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A deep access pit to repair tunnel-boring machine Bertha will be finished a month late because crews need extra time to grind and chisel through concrete.

The 120-foot-deep vault in front of the machine should be done in August instead of July, project director Chris Dixon of Seattle Tunnel Partners (STP) said Monday. After that, it will take a few weeks to excavate the dirt inside, and prepare a crane to lift the tunnel drill’s front-end parts, before the drive bearing and other parts can be fixed.

He said the stalled Highway 99 tunnel drill still will be ready to resume its dig to South Lake Union by March, as previously announced. The repair plan included a two-month cushion, known as “float,” that is now down to one month, Dixon said.

The state Department of Transportation seems more ambivalent.

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An official tunnel update vouches for March, but program administrator Todd Trepanier said Monday afternoon that this latest delay “has increased our concern in the ability for STP to resume mining in March,” given the fact repairs haven’t even begun yet.

The ring-shaped pit will allow a giant crane to hoist Bertha’s entire 2,000-ton front end, so the 57-foot, 4-inch diameter cutterhead and its drive system can be laid on the surface and repaired. The main bearing and its leaky seals will be replaced, and gaps within the cutting disc will be widened to reduce clogs when muck passes through.

Bertha sits in wet soil near the waterfront, after digging only 1,025 feet of the 9,270-foot route since its launch one year ago.

The latest challenge is caused by the geometry of the vault.

Malcolm Drilling, the pit subcontractor, is installing the pit walls as a barrier against groundwater that presses inward at triple atmospheric pressure.

This requires crews to install an initial series of 10-foot-diameter columns 6 feet apart, then to install another set of columns between those. The secondary columns require carving a 2-foot-wide arc from each of the adjoining columns.

Drilling is done with spinning steel tubes with teeth on the ends — but the excess concrete wasn’t flaking away as expected.

“We thought they would chew through the concrete better than they’ve been able to,” Dixon said.

So Malcolm brought in a pair of gravity-driven chisels and dropped them inside the tubes to break the recalcitrant concrete — which adds an extra step, several times per column.

Only then can the fragments be scooped out, using clamshell shovels.

“You’re constantly changing the equipment,” Dixon said.

Asked why STP didn’t anticipate this problem, he said Malcolm knew chiseling might be needed, but the concrete was expected to crumble more easily. The pilings in Seattle turned out more difficult to cut than piling half this size elsewhere, Dixon said.

Also, 11 columns have been added to the original design, for a total 84. So far, 53 of the 84 buried columns are completed, he said.

The extra columns will form a second layer along the south side of the ring. Engineers are expecting Bertha is healthy enough to bore through the concrete wall to reach the open-air access shaft. When the machine drilled through a concrete wall earlier in Sodo, it made slow, steady progress but the cutting teeth eroded quickly. Dixon predicted the machine will grind through the double wall just fine.

He said STP refuses to take shortcuts in building the circular ring, which is designed to reinforce itself against compression from all sides.

Malcolm Drilling managers in Kent weren’t available for comment Monday.

Typically, these overlapping walls contain 3- to 4-foot-wide pilings that are 50 to 100 feet deep, and it’s only recently that depths beyond 100 feet became possible, said a Tunnel Business Magazine article this spring by Rob Jameson, a vice president at Malcolm, and Eric Lindquist of Brierley Associates, which is engineering the rescue pit in Seattle. The companies also collaborated to design a 115-foot-deep ring in rock and sand at a San Francisco water project.

Barring further major problems, STP expects to finish the four-lane highway link from Sodo to South Lake Union in fall 2016, under a $1.44 billion contract. The partnership, led by Dragados USA and Tutor Perini, has filed a $125 million claim related to repair delays, which the state Department of Transportation denied, setting the stage for prolonged negotiations or a legal battle.

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

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