A buried steel pipe is mostly to blame for stopping the giant tunnel-boring machine, Bertha, which has been stuck since Dec. 6 along the Seattle waterfront near South Main Street.
The pipe is an 8-inch diameter, 115-foot-long “well casing,” used to measure groundwater during studies in 2002 on the Alaskan Way Viaduct replacement project, tunnel project officials said.
Matt Preedy, the deputy project administrator for the state Department of Transportation, said he had no estimates about how much time and money it will take to remove the rest of the pipe and to repair damaged cutting tools on the face of the machine.
Nor does the team have a strategy yet for how removal should take place. One possible method is to send tunnel-trained divers to work near the cutter face, under extreme pressures that are exerted by groundwater.
The well site was listed in reference materials provided to bidders as part of the contract specifications, DOT says.
“I don’t want people to say WSDOT didn’t know where its own pipe was, because it did,” said DOT spokesman Lars Erickson.
However, Chris Dixon, project director for contractor group Seattle Tunnel Partners (STP), said the builders presumed there would be no pipe in the way, because casings are customarily removed after use.
Dixon said the tunnel-machine crew first noticed metal pieces in Bertha’s conveyor system in early December – when Bertha’s rotation actually shoved a segment of pipe through the surface, prompting crews to remove a 55-foot-long piece. However, the machine kept grinding forward just fine, Dixon said, leading STP to have what he called “a false sense of security” that things would be OK.
But then on Dec. 6, the cutting face rotated without catching soil. The team later found unusual damage to cutting teeth. On Thursday night, an inspection found a pipe fragment jutting through spaces between spokes of the cutter.
A modern tunnel machine can chew through dirt and concrete, but not steel. Even fiberglass rods caused a snag that delayed work several days this summer. Steel could become tangled in the spokes of the rotary cutting head and in a conveyor screw that pushes dirt from the cutter face onto a belt that moves out the rear of the machine. Dixon said Friday that “we don’t know” if any moving parts are jammed.
Downtown Seattle contains some of the most frequently poked and studied ground on earth, which makes the blockage all the more confounding. Five-foot diameter holes were drilled alongside the tunnel path to install concrete pilings that protect the old viaduct; the contractors have used ground-penetrating radar; and geotechnical experts drilled test holes, which didn’t hit this particular object.
For the past four weeks, DOT didn’t mention the pipe during several news interviews and press conferences. Asked about this, Preedy said Friday that initially, the project’s expert review team thought the pipe was a secondary issue, and that a giant boulder seemed more likely. At 60 feet down, the top of Bertha is in glacial soil, beneath the extent of fill soils and debris that early Seattle settlers dumped near Elliott Bay.
The $1.44 billion tunnel construction, from Sodo to South Lake Union, is about three months behind schedule. But Bertha in November was advancing as fast as 50 feet a day, prompting Preedy to say it’s possible to regain time after the steel is removed.