Barnhart/Dawn Paul Barnhart's modular lift tower placed the TBM's 745-ton cutterhead on May 31. Related Links: Bertha to Dig This Summer Follow Bertha on Twitter Lowering a 7,000-ton tunnel-boring machine piece by piece into an 80-ft-deep launch pit sloped 3° forced Barnhart Crane and Rigging to think outside the boom. The subcontractor working at the Alaskan Way Viaduct in Seattle opted to use a modular lift tower instead of a conventional crawler crane, saving millions of dollars and valuable site space.The largest tunneling machine built for North America, Bertha—a 57.5-ft-dia unit—will this summer bore a 1.7-mile-long tunnel to replace the
Arch dams, classically curved monoliths situated in narrow valleys, are not as common as other types of dams, such as embankment dams and gravity dams. ENR takes a look at some of the biggest arch dams in the world, both completed and under construction.