The $155-million Sound Transit U230 Tunnel project is located in the most densely populated neighborhood in the northwestern U.S. Developed to create a vital connection between downtown Seattle and the Capitol Hill neighborhood, the 3,800-ft-long, 18-ft, 10-in.-dia twin tunnels are on a steep downhill grade and entirely on a curve.
Tunneling crossed under 32 structures and multiple utilities, many of which are more than 80 years old. The tunneled portion also included two crossings of Interstate 5, which carries approximately 250,000 vehicles per day through downtown Seattle. These crossings were carried out with only 13.5 ft of cover.