Free the tunnelling machine!

In July 2013 Seattle began digging a new highway-tunnel to replace its existing and earthquake unstable viaduct. The biggest tunnelling machine ever, designed and constructed in Japan, got nicknamed after Seattle’s only female mayor: Bertha. With 17 meteres in diameter and more than 700 cutting tools on its front head, Bertha (controlled by more than 20 people) would do the job of tunnelling through 2830 meters of soil and rock.

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Bertha before drilling began in July 2013. Photo credit: Ted S. Warren

But in December 2013 she got stuck. The machine’s cutting blades encountered a 36 meter long steel pipe left over from recent earthquake investigation. The pipe damaged the cutter blades and knocked her down. It was known that Bertha’s route also featured some man-made fills and unstable debris, but why this couldn’t have been prevented remains unclear. The machine got overheated and also unexpected amounts of groundwater are delaying continuation.

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Meanwhile the finalising of the interior of the 300 meters that already have been dug continues

It has been more than a year now, and Bertha is ever still stuck underneath downtown Seattle at only 10% of its journey.. But freedom is near.

In October 2014 engineers started a rescue operation: a big access shaft was dug towards the point where the cutting machine had to give up. This shaft was completed two weeks ago, on January 30th 2015. Now the next step for Bertha is to tunnel through the concrete walls of the shaft, from where the front end of the machine will be lifted up by a crane. At ground level motors will be checked and components will be replaced.

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A view into to the Bertha access pit in January 2015. Photo credit: Washington State Department Of Transportation

After re-instalment Bertha will be ready to resume tunnelling her remaining 2400 meters.

Sources: City Metric, WSDT
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Posted on February 14, 2015

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