Steel pipe, plastic and big rock found inside tunnel machine
Workers have found a steel fragment, plastic and a large rock or boulder inside the cutting face of stalled Highway 99 tunnel machine during inspections.
Seattle Times transportation reporter
Workers found a tangled fragment of steel pipe, plastic and a large rock or concrete piece during the first 35 hours of inspections inside the cutting face of Highway 99 tunnel machine Bertha.
The discoveries, made since Friday, suggest more junk might appear in the next few days.
Seattle Tunnel Partners is trying to determine why the machine has been stalled since Dec. 6, when the cutter head quit grabbing dirt. Three days earlier, the machine hit a pipe along the central waterfront.
On Tuesday, the state Department of Transportation (DOT) issued its first status report since inspections began to look for obstructions or damage inside the cutting machine.
The state update Tuesday said:
“While the inspections are under way, it is too early to speculate on what led to the tunneling stoppage.
“Over the weekend, crews inspected a portion of the cutterhead as well as the cutting tools they were able to access. They also cleaned the spokes and removed a bent piece of metal well casing and plastic PVC pipe. Crews also identified a large boulder or piece of concrete material in a cutterhead opening.”
DOT would not elaborate or make staff members available to answer questions Tuesday about the size or importance of the buried objects.
The search for answers has dragged on because the machine’s 5-foot wide mixing chamber, where dirt enters the conveyor system, was flooded with mud and groundwater.
Ten temporary wells had to be drilled just to pump enough water away from soil around the machine that workers could view the cutter from inside.
Teams of five or six workers are cleaning and inspecting the rotary cutter at 1.4 times normal atmospheric pressure, more than 60 feet underground. Compressed air is being blown into the area to form a sort of bubble surrounding the cutter head — a work zone.
On Sunday, a gradual loss of air pressure occurred, so the chamber needed to be resealed and refilled with air, the update said. DOT spokeswoman Laura Newborn said this air loss wasn’t unusual, nor did it pose a safety hazard.
The state wouldn’t describe the rock or concrete piece in detail Tuesday. The $80 million Hitachi machine was designed without an internal rock crusher, on the premise that boulders 3 feet or bigger would stay lodged in the soil, and get cracked apart by discs on the spinning face. But it’s possible for STP to break apart a recalcitrant rock using power drills or hammers.
Steel pipe was left behind from groundwater research conducted for the state years ago, before excavation began on the new four-lane tunnel — but DOT didn’t divulge for four weeks that STP hit it on Dec. 3.
STP has blamed the stoppage mainly on the 8-inch-diameter pipe. Steel is capable of gouging cutting tools or becoming tangled in moving parts.
State officials have called the pipe only a partial factor but won’t say what else they think is wrong. Transportation Secretary Lynn Peterson last week said the state had concerns since summer about how STP was operating the machine.
Plastic PVC pipe was used in previous groundwater studies and wouldn’t ordinarily be considered a threat to a tunnel drill.
It’s likely several days of inspections remain.
Groundwater and muck cover the lower half of the 57-foot-diameter tunnel drill. Once the top half is inspected, the plan is to rotate the cutter 180 degrees, to inspect the half that was submerged.
Inspections began Friday afternoon. The work proceeds slowly, in part because scaffolding and safety equipment must be set in place for people to enter the work zone near the crown of the machine, and descend nearly 30 feet.
Mike Lindblom: 206-515-5631 or firstname.lastname@example.org. On Twitter @mikelindblom