While tunnel machine Bertha takes a very long sabbatical, most of the Highway 99 tunnel labor force will stay busy this spring.

More than 200 of the 300 construction-trades workers have avoided layoffs, state officials say. Only 25 to 30 people work inside the giant drill anyway.

The rest are building a double-deck highway entrance in Sodo, setting the foundation for an operations building, or excavating the tunnel’s north portal near Seattle Center.

“Clearly, there’s a lot of work going on here,” said Matt Preedy, deputy program administrator for the Washington State Department of Transportation. “They’ve not had any significant downsizing yet at this point.”

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What’s unclear is whether there’s enough work to sustain those jobs through summer. Seattle Tunnel Partners (STP) has estimated that digging might not resume until at least Sept. 1, after a complicated repair or replacement of Bertha’s bearing assembly.

As some people are laid off, others would be hired, for instance electricians to string the wire and plumbers to fasten the pipes for the operations center, which will house the tunnel’s electronic-control systems, maintenance equipment and giant ventilation fans.

STP has laid off about 20 operating engineers — many of whom worked inside the tunnel drill — and 15 laborers since fall, leaving about 40 of each trade, said Lee Newgent, executive secretary for the Seattle Building & Construction Trades Council.

The ironworkers’ workforce varies between 10 and 80 people, with about 60 currently on site, said Jeff Glockner, business manager for Ironworkers Local 86. The tunnel job has gone smoothly for them so far, he said.

Layoffs come at a price. STP might have to train new people when work resumes.

STP and Hitachi-Zosen, the drill’s manufacturer, are expected to announce their strategy next week to repair the $80 million tunnel machine, stranded underground near South Main Street since December.

The rescue mission requires equipment operators to dig a 120-foot-deep vault in front of the drill to reach the bearing.

Behind the job-site fence, it’s loud enough that a supervisor had to shout and wave Friday to shoo visitors away as a crane moved a load of steel overhead.

Down at the Sodo tunnel entrance, workers are cleaning out leftover parts from the pit where Bertha started its journey July 30.

A team used cutting torches and jackhammers to break some temporary concrete rings and a steel frame — which Bertha used to make its initial thrust north. These performed like starting blocks in an Olympic sprint, only slower, to propel the 7,000-ton machine 19 inches per hour.

This month, the laborers who have installed 149 permanent rings, to form the highway tube, are breaking the 13 temporary rings in the Sodo launch pit.

Bertha’s initial thrust force
was so great that part of a concrete ring became embedded in the steel brace.

Next, the workers will install a seismic expansion joint — a gap where the tunnel tube begins, to isolate it from near-surface earthquake vibrations.

By doing the work now, Preedy said, STP can save a month later, when Bertha stops for maintenance in a so-called “Safe Haven 3,” just before burrowing under the Alaskan Way Viaduct.

Some operating engineers, who usually work in Bertha’s control room, are repairing the conveyor system, he said. Abrasive dirt and concrete prematurely eroded the inside of the conveyor tube used for removing dirt. So the tube must be reinforced.

Meanwhile, ironworkers at the future Sodo operations center are fashioning a four-layered web of steel rebar where a 5-foot-thick concrete floor will be poured. The heavy floor will resist uplift from the tremendous groundwater forces, comparable to what might keep a ship afloat, Preedy said.

And work continues on the double-deck highway ramp from the surface to the tunnel. In a few months, ramp work will extend farther south — and four traffic lanes near the stadiums will have to be relocated. Motorists can expect a four-day weekend closure, Preedy said.

For the building trades, that’s 12 shifts of gainful employment.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com. On Twitter @mikelindblom