall $3.1 billion budget to replace the viaduct, which has sunk several inches in spots, since the Nisqually earthquake of Feb. 28, 2001.

The extent of any taxpayer losses may not be known for years.

The state’s online update included this reminder: “Like all large construction projects, the schedule for this project changes frequently. WSDOT cannot verify any of the dates shown in this schedule.”