The Civil Engineering Contractor March 2018 | Page 15

Bertha, the world’s biggest tunnel boring machine
(TBM), was used on the Alaskan Way Viaduct
tunnel. At 17.68m diameter, the tunnel is the
biggest ever drilled, extending 2.7km beneath 158
downtown structures at depths up to 60m. Bertha’s
tunnel rerouted a crumbling elevated highway
underground, while keeping vehicular traffic
flowing. The five-story-high drill for the work was
custom-built by Osaka-based Hitachi Zosen.
The project was plagued by legion challenges,
including Bertha hitting an eight-inch-wide steel
well casing after boring only about 304.8m of the
planned route.
On 4 April 2017, Big Bertha reached its
disassembly berth. In total, 1  426 concrete rings
were built and installed, and the machine ground its
way through 2 825m of earth. nn
CEC March 2018 - 13