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Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →In a July 23, 2013, on-site report, New Atlas took readers inside Bertha, the custom tunnel boring machine built for Seattle’s SR 99 project. The machine was then described as the world’s largest TBM; that is a historical description, not a verified current ranking. The walkthrough explains how Bertha excavated soil, managed pressure at the tunnel face, protected the work area and installed the concrete lining.
Why Bertha was built
Bertha was designed to bore a tunnel under central Seattle as part of the project to replace the earthquake-damaged Alaskan Way Viaduct. New Atlas’s 2013 account linked the project to damage caused by the February 28, 2001 Puget Sound earthquake. It said engineers considered more than 90 alternatives before selecting a bored route beneath downtown and described the planned tunnel as 1.7 miles (2.7 km) long. These are figures and project context reported at the time, not a summary of the project’s eventual cost, schedule or transport outcomes.
At the machine’s dedication, the report quoted then-Washington Governor Jay Inslee: “This is a historic day. When I look down into this pit, I don’t just see a big machine. I see determination, I see innovation, I see teamwork, I see a symbol of a community that worked together to move us forward.”
How large was Bertha?
New Atlas reported in 2013 that Bertha measured 326 ft (99.3 m) long and 57.5 ft (17.5 m) wide, and weighed 7,000 tons (6,350 tonnes). Its article called Bertha “the world’s largest TBM.” A 2017 Federal Highway Administration report later described Bertha as the largest-diameter TBM to that point. Those sources establish the historical superlative, but not which machine holds the record today.
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How Bertha worked
A tunnel boring machine combines excavation, spoil handling, ground support and lining work. In New Atlas’s 2013 walkthrough, Bertha’s components worked as a sequence rather than as competing methods: the cutterhead loosened ground, a slurry system handled excavated material and pressure, the shield protected the working area, and the machine advanced against the tunnel lining.
Cutterhead: breaking and moving ground
The cutterhead covered the front of Bertha and rotated to work through the ground. New Atlas reported 260 teeth, arranged to break softer material, guide loosened soil through openings and grind larger boulders. The report said the machine used 25,000 bhp (18,600 kW) and projected progress of about 35 ft (10 m) per day. Those were contemporaneous descriptions and an expected rate published before the drive, not verified final performance figures.
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Slurry system: handling soil and face pressure
Behind the cutterhead, Bertha controlled pressure on the excavated material and mixed soil with water and other additives to form slurry. The slurry could then be carried away through a pipe. New Atlas said the project expected 850,000 cubic yards (650,000 cubic metres) of excavated soil to pass through that pipe. That quantity was an expectation stated before boring, not a verified final total.
Shield and hydraulic jacks: protecting and advancing the machine
A steel shield behind the cutterhead sheltered workers and machinery and protected the tunnel’s sides in the active work area. Hydraulic rams pushed Bertha forward by bracing against the most recently installed concrete lining ring. This let the machine advance while the completed lining supported the tunnel behind it.
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Lining erector: building the tunnel wall
New Atlas described each lining ring as ten concrete panels fitted together with gaskets to keep water out. Once the ring was in place, grout filled the gap between the outside of the lining and the surrounding ground. The erector’s task was therefore distinct from excavation: it assembled the permanent tunnel lining as the TBM progressed.
Control room and trailing gear
Operators in the control room monitored machine systems and camera views, steered the TBM and watched for signs of ground movement or obstructions at the cutterhead. Behind the shield, articulated trailing gear carried supporting equipment and supplies, allowing the operation to continue as the front of the machine advanced.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Further reading on Bertha and the SR 99 project
Big Bertha for a child-friendly explanation
Amanda Abler’s illustrated children’s book, Big Bertha: How a Massive Tunnel Boring Machine Dug a Highway under Seattle, with illustrations by Katy Wu, was published September 17, 2024. The publisher describes it as a 48-page hardcover covering Bertha, how it worked and engineering background. See the publisher’s book page for its description and retail options.
Supertunnel for project photography
Supertunnel is a photographic book about the Seattle SR 99 project, with commentary and perspectives on the work. Its official website describes the book and identifies Amazon as a purchasing option; it also mentions a matching puzzle set.
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