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.
The University Link project, according to Gray, is between six and nine months ahead of schedule and roughly $108 million under budget.
“The U Link sets the bar kind of high,” said Davis, the executive project director for the Northgate extension. He adds, “The challenge in any tunneling job, is unknown conditions.” Asked how the next segment of tunneling would differ from the University Link project, he said: "If there’s going to be big differences, it’s going to be in the ground that we encounter.”
The soil is expected to be especially soft on the first section of the drive, between the Maple Leaf portal and a future station-site near 12th Avenue NE and NE 65th Street. Roger Johnson, a Sound Transit tunnel inspector, was at the portal-site last Friday. Standing near Brenda, the 21-year tunneling industry veteran said, "I think this is going to be a fairly sandy run."
Looking north at the Maple Leaf portal site, from atop the headwall where Brenda will begin digging. Photo: Bill Lucia
Both Brenda and Bertha are classified as earth pressure balance boring machines, which are designed specifically for mining in soft and unconsolidated soil. Brenda has been retrofitted and modified since the Capitol Hill to downtown job, in part to make the machine better suited for the ground conditions at the north end of the drive.
One component that the contractor decided to change, Johnson said, is the "screw conveyor." The mechanism looks like a corkscrew-shaped ribbon. It moves excavated muck from the front of the machine to its rear, dumping it onto a belt conveyor that carries the material out of the tunnel for disposal. The modified screw conveyor is designed to be less prone to clogs in soft dirt.
When boring machines dig through soft earth, it’s not uncommon for the ground above to settle slightly. And in rare instances boring can cause holes to form above ground. After Obayashi Corp. dug a stretch of Sound Transit light rail line under Beacon Hill in 2008, voids began to appear in the neighborhood. The transit agency spent more than $1.5 million repairing the holes and also ended up buying a home with a cracked foundation.
Davis notes that compared to the machines used on the Beacon Hill job, Brenda and TBM 2 rely on more advanced monitoring technology to make sure soil is being excavated at rates that reduce the risk of surface holes. While Davis said that ground settlement of greater than an inch is not expected on the Northgate project, he also acknowledged, “You can always hit a pocket of something you don’t expect.”
Sensors will be placed above ground along the tunnel route to monitor settlement, and officials already know there will be some sensitive areas. “We’re directly under the Neptune Theater," Davis said. "We’re under a couple old apartment buildings, where we’re looking at ground improvement techniques.” Those techniques could include injecting cement-based conditioners into the soil, or freezing the ground near the buildings as the Brenda and TBM 2 pass through. “We don’t expect to see any structural damage,” Davis said. “Maybe some aesthetic cracking.”
Bertha's main bearing was compromised after a set of protective seals failed, allowing grit inside. On a boring machine, the main bearing enables the cutter-head to spin and dig. Brenda's main bearing is new. And the Northgate extension contract requires JCM to have a spare on hand. To meet that requirement, the contractor refurbished the bearing used during the Capitol Hill to downtown work. “The problem with the bearing,” Davis said, “is that even on a small machine like this it takes six to eight months to order a new one.”
JCM also plans to have an entire spare machine on standby. Both Brenda and TBM 2 will surface near 12th Avenue NE and NE 65th Street, before continuing on the last leg of the drive toward Husky Stadium. This pit stop will provide an opportunity to make repairs and, if necessary, swap in the spare machine.
Johnson expressed cautious optimism about Brenda and the upcoming dig. “It did a good job,” he said, referring to the machine's performance on the U-Link tunnel. He then adds: “I hope it does a good job again.”
