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Thursday, 22 June 2017 12:00

The largest tunnel boring machine (TBM) in the world finished drilling underneath Seattle

Written by  TheStructuralEngineer.info

The tunnel will become a double-decker highway, scheduled to open in 2019

It was in April of 2013 that the 57-foot wide boring machine arrived in Seattle after leaving Japan three weeks before. The tunneling machine, nicknamed Bertha after former Seattle mayor Bertha Knight Landes, was sent to the US city in more than 40 pieces, weighing 6,700 tons in total, and four months later it was reassembled and ready to start digging.

Nearly four years later, on April 4, 2017, and 3 years behind schedule due to a major repair, Bertha broke through into her disassembly pit, marking the end of the 1.75-mile long tunnel beneath Seattle. Measuring 325 feet long, Bertha not only drilled the tunnel but also installed a concrete reinforcement wall throughout it. Now, the aging Alaskan Way Viaduct, an elevated highway that was partly demolished after being damaged by a 2001 earthquake, will be moved below the surface and a new public space will be built in its place. During the following months, workers will be disassembling the machine and removing it from the tunnel using cranes. 

The $80 million  TBM was manufactured by the Japanese firm Hitachi Zosen Corp., a company that has successfully built more than 1,300 tunneling machines, a number of them for large-diameter tunnel projects.

The delay

The project was initially to be completed in two and a half years, but in December 2013, as Bertha was digging, it seems that an 8-inch steel pipe that was in her way caused the machine to overheat and damaged its blades at the front. The process of fully assessing the situation and fixing the TBM finally ended in December 2015, as the technicians had to pull the cutting head back up to the surface and do repairs on site. An estimated $223 million in cost overruns were reported as a result of the two-year stoppage.

The new highway

The new double decker highway, called State Route 99 (SR 99), is expected to open in 2019, having an estimated cost of $2.1 billion. It will have four lanes, lighting, ventilation, and fire detection systems and inspectors will test more than 8,500 components over the next two years, before signing off and letting traffic through.

Source: Wired  

seattle tunneling machine 1

  

seattle tunneling machine 2

Photo Source: The Seattle Times

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