Engineers never planned to lift "Bertha"—at 57.5 ft in dia, the world's largest active tunnel-boring machine—out of its bored tunnel. But when it came time to remove for repair her 2,000-ton cutter-drive unit from 120 ft below downtown Seattle, heavy-lift specialist Mammoet created a custom gantry system for the job.
Due to the site constraints and soft waterfront soil conditions, Mammoet didn't have the typical foundation with which to work, forcing the Dutch-based firm to get creative with a fully hydraulic skid base that was 116 ft wide, 90 ft long and 105 ft tall—with a distance between the towers of 75 ft.