World’s largest boring machine, called “Bertha” just finished drilling a 9,720 feet (2,963 meters) tunnel underneath the city of Seattle. Bertha has been boring underneath the Seattle grounds for four years now, finishing the bore on April 5th through a wall of reinforced concrete near the Space Needle observation tower. The tunnel aims to connect the southern and the northern halves of the city.
The project of building a tunnel in the ground below Seattle’s skyscrapers began in April 2013. The purpose was to replace a 64-year old viaduct that had been declared unsafe after an earthquake in 2001. The plan was to build an underground tunnel to link the SR 99 motorway replacing the elevated roadway in favor of a waterfront park.
The project was in the hands of Washington State Department of Transportation (WSDOT) and a contractor Seattle Tunnel Partners (STP) who commissioned Hitachi Zosen Corporation of Japan to build the world’s largest tunnel boring machine measuring 326 feet in length and 57.5 feet in width. The original machine weighed 7,000 tons, which was later modified after it malfunctioned at the beginning of excavation.
As Bertha bore through the ground, there was no sure way of knowing what exactly would happen towards the end. The concrete columns could have come apart in small sections or the entire plug could have come out in one piece, according to a spokesperson from WSDOT. Bertha completed the bore, releasing jets of muddy water into a 90-foot pit that had been dug to receive it. The disc cutting through the concrete created clouds of fine dust reducing visibility. The machine paused while a spray of water cleared the dust. It took another 20 minutes to drill through the rest of the tunnel, and the machine emerged with blunted edges and a worn out surface.
The largest boring machine has finished digging a unique tunnel but the question of how the machine will be removed from the ground remains. The massive metal block sits blocking the tunnel, and the authorities say it will take over five months to dismantle the machine into pieces weighing less than 20 tons so it can be removed, and carried across the streets. A few of the machine components will be sold back to the manufacturer, and most of the steel will go to an iron foundry to be reused for building the tunnel motorway. The pit through which Bertha was lowered into the ground will be used as a ramp that connects the tunnel to the ground road network.
The Washington State Governor Jay Inslee commented on the completion saying:
“This is a historic moment in our state’s transportation history. 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.”
Watch the video shot from a drone, showing the big steel giant as it bores through the ground.