Nearly three days after an unknown object blocked tunnel-boring machine Bertha, project managers haven’t yet determined the size or how to remove it, according to the state Department of Transportation.
“We don’t know what it is. We don’t know whether it is man made or natural,” DOT spokeswoman KaDeena Yerkan said.
Drilling halted Friday night, about 60 feet deep, along the Seattle waterfront between South Jackson Street and South Main Street. In normal conditions, the team might try a “hyperbaric intervention,” meaning that the tunnel machine could retreat 18 inches, and then divers would explore gaps around the cutting face, at high pressure. (Tunnel projects keep specially-trained divers on call, to work in air and soil that exceeds atmospheric pressure – Bertha includes three hatches where drivers can move to the machine face.) But in this case, there is watery sand and weak fill soil immediately above the machine. So if high air pressure were exerted in front of the machine, the air would push or burst through the soil, said Yerkan.
The problem was discovered Friday night, and reported to state DOT Secretary Lynn Peterson, who was updated on Monday.
“STP (Seattle Tunnel Partners) has not made a decision on how they’re going to move forward yet,” Yerkan said. “They’re talking to their experts, we have been talking to ours.” Chris Dixon, STP’s manager, hasn’t yet responded to messages requesting comment.
The cutting face, at 57 feet, 4 inches, is the widest in the world. It’s equipped with steel cutting discs to scour and crack boulders, but apparently can’t defeat the large obstruction. Fragments less than three feet diameter can slip through openings in the cutter head, and be removed out the back of the conveyor system.
The tunnel route was intensively sampled by soil engineers from Shannon & Wilson long before the project started, but apparently their narrow test shafts didn’t strike this object. The soil at 60 feet down is considered clean, glacial sediment, but most of the soil above is unstable fill, including wood debris from industries more than a century ago, and spoils from the Denny Regrade in 1898.
One scenario might be to simply excavate from the surface, and pluck out what’s in the way — since the soil above is useless anyhow. A blue crane was being assembled nearby on Terminal 46 Monday, but it’s unknown whether that’s related to the drilling.
Another response could be to send in crews with pneumatically driven drills, hammers or other tools to break the large object. But that task would be hampered by the loose soil, Yerkan said.
Since its start on July 30, the deep-bore tunnel project has advanced more than 1,000 feet, or close to one-eighth of its total distance from Sodo to South Lake Union.
“The machine is running well, it’s functioning,” Yerkan said. There have been no reports of damage to the drill face, where eroded steel discs were recently replaced during routine maintenance.
By early 2014 the machine is supposed to dive under the Alaskan Way Viaduct, causing a temporary highway closure and potential risks of vibration to old brick buildings nearby.
Seattle Tunnel Partners, led by the US branch of Spanish-based Dragados and by California-based Tutor-Perini, has been paid $730 million as of September, or about half the total $1.44 billion contract value, according to a state chart, released under a public-document request.
Asked why officials waited two days to disclose the problem, Yerkan offered two theories: some project staff were gone during the weekend, so Monday was the logical time to regroup for an update; and the team may have wanted to come up with some progress or strategy to offer, before reporting to the public there was a hitch.