State Route 99 Tunnel

FlatironDragados, as the lead in a joint venture with Tutor Perini and lead designer and engineer HNTB, constructed Seattle's State Route 99 Tunnel for the Washington State Department of Transportation. The project replaced the aging Alaskan Way Viaduct with a 1.7-mile-long, 57.5-foot-diameter, double-deck tunnel to improve safety, reduce traffic congestion and reconnect the city with the waterfront.  

Key features:  

• Engineered with state-of-the-art seismic reinforcements
• Excavated with one of the world’s largest tunnel-boring machines, named Bertha. 
• Installed more than 1,400 concrete tunnel rings, each weighing more than 36 tons
• Lined with 2-foot-thick, 7-foot-long precast segments
• Located as deep as 165 feet below sea level beneath heritage buildings
• Equipped with advanced ventilation, fire suppression and traffic monitoring systems

The project enhanced mobility by moving State Route 99 underground, creating new public spaces and improving pedestrian and bicycle access along the waterfront.