Seattle will no longer be a one tunnel-boring machine town. While Bertha, the massive rig digging the Highway 99 tunnel sat idle under downtown in recent months awaiting repairs, a Sound Transit contractor has been quietly assembling a similar but smaller machine, which will soon begin tunneling at the north end of the city.
Unlike the oh-so-much-better-known Bertha, the machine on the Sound Transit job comes with a proven track record, having successfully completed other digs for the agency in recent years. The new Sound Transit project, known as the Northgate Link Extension, involves digging twin, 3.4-mile light rail tunnels beginning near NE 94th Street and First Avenue NE, and ending at University of Washington’s Husky Stadium. “Brenda,” the first machine set to launch, is scheduled to start mining in June. Another machine, known for now as TBM 2, is slated to begin digging the second of the twin tunnels sometime around October.
Workers are putting the final touches on Brenda, a 21-foot diameter boring machine. Photo: Bill Lucia.
When the Northgate extension is completed, Sound Transit's Link light rail service will run all the way from NE 103rd Street to just beyond Seattle-Tacoma International Airport to S. 200th Street in SeaTac. The light rail currently runs from the airport to Westlake Station in downtown Seattle. The total budget for the Northgate extension is $2.1 billion. Including the aboveground portions, the new stretch of light rail track will run 4.3 miles. Sound Transit is planning to have tunneling completed by 2016 and the extended rail service operating by 2021.
Getting the project done will rely partly on Brenda. With a 21-foot diameter cutter-head, the machine looks like a pint-sized replica of Bertha. At 57.5-feet wide, Bertha is the world’s largest tunneling rig. But a damaged main bearing assembly has left the machine hamstrung after about 1,000 feet of digging. Bertha has barely moved since early December and will be partially unearthed to undergo repairs.
Hitachi Zosen Corp. manufactured Brenda and Bertha in Osaka, Japan and both machines use similar technology to chew through the earth.
Assembling Brenda has taken about three and a half months so far, according to Sound Transit's executive project director for the Northgate Link Extension, Don Davis. It should be completely put together in about one week, a spokesman for the agency, Bruce Gray, said Friday. After that it will be tested. The machine is currently parked across from a shopping plaza in Maple Leaf, just south of NE 100th Street, next to Interstate 5. A few blocks further south, near NE 94th Street, is a roughly 70-foot concrete headwall where Brenda will begin tunneling.
On Friday, hard-hatted workers came and went from the rear of Brenda’s can-shaped forward shell. The sun shone on the freshly painted green-and-yellow cutter-head. Valve fittings were laid out on a shop rag, a worker cut a piece of metal with a torch, hoses hung down from the system that will be used to inject soil-conditioning grout in front of the machine as it mines forward. Extending rearward was Brenda's "trailing gear," which houses the operator cabin, parts of the electrical and soil conveyor systems and other support equipment. With the trailing gear, the machine is about 300-feet long.
The Northgate project will actually be Brenda's third trip underneath Seattle. In 2011 and 2012, the machine dug a pair of Link light rail tunnels from Broadway and East Denny Way in Capitol Hill to a location near Pine Street and Interstate 5 in downtown.
Each of the two tunnels was a segment of Sound Transit's University Link Extension, which will eventually connect the existing light rail service that ends at Westlake Station with the University District. The two tunnels Brenda mined were both about three-quarters of a mile in length. The first was completed in 134 days, according to Gray. After the machine finished the first drive it was trucked in pieces back to Capitol Hill and re-launched. The second tunnel took 96 days to dig. Jay Dee Contractors, Frank Coluccio Construction and Michels Corp., or JCM, the contracting consortium that constructed those tunnel segments, was also awarded the Northgate contract.
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