A Sound Transit tunneling machine will break through to the future Capitol Hill light-rail station this week, but not before passing under an 88-year-old brick building. The owner worries that vibrations or soil settling may crack the masonry.

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For the last 1 ½ years, Franklin Tseng has worried that shifting soils from Sound Transit’s tunneling project would crack his brick apartment building atop Capitol Hill.

Finally, the day of judgment has arrived.

A 21-foot-wide drill on Monday is expected to pass beneath the five-story Capitol Building, built in 1924 at the corner of Broadway and John Street. By Tuesday night or early Wednesday, the machine should break through at its destination, the future Capitol Hill Station.

The station, scheduled to open in 2016, will bring light-rail into the most densely populated neighborhood in Washington state. The Link line, which now runs from Seattle-Tacoma International Airport to Westlake Center, will continue north to the transit-dependent University of Washington. This three-mile extension is expected to carry at least 70,000 riders per day by 2030.

Tunneling projects have long penetrated Seattle’s glacial soil to make way for light rail, freight trains, sewers and highways. Next year, a machine will embark on the Highway 99 tunnel beneath downtown.

In this case, the combination of an old brick building and shallow tunnel depth — the twin tubes will be only 45 feet below Tseng’s basement — make for a particular challenge.

Sometimes in tunneling projects, soil or the water table will move, causing buildings above to sink.

Because the Capitol Building is unreinforced masonry, even a settlement of less than 1 inch could crack it down the middle, where the tunnel passes under the building’s east side, says Tseng, a licensed civil engineer.

“It’s so shallow, probably half the building will settle and half not,” he said.

Sound Transit answers that it has carefully prepared for the Capitol Building and done everything possible to avoid damage.

The $1.9 billion rail extension is on budget and on schedule to open in 2016. Last May, the lead tunneling machine, nicknamed “Togo,” left the UW Station pit to drill the southbound train tube, followed by its twin, “Balto,” grinding the northbound tube — they’re named after two heroic huskies who delivered serum to avert a diphtheria epidemic in Nome, Alaska, in 1925. Farther south, a third machine has drilled from Capitol Hill to the Downtown Seattle Transit Tunnel, and is making a second trip.

Last fall, neighbors in the Montlake area felt vibrations from supply trains that carried concrete segments for the tunnel wall. And after Beacon Hill tunneling a few years ago, it cost $4 million to investigate and fill several voids, one of which nearly swallowed a house. (Sound Transit recouped the money by withholding it from final payment to the contractor.)

On the other hand, tunnel machines recently maneuvered a mere 20 feet beneath both Interstate 5 and the Montlake Cut.

“Both of those drills were shallower than what we’re doing under his building, and we accomplished those without incident. So that gives us a great deal of confidence,” said transit spokesman Bruce Gray. A machine nicknamed “Brenda” is now passing uneventfully beneath masonry buildings, including the Egyptian Theatre, he said Friday.

If the Capitol Building is damaged, Sound Transit’s insurance or funds in the construction budget would pay, Gray said.

Tseng’s corner building contains 52 apartments and two clothing stores. Tseng grew up in Seattle, but lives mostly in Hong Kong now. His family bought the structure in 1991. Its assessed value is $4.8 million — Tseng says he’s been offered far more but intends to keep the building for the next generation.

Sound Transit paid Tseng $230,000 for the right to run trains below his building, but he says he’s spent more than that on engineers to study the potential damage from tunneling.

A core dispute is over how the building might react if the soil moves a small amount. One geotechnical engineer hired by Tseng said the west and east sides may settle at different rates, cracking the whole structure. Another predicts the upper brick masonry might separate from the solid foundation.

“Anything can happen, and we don’t know how big the vibration is,” Tseng said. “I’ve got four engineers telling me I’ve got problems.”

Tseng contrasts the transit drilling with the Highway 99 tunnel project, where the state’s engineers published a roster of fragile downtown buildings and demanded that bidders explain how to protect each.

He wanted Sound Transit to spend $1 million or more on compensation grouting, in which tubes are inserted around a building, then concrete grout is pumped in whenever soil settling is detected. Or chemical grout could be injected to harden the soil.

Joe Gildner, deputy light-rail director, said excessive grouting could backfire by causing a structure to float upward.

He says Sound Transit has learned the “hard lessons” from Beacon Hill — and taken sufficient precautions this time.

At the station pit, workers have poured a concrete-grout wall, a full 40 feet thick at the breakthrough point, which should preclude any soil slide from spreading across John Street to Tseng’s building, Gildner said. Sound Transit and contractor Jay Dee-Coluccio-Michels plan to drill 24 hours continuously in the final stretch, to reduce risks related to stopping and restarting.

Sound Transit has installed eight sensing devices for the building, including probes that would register any soil shifting at the tunnel depth.

A half-block away, the tunneling hasn’t disrupted construction of a 224-unit, eight-story building. Project engineer James Wilkins said there have been no tunneling effects, and he isn’t expecting any.

“They’re professionals. I’m thinking they’re doing everything according to their plan,” he said.

Last week, air from the pressurized cutter head reached the surface and blew out some mud. Gray said the pressure has since been reduced and is not a threat to structures.

The Capitol Building has survived earthquakes. Still, Tseng was worried enough to fly in from Hong Kong this month. He will be sitting quietly in the basement office, to feel any vibrations heading his way.

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