Augers, tubes and a crane are standing by at the Highway 99 tunnel job site, to help workers drill 60 feet deep and remove the unidentified buried object that blocks boring machine Bertha.
The job is likely to take several days. It will take until Friday for the Seattle Tunnel Partners contracting team to identify the object and decide how to proceed, said Linea Laird, chief engineer for the state Department of Transportation.
It could be a large boulder. A more exotic scenario is that Seattle settlers maybe buried a boat hull or a train car. Not only will tunnel workers need to find the buried object, but it must next be lifted or broken.
The team is considering whether to drill vertically from above, horizontally from the tunnel machine, or both, said Laird. Bertha is equipped with small drills that can extend forward, and workers could extend handheld pneumatic tools through gaps in the rotary cutting face, she said.
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There’s more to the task than simply busting the clog. Afterward, crews will probably have to inject concrete grout into the Earth, to fill resulting gaps. Bertha requires soil resistance as it moves ahead, to help it hew to a precise course.
The tunneling machine ran into the object Friday evening between South Jackson Street and South Main Street. The stall happened shortly after drilling passed the 1,000-foot mark, on the 1.7-mile route from Sodo to South Lake Union. Last week the machine kept an above-average pace of 52 feet per day, said DOT spokeswoman KaDeena Yerkan.
Highway 99 Administrator Todd Trepanier said that by Saturday, the drilling effort “was stopped after unanticipated and increasing resistance was experienced, possibly due to an obstruction.”
Contractors have been paid at least $730 million to date, of the $1.44 billion contract to complete the four-lane tunnel by the end of 2015. Before construction, soil engineers tested hundreds of sites. Three to five samples were taken per city block along the waterfront, according to a geotechnical report. Unsurprisingly, the mystery object sits 120 feet south of South Main Street, between test points.
Boulders of more than 8 feet in diameter, left by glacial flows, are within the tunnel route, the report says.
The tunnel drill’s rotary cutting face is designed to scour and crack them by using two dozen or so steel cutting discs, shaped somewhat like the plowing discs farmers use. Bertha’s discs have a double-edged shape instead of the common single edge. Twin blades are meant to bite chunks in the rock edge, to break it quickly, said Aaron Shanahan, cutter engineer for the Robbins Co. in Kent, which supplies them. But the shallow, weak soil near Elliott Bay makes buried objects fickle, so they might jiggle or rotate when the machine makes contact.
“The boulder has to be held by something to be broken up,” he said. “Any boulder in soft sand is going to be difficult to handle, because it has little support.”
In that event, the powerful drill face might spin weirdly without success.
“The torque will start going up in the machine, and the operator will shut it down,” to avoid damaging the $80 million machine, said John Reilly, former president of the American Underground Construction Association.
Steven Kramer, a geotechnical engineering professor at the University of Washington, said that normally, there ought to be enough good soil to hold an object firmly, but “soils are deposited in erratic ways” around here. Soil studies indicate that fill dirt from the Denny Regrade, circa 1898, is about 25 feet deep, and very soft above the tunneling machine.
The possibility of a buried rail car seems remote because settlers would have had to excavate 30 feet or so, before the Regrade, to bury something deep enough to meet Bertha in 2013. DOT’s geotech studies didn’t locate big man-made objects elsewhere, Laird said.
Bertha is unable to chew through steel. A railcar or metal beam would need to be plucked out. It’s unknown whether the blockage will lead to cost overruns. Tunnel builders might conceivably file a claim based on “differing site conditions” that state studies didn’t predict. The Highway 99 contract includes contingency funds for such cases.
Mike Lindblom: 206-515-5631 or firstname.lastname@example.org. On Twitter @mikelindblom