Seattle Tunnel Partners says Bertha should resume drilling the Highway 99 tunnel this week, after problems that included a barge mishap and, now, a sinkhole.

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The manager of the Highway 99 tunnel dig promises to keep a closer watch on the soil removed by boring-machine Bertha, after a sinkhole formed Tuesday night near the waterfront.

That incident happened the same day as a Seattle Tunnel Partners (STP) barge listed while being loaded with tunnel muck alongside Terminal 46. The tilting barge dislodged at least 15 concrete pilings and spilled soil into Puget Sound.

Such foibles raise the question about whether more trouble awaits Bertha — which just started digging again after extensive repairs and will soon burrow under the Alaskan Way Viaduct.

“What can I say? Some things happen. We still remain confident in everything that we’re going be doing forward, to complete the tunnel drive,” STP project manager Chris Dixon said Wednesday.

He said dirt will be manually measured every 6½ feet, while the machine stops to assemble each new ring of the tunnel tube. Those calculations will be compared with the theoretical amount of soil that Bertha ought to remove per ring.

Dixon said STP was performing the manual measurements earlier in the project.

If soil flow is bungled as Bertha moves north, sinkage or voids could destabilize the viaduct and brick buildings in Pioneer Square.

“We’re certainly disappointed they [STP] allowed it to occur,” said Brian Nielsen, deputy Highway 99 administrator for the Washington State Department of Transportation (WSDOT).

Voids can occur when too much dirt falls into the rotating cutterhead of a cylindrical drill, and the soil overhead settles.

STP’s bid proposal calls for multiple soil checks, including both scales and lasers. The state seeks to prevent the kinds of voids that appeared in the Brightwater sewer project, and almost swallowed a house above Sound Transit’s Beacon Hill Tunnel.

Tuesday’s sinkhole was 35 feet long, 20 feet wide and 15 feet deep, and filled by 250 cubic yards of concrete-sand mix.

Dixon wouldn’t quite call it an oversight, but said:

“The focus and everybody’s attention has been on the machine and how it’s performing, and whether it’s operating within tolerances, whether we’re controlling the heat in the machine, checking the gaps on the seals, making sure the machine is repaired and able to complete the tunnel drive.”

Bertha is two years late on its journey from Sodo to South Lake Union, where contractors are building the world’s widest single-bore tunnel, at 57 feet, 4 inches diameter, to replace the old, seismically vulnerable viaduct. The four-lane, tolled tunnel is now estimated to open in April 2018.

A much smaller sinkhole opened on the project in November 2013, but it caused little alarm because of Bertha’s location in shallow fill soil.

WSDOT said neither of these harmed the nearby viaduct, which is protected by an underground fence of concrete pillars that STP poured early in the job, for just this purpose.

But what’s to prevent future, more destabilizing voids while digging below downtown?

If future soil measurements don’t jibe, Bertha’s operators could extend small probe drills from the cutterhead to search for voids. Grout can be injected to fill gaps in front of the drill, Dixon said.

Also, the machine is descending toward tighter, less watery glacial clays. “We’ll be in much better soil, with every foot we advance on Alaskan Way.”

Bertha relies on a conveyor-belt system to carry the excavated dirt out the rear of the machine and to the waterfront, where a barge awaits at Terminal 46.

STP will investigate what led the barge to tilt early Tuesday, during soil loading. Dixon said he doesn’t have an explanation yet.

WSDOT leases the 5-acre loading area at Terminal 46 from the Port of Seattle for about $1.1 million per year. As with any tenant, the port expects the tunnel project to pay for damage to the pilings, which were installed in 2008, Port spokesman Peter McGraw said.

For now, it’s the dock damage, not the sinkhole, that’s making Bertha pause.

“It could be a couple days,” Dixon said.

He’s pursuing two methods to get Bertha grinding again:

Contractors have ordered rubber bumpers, five to six feet in diameter, and hot-dog shaped. These would form a cushion between soil barges and the wood-timbered main pier at Terminal 46. That would allow shipment to resume to a quarry near Port Ludlow, where soil is being dumped.

Or, concrete-contaminated soil that’s on the pier can be removed by truck so that Bertha’s conveyors can drop clean soil there.

Some lawmakers were notified about the barge incident by senior transportation officials.

“It sounds to me like they’re dealing with the situation as best they can,” said Rep. Ed Orcutt, of Kalama, Cowlitz County, and ranking GOP member on the House Transportation Committee. “I have concerns about what caused all this,” he said. “I don’t know who failed at what.”

Bertha news

Despite the setback, Dixon said he expects Bertha to reach a designated stopping place, at about Yesler Way, for inspections at the end of this month, to keep to the post-repair schedule.

Then the machine will burrow under the foundations of the viaduct. State transportation officials expect to close the elevated highway for two weeks in March as a precaution.

Todd Trepanier, Highway 99 program administrator, told the Seattle City Council this past month that halting traffic will make Bertha’s electronic measurements more accurate, by reducing vibrations — and would make it easier to inject grout in case the ground sags.

That kind of worry just took on extra importance.