A 7-foot-deep sinkhole opened above the Highway 99 tunnel machine early Thursday and was quickly filled.
The job site is between the Alaskan Way Viaduct and the Seattle waterfront, near South King Street.
There was no damage to buildings and roads, and no utility outages.
But a city electrical vault began to flood with concrete slurry, as workers were filling the hole nearby.
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Tunnel machine Bertha had just resumed drilling at 9:30 p.m. Wednesday, after a two-week pause for adjustments and to receive a new set of cutting teeth. The drill was passing through a temporary concrete wall that provided a “safe haven” for maintenance work.
As it pushed through the wall — into very weak fill soils — the forward pressure exerted by the machine was too low for the sudden change in conditions, and the ground slumped downward, said Dave Sowers, Highway 99 engineering manager for the state Department of Transportation (DOT).
Not quite one-half of 1 percent more soil than expected poured into Bertha through the cutter face, he said.
It’s difficult to achieve perfect balance, Sowers said. Too little pressure and the ground sinks. Too much, and dirt or fluids will burst upward.
“It certainly isn’t something we want to happen, but it’s not unexpected,” Sowers said.
Previously, a concrete slab protected and compressed the tunneling route from above. Now, Bertha is beyond the slab, with only 35 feet of soft soil above it.
If there has to be soil failure, this may well be the best place.
The viaduct is protected from tunnel vibrations and soil slides by a row of deep concrete pillars along its west flank — inserted specifically to protect against these sorts of problems during the first phase of tunneling.
But once the machine passes under the viaduct, in early 2014, even a slight soil settlement could jeopardize downtown buildings. Hundreds of monitoring devices have been installed to try to catch problems early.
The Thursday incident happened at 5:45 a.m. and crews filled the hole in about 45 minutes, Sowers said.
The affected area is 15 feet by 20 feet, DOT spokeswoman KaDeena Yerkan said.
Bertha continued to have one of its better days, advancing 33 feet as of 5 p.m. Thursday, officials said. The drill has traveled more than 460 feet since July 30, on its voyage to South Lake Union.
Rotten timbers, dumped a century ago near the waterfront, may have contributed to the unstable soil.
Sowers blames underground pressure and says contractors will learn.
“This hasn’t changed our opinion of STP (Seattle Tunnel Partners). We think they’re doing a good job.”
“The hole exposed an abandoned electrical conduit, and apparently fluid flowed through the conduit afterward. We have a vault being filled with slurry,” said Scott Thomsen, City Light spokesman.
Mike Lindblom: 206-515-5631 or firstname.lastname@example.org