Workers have poked the soil in six locations in front of the stranded tunneling machine Bertha, but their probes haven’t bumped into any huge obstructions yet, a senior engineer said Monday.
Those results increase the mystery over what might be blocking the world’s biggest tunnel machine. Project leaders initially speculated that an unusually wide boulder, deposited by ancient glaciers, might be in the way.
Progress on the Highway 99 tube has stalled since Dec. 6, when the rotary cutting blades of the 54.3-foot-diameter drill spun without scouring much sediment between Pioneer Square and Elliott Bay.
The probe holes are being drilled 110 feet deep and spaced five feet apart, from east to west. A hole-digging rig was out Monday morning to take another poke, and more test holes will be drilled Tuesday.
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“We’re not hitting anything unexpected,” said Dave Sowers, tunnel-engineering manager for the state Department of Transportation. If a school-bus-size boulder were there, he said, the probes should strike it.
Meanwhile, contractor team Seattle Tunnel Partners intends to send workers into Bertha’s flooded front end Thursday to inspect for the blockage through gaps in the cutting head, Sowers said. Many crew members were gone on holiday, so contractors chose to wait for their return before starting the inspection, he said.
The tunnel team hopes to remove enough groundwater so the inspection can be done at normal air pressure. Ten wells have been installed around the buried machine to extract the silty water that permeates this stretch of the route.
Bertha’s cutting head, and a 5-foot-wide chamber where excavated soil enters a conveyor system, are flooded with roughly 90,000 gallons of water and mud.
The goal is to lower the water table enough so the top half of the cutter is exposed for a clear view, Sowers said. The cutter then can be rotated so that the other half can be viewed. “You’ll be able to see whether spokes are clogged,” he said.
The soil contains enough groundwater to create hyperbaric pressure, similar to working deep in the ocean. Tunnel-trained divers eventually may be needed to hammer or blast away a boulder, Sowers said.
Chris Dixon, project manager for Seattle Tunnel Partners, has said the team notified Ballard Marine Construction to have divers available in January.
The white-colored well tops protrude a few feet above the surface, clamped to thick hoses alongside the Alaskan Way Viaduct. Silty water is dumped into a green holding tank, where sediments can settle out. The remaining water is released into the city stormwater system.
The four-lane tunnel from Sodo to South Lake Union is supposed to open for traffic by the end of 2015, a timeline Sowers said is still possible.
Officials say it’s too early to know whether the stoppage will lead to overruns on the $1.44 billion construction budget, or how big those might be.
While Bertha is stalled, the builders are trying to gain time on other tasks. About 20 blades are being replaced, by workers who reach them by crawling within the hollow spokes of the cutter. Temporary concrete rings and steel frames, used by Bertha to make its initial push, need to be removed from the launch pit in Sodo.
Mike Lindblom: 206-515-5631 or firstname.lastname@example.org. On Twitter @mikelindblom