In 2001, a magnitude 6,8 earthquake rattled Seattle, cracking the ageing Alaskan Way Viaduct. Finally, in 2009 local and state leaders decided: the viaduct would fall. In its place a waterfront renaissance would knit the city’s core and its shoreline together.
The costliest and most complicated puzzle piece – the one that would make all of this possible – would also be one of the least visible. A 3 km tunnel would replace the hulking viaduct. Just 2,7 km of that tunnel bored through the Earth.
That’s where the Bertha tunnel boring machine would come in. On 30 July, she got to work. On 3 December, when Bertha had dug just 312 metres, the unthinkable happened. Bertha hit something. A few days later her temperature rose, then her cutting head stopped spinning. With Bertha, essentially, stuck in the mud 20 metres under the earth, the project team had to figure out how to reach Bertha and get her moving again.
Here’s a look at what it will take to rescue the world’s largest tunnel-boring machine ever built:
1. Seattle Tunnel Partners sank 73 enormous, connected concrete pillars in a huge ring in front of Bertha’s face. A few days ago, workers started excavating the ring to create a shaft wide and deep enough to swallow an 11-storey building – a vertical rescue tunnel.
2. Though she’s running hot, Bertha can still move. Soon Bertha will chew through the front of the protective wall until her head rests on a concrete cradle in the rescue pit.
3. Finally, a custom crane called a modular lift tower will raise Bertha’s 1 800-ton face, tilt it, and set it down. Workers will replace Bertha’s bearing assembly and add 77 more tons of steel ribs and plates.
Watch this animation showing the step-by-step process for rescuing Bertha from the depths (with narration by the tunnel’s project manager, Chris Dixon)…
Read more about the rescue mission for Bertha in PM’s November 2014 issue – on sale now.
Thumbnail image credit: Washington State Department of Transportation