Bertha breaks through: World's largest boring machine completes tunnel under Seattle
The normally sedate city of Seattle sounded more like the set of a science fiction epic today as the world's largest boring machine roared and screeched its way into the open air. The tunnel boring machine, better known as "Bertha" cut through a wall of reinforced concrete in the shadow of the city's iconic Space Needle at about 11:15 am, completing the last few feet of a tunnel more than 9,720 ft (2,963 m) long as part of project to build a new road link between the southern and northern halves of the city. New Atlas was there to witness it.
Four years ago, in April 2013, Bertha began the task of digging under the skyscrapers of Seattle to replace the 64-year old viaduct that had been declared unsafe after a 2001 earthquake. The decision was made to remove the elevated roadway to make way for a waterfront park and replace it with an underground tunnel linking the SR 99 motorway.
To do this, the Washington State Department of Transportation (WSDOT) and main contractor Seattle Tunnel Partners (STP) commissioned the Hitachi Zosen Corporation of Japan to build the world's largest tunnel boring machine. Measuring 326 ft (99.3 m) long and 57.5 ft (17.5 m) wide, it originally weighed 7,000 tons (6,350 tonnes), though it was considerably modified after a major malfunction was discovered shortly after excavation began.
Today's breakthrough had a strong element of suspense because digging a tunnel using a giant mechanical earthworm is hardly an exact science. A WSDOT spokeswoman said that they weren't even sure of what would happen – whether the concrete columns would come away in small sections or the entire plug would simply be shoved out and collapse.
At first, the only sign that Bertha was approaching was a deep grinding, screeching noise that came from behind the plug that dominated the south end of the 90-ft-deep pit that had been built to receive the steel behemoth. On one seam of the plug, a trickle of foam was seeping out as the soapy conditioner used to make the soil easier to dig through tried to escape.
At 9:45 am, the scene suddenly changed as a sudden jet of suds and muddy water shot out of the base of the plug, flooding the pit. At the same time, the giant cutting disc of the machine apparently made contact with the inner surface of the plug and clouds of very fine white dust rose and was trapped in the confines. This reduced visibility to zero for many minutes and members of the press were asked to retreat to cleaner air while Bertha paused and the dust cleared under sprays of water.
Then at about 11:15 am EDT, Bertha broke through as section by section chunks of concrete and rebar were ground up and fell away into the water below. Within 20 minutes, most of the plug was gone and the cutting disc, once a bright mosaic of green, red, and yellow covered with sharp cutting heads, was now revealed as a worn metal surface with blunted edges.
According to the WSDOT, Bertha's breakthrough is a good news/bad news event. On the one hand, a tunnel unique in the world has been dug, but it has a dirty big digging machine blocking it. This means the contractors will spend the next few days removing the giant support struts from the pit, then move Bertha into it, where over five months will be devoted to dismantling it and cutting it into pieces of under 20 tons, so they can be moved through the city streets.
Some components will be salvaged and sold back to the manufacturer while much of the steel will go to a local iron foundry to be melted down and reused for building the motorway inside the new tunnel. Meanwhile, the receiving pit will be converted into a ramp connecting the tunnel to the road network.
"This is a historic moment in our state's transportation history," said Washington State Governor Jay Inslee. "Innovation and perseverance are the engines that keep Washington in the forefront. There is still more work ahead but this moment is one worth celebrating."
The video below shows a drones-eye view of Bertha breaking through.