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The Highway 99 tunnel team is deploying a $20 million army of engineers, gauges, wires and gnome-sized computers to serve as the last line of defense against a catastrophic soil collapse.

Drilling will begin this summer on the two-mile tube below downtown Seattle, a few weeks delayed from the initial June 3 goal. The dig will start at Pioneer Square in shallow fill soil, gradually dropping to 250 feet below street level by the time it passes beneath the Pike Place Market en route to South Lake Union.

The first building above the path is 1 Yesler Way, a brick triangle that’s home to Al Boccalino restaurant and Runberg Architecture Group.

Technicians and electricians stopped by over several weeks to install on the roof an automated survey machine, which employees there call R2-D2, after the little droid in “Star Wars.” To check for motion, the machine continually pivots and bounces invisible laser beams off amber prisms that are mounted on the surrounding buildings.

“I don’t think this building’s going anywhere, honestly,” said Runberg office manager Anne O’Rourke.

Up on First Avenue, drilling rigs near Seattle Art Museum this week reamed holes to lower some of the project’s 120 extensometers. These are bundles of long wire attached to probes resting within the soil. If excavation causes a void, soil pressure will change and the probes will send signals to a control room in Sodo.

The team has other tools, most of them sensitive to within one-thousandth of an inch, said David Sowers, Highway 99 engineering manager for the state Department of Transportation:

Crack gauges that straddle existing cracks to measure whether they widen during construction.

Inclinometers, which are hollow rods inserted vertically along each flank of the tunnel bore. If soil moves, the meters will be nudged sideways.

Liquid level sensors, water-filled tubes placed in buildings and the BNSF train tunnel, to function like carpenter’s levels.

Tiltmeters, which are a foot long and mounted in basements to transmit data about changes in wall or foundation position.

Deep survey markers, as low as 300 feet, to help calibrate the other metering devices or the path of the drilling machine. These would come in handy after an earthquake, said Sowers.

Satellite-based interferometric radars, a recent technology meant to detect ground shifts to 1/8-inch accuracy. This is done by measuring whether peaks and valleys in radar waves bouncing back from the street line up with past measurements.

Satellites aren’t a primary tool, as the prisms are more sensitive. “But it gives us verification,” said Sowers.

A total of 200 buildings along the route will be equipped, and nearly 700 devices placed in the streets and sidewalks.

But the primary defense will be to measure the soil inside the machine, as it’s being removed by conveyor belt, as well as the earth pressure against the rotating cutter head.

A few years ago, soil-measuring errors caused a void that nearly swallowed a house above Sound Transit’s Beacon Hill Tunnel. Contractor Obayashi Corp. had to reimburse taxpayers $4 million to locate and fill gaps.

Soil sinkage has caused disasters elsewhere, notably the March 2009 collapse of the city archives building and nearby apartments in Cologne, Germany, which killed two people. Seattle tunnelers are limited by contract to 1/2-inch of soil settling for fragile sites such as Al Boccalino, and 1 inch elsewhere.

“I don’t expect the project to go perfect,” says Sowers. “But I also don’t expect us to see large deformation in any of the buildings.”

At this point, says O’Rourke, workers of the businesses at 1 Yesler Way worry less about the tunneling risks than what will follow in 2016, when the Alaskan Way Viaduct is demolished a few feet away.

Mike Lindblom: 206-515-5631 or