Bertha One of the biggest (TBM)
A tunnel boring machine (TBM), also known as a “mole”, is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They may also be used for microtunneling.
They can bore through anything from hard rock to sand.
Tunnel diameters can range from a metre (done with micro-TBMs) to 19.25 metres to date.
Tunnels of less than a metre or so in diameter are typically done using trenchless construction methods or horizontal directional drilling rather than TBMs.
Bertha is a 57-foot-diameter (17.4 m) tunnel boring machine built specifically for the Washington State Department of Transportation’s (WSDOT) Alaskan Way Viaduct replacement tunnel project in Seattle.
It was made by Hitachi Zosen Sakai Works in Osaka, Japan, and the machine’s assembly was completed in Seattle in June 2013.
Tunnel boring began on July 30, 2013, with the machine originally scheduled to complete the tunnel in December 2015.-Justplay
On December 6, 2013, work was halted approximately 1,083 feet (330 m) into the planned 9,270-foot-long (2,830 m) route because of an unexpected impediment.
Initially, it was thought that the machine had damaged several of its cutting blades after encountering a steel pipe that was used to measure groundwater in 2002 around the Alaskan Way Viaduct.
However, subsequent investigations revealed that portions of the main bearing seal system were damaged,
which caused the bearing to overheat during operation.
Over the next two years, a recovery pit was dug from the surface in order to access and lift the machine’s cutterhead for repair and partial replacement in 2015.
Bertha resumed tunnel boring on December 22, 2015, but was stopped in early January 2016 after a tethered barge in Elliott Bay damaged nearby piers and a sinkhole opened near the project site.
Governor Jay Inslee halted all work on the tunnel on January 14, 2016, citing concern over public safety after the sinkhole incident.
Digging resumed on February 23, but was halted again for maintenance and inspections late in March.
Bertha resumed tunneling on Friday, April 29, 2016, digging under the Alaskan Way Viaduct.
Tunnel boring was completed on April 4, 2017, with Bertha’s cutterhead breaking into a disassembly vault at the tunnel’s north portal in South Lake Union.
In December 2015, WSDOT estimated that the tunnel would be completed and open to traffic in early 2018.
The estimate was revised in July 2016 to open in early 2019;
an estimated $223 million in cost overruns stem from the two-year delay of the machine.
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