1. Built environment
February 2, 2015updated 20 Jul 2021 1:02pm

Bertha, the world's biggest tunneling machine, has been stuck under Seattle for over a year

By City Monitor Staff

Meet Bertha: 

Bertha is the world’s largest tunnel boring machine. She weighs 7,000 tons and, at 17.5 metres tall, is roughly the height of a three storey house. Unfortunately, she’s also currently stuck in a tunnel 18m below Seattle. 

Named after Seattle’s only female mayor, Bertha was built in Japan and arrived in Seattle by boat in 2013. Her job (yes, we’re sticking with “her”) was to burrow for two miles along the waterfront to make way for a new, four lane, double-layered road, which would replace the earthquake-damaged Alaskan Way Viaduct:

The tunneling began in July 2013, and by December 2013, Bertha had dug through 1,000 feet of the route (roughly ten per cent of the total). What happened next is up for debate: local media reported that the machine hit an unexpected metal pipe, while the Washington Department of Transport’s official line is that she “heated up”, which damaged seals inside the machine. 

Whatever the cause, fixing a machine the size of a house stuck underneath a major city is no easy thing. That’s why, over one year on, Bertha is still there. At the moment, workers are building a giant pit so they can free the machine’s front end, and lift it up to the surface for repairs. But according to Global Construction Reviewthe project has already cost the city $2bn, and state officials aren’t happy.

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Earlier this week, two Republican senators in the state put forward a bill to give up on the project altogether, as extracting the machine would be too difficult. Luckily for Bertha, however, Curtis King, the chair of the transport committe,e said this was not realistic, and that he would not hear the bill. As he put it:: “Now is not the time to be talking about, let’s bury it, let’s save ourselves,”

As it stands, then, the workers will continue to dig until they can hoist her front end out of the ground.

Until then, tunneling fans can visit this Bertha model at the tunnel project information centre (human is not to scale): 

 Image: Dennis Bratland at Wikimedia Commons.

This helpful clip also explains how the tunnel boring process works. Essentially, giant blades at the front tear up the earth and rocks, then the debris is carried away by a complex conveyor belt system at the back:

Not shown: Bertha choking on a bit of pipe and dying. 

This article was updated on 2/2/2015 to reflect the fact that Curtis King was not one of the bill’s proposers but is chair of the Senate Transportation Committee.

Images: WSDOT.

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