Drill, Baby, Drill! How The Largest and Most Exciting Boring Machine Will Bury a Seattle Freeway

The tunnel will be able to withstand an earthquake of roughly 9.0 on the richter scale. The quake that damaged the Viaduct was a 6.8. After a 2001 earthquake damaged a double-deck section of State Route 99 running through Seattle, city planners decided to just bury it. Rather than rebuild the so-called Alaska ­Viaduct, they are replacing a two-mile stretch with a tunnel. So how do you dig a nearly-60-foot-diameter tunnel under downtown ­Seattle? With the world's largest tunnel-boring machine (TBM). "Bertha," as Seattle has named the machine, is currently being assembled in a pit at the head of its path and starts chewing through buried flannel shirts and Starbucks cups this summer. spare us the cutter Cutterhead With teeth optimized for digging the softer soil found under Seattle, Bertha's cutterhead grinds through the earth, collecting displaced dirt and rock in the excavation chamber directly behind her head. Conditioners are added to the soil in the chamber to create a goop. Articulation and Shove Jacks The task of steering such massive equipment beneath a populated downtown area falls to articulation jacks between the cutterhead and the rest of Bertha's outer shield. Shove jacks pushing against completed tunnel segments maintain face pressure and move the TBM forward. Concrete-Panel Erector Arms Giant arms fix the concrete-wall sections in place, building the tunnel one ring at a time. Each ring is completed inside Bertha's shield and sealed with grout as it is pushed outside. Most TBMs have just one erector arm, and it's only Bertha's sheer size that allows for the use of two. Soil Transportation Screw and Belt Conveyor The soil transportation screw essentially acts as a giant rotating scoop that moves the paste-like soil mixture from the cutterhead to the belt conveyor. The conveyor in turn transfers the muck out of the tunnel, where it is placed on barges headed to a disposal site. Trailing Gear and Supply Train Located behind Bertha's head are three "cars" that make up the TBM's trailing gear. The first car includes the control room, lunchroom, and workshop for Bertha's 25-member crew. The boring process is automated; the operator and two engineers provide technical oversight. Other tasks include maintenance, surveying, and operation of the tunnel-wall-segment delivery and installation. The second car contains the hydraulic power packs, electrical control panels, and backfill grout equipment. The third car carries high-voltage transformers, a water-regulating tank, and a temporary power generator. The State of the Unions Roads: An Investigative Report C/D Video: We Assault the Tunnels of Detroit with Nine (!) Mercedes AMGs Aaron Robinson: One Highway, Two Plans For Milking It What's Next? Seattle Tunnel Partners, the contractor that owns Bertha, hasn't released any official statement as to what the future holds for this TBM after the project is complete. Some components might be reused for future boring needs, but others will be sold or recycled as scrap metal. GETTIN' BERTHA'S DIGITS 25 Number of crew working inside the TBM 35 Number of feet Bertha will tear through on an average day 326 Bertha's total length in feet 7000 Bertha's weight in tons 9000 Length in feet of the belt conveyor by the time the project is complete 900,000 Cubic yards of soil Bertha will excavate by the time she completes the tunnel $80,000,000 How much Bertha cost to build 88,000,000 Pounds of total forward pressure generated by the machine's shove jacks from Car and Driver Blog http://blog.caranddriver.com

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