Sound Transit's tunneling work inside Beacon Hill has opened up seven underground voids.

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Sound Transit’s tunneling work inside Beacon Hill has opened up seven underground voids — one of which recently tore a 21-foot-deep hole beneath a neighbor’s front yard.

The gaps were a delayed result of deep sand flows that happened as a giant grinding machine bored through the hill. The twin train tunnels run mainly through stable clay, but the machine hit some pockets of sand, said transit spokesman Bruce Gray. The sand flows wound up being excavated though the machine’s conveyor belt, he said.

All the voids are within the two blocks east of the hilltop station, and all have been filled, Gray said.

Voids and shifting soil remain a concern as Sound Transit prepares to bore a three-mile tunnel from Westlake Center to Husky Stadium. Sandy patches exist within Capitol Hill.

Gray said Sound Transit will increase monitoring and stop boring to investigate if any anomalies happen in soil removal.

Washington state has just decided to bore a Highway 99 tunnel from Sodo to South Lake Union, to replace the Alaskan Way Viaduct. Soil research is just starting there, but officials know they face soft-fill soil near the Sodo portal, followed by strong glacial tills downtown, mixed with sandier areas.

“It’s certainly one of our more sensitive issues, and one of our issues that has to be managed very closely,” said John White, the Highway 99 tunnel-program director.

A sinkhole swallowed a Kenmore driveway in March during construction of a tunnel for King County’s Brightwater treatment project.

Soil settlement is a well-known bugaboo for tunneling firms. The topic surfaced Monday at an Underground Construction Association forum in Seattle, full of would-be Highway 99 contractors.

In an extreme case, contractors in downtown Shanghai, China, rebuilt the foundations of the historic Suzhou Creek Bridge, and injected grout into soggy soils near the Bund — the European-style downtown — before drilling began.

Sound Transit will open its first light-rail line July 18, passing through Beacon Hill on its route between Westlake Center and Tukwila.

Holes in the yard

Christine Panganiban was tending the daffodils and tulips in her front yard March 29 when she noticed a small hole next to her yellow house at 18th Avenue South near South Lander Street.

“I stuck the shovel handle in the hole and couldn’t feel the end of it,” she said. A neighbor shone a flashlight inside. “I realized there was only 5 or 6 inches of earth between me and the hole. I started backing away from it,” she said.

The hole turned out to be 21 feet deep. Sound Transit found a second void in her side yard, and filled that too. The discovery of the hole was reported last month by Tuesday, crews were in Panganiban’s front yard, drilling a test shaft at an angle, to search for possible voids beneath the yellow house. A second drilling crew worked near the station.

Only one of the seven voids caused a hole on the surface, while the rest appeared 20 to 65 feet down, said Gray.

Besides test drills, consultants have used a “super-, super-, supersensitive” accelerometer in 100 spots to find any underground gaps, said Kirt Hansona, senior engineer with the Shannon & Wilson firm.

The soil testing and filling is costing $1 million.

The southbound tunnel was finished in May 2007, the northbound twin in March 2008. So far the construction costs for the twin tubes and two stations total $307 million for the one-mile Beacon Hill segment.

As boring proceeded, there were times when more soil than normal passed through the conveyors. Contractors attributed the spikes to a measurement glitch, but in hindsight they turned out to be sand flows, said Gray. Sound Transit monitors buildings on the hilltop for settlement, but none occurred, he said.

Inside Capitol Hill, “any time we see any sort of spike, we’ll likely stop and investigate what’s happening,” Gray said.

For the Highway 99 tunnel, White said there’s a good chance builders will replace some fill soils in Sodo with lightweight concrete, where the highway enters a shallow south portal. Bidders will be expected to design a custom-boring machine to match downtown conditions.

If a boring machine hits sand, it could release grout to harden the soil, White said. Contractors will be expected to slow down in soft soils, so as not to excavate too much. And the state would do its own monitoring of surface settlement and soil removal — as a “check and balance” to the contractor’s own measurements, White said.

“There are plenty of projects delivered in sand, where there haven’t been big settlement problems,” he said.

Mike Lindblom: 206-515-5631 or