Call it a birthday. Or an anniversary. Either way, Seattle’s State Route 99 tunnel has hosted public traffic for an entire year. After what seemed like a tedious slog to get one of the world’s largest single-bore tunnels dug, the last couple of years have raced by. 

With the racing by comes plenty of racing through, as Seattle-area commuters now use the tunnel at a clip of more than one million trips per month, with the busiest travel times the three-hour windows of typical commuting mayhem between 6 and 9 a.m. and 4 and 7 p.m. 

The $1.3 billion tunnel replaced the seismically vulnerable State Route 99 Alaskan Way Viaduct. While debates went on for years on the best way to improve travel in Seattle, the tunnel was ultimately the decided approach and work started in 2011 to create a plan and tunneling machine that could do the job. That machine, though, failed quickly, leading to a nearly two-year delay as crews fixed the machine 11% of the way into the planned 1.7-mile bore. Once the machine got moving again, the rest of the project went well, with the tunnel reaching 200 ft deep at its lowest point and excavated at 57.5 ft in diameter.

Crews were building the highway inside the tunnel even while it was being dug, designing in world-class ventilation, fire suppression and intelligent traffic systems. 

“Not only is the tunnel an important milestone in transportation history, it also demonstrates how transportation infrastructure can work in harmony with the physical environment to usher in a new era of safety, environmental control and aesthetic integration into the surrounding city,” says Brian Russell, HNTB engineer and lead tunnel designer.

The project started and finished in downtown Seattle, tunneling under 157 buildings along the way. In the beginning, engineers focused on protecting structures, and after the work reached deeper soil conditions, contractor Seattle Tunnel Partners—a joint venture between Dragados USA and Tutor Perini Corp.—also focused on avoiding ground loss by installing tunnel liners and filling voids with grout throughout the boring process.

“While the project certainly had its challenges, there were many lessons learned that could benefit other cities,” says Brian Nielsen, the project’s administrator for WSDOT and now the agency’s deputy regional administrator for the Northwest region.

“Ultimately, the SR 99 tunnel is a model for megaprojects and mega-tunnels in urban areas. Now that the country’s only double-decker highway tunnel is open, we are pleased to report it is performing as promised, carrying tens of thousands of vehicles each day safely through downtown Seattle,” Nielsen says.

The completed tunnel features 1,152 panels for the double-deck roadway and 16 electrical rooms spaced along the alignment to control lights, cameras and operation of 9,270 ft of electrical, mechanical and plumbing.

The 95 miles of wiring, 21 miles of sprinkler pipes, 15 miles of lights, 13 miles of fiber-optic cables and eight miles of heat detectors are interconnected with a safety system that uses cameras and electronic sensors to monitor conditions in the tunnel.

If needed, the system can shift traffic patterns or go into emergency mode and activate jet fans to pull in fresh air using wall louvers to ventilate the tunnel. The tunnel also includes a separate safety corridor.

The system samples air every few seconds to detect an increase in emissions, while the fire-suppression system has eight miles of fiber-optic heat sensors in the ceilings.

In the year since the tunnel has been open, WSDOT was able to remove the double-decker concrete viaduct that opened in 1953, not only removing the seismic risk from the downtown core, but also opening up the Seattle waterfront and paving the way for planned waterfront redevelopment. 

“It has allowed the city of Seattle to really develop the waterfront and change the downtown core,” Russell says. “I actually drove it many times just to see how well it worked. It is just amazing how long it is, but how fast I get through downtown.”

Follow Tim Newcomb on Twitter at @tdnewcomb.