If a sensor is triggered, the fans activate and the system notifies tunnel staff. 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. The tunnel also includes a separate safety corridor. 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 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. 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 quantity of concrete placed under the logistical constraints of working underground, under a tight schedule and with high-quality expectations, is a true triumph of collaboration and advanced planning,” Russell says. The project used precast concrete tunnel segments for the liners, precast panels for the northbound roadway decks, precast bridge girders at the north end of the tunnel and cast-in-place concrete for drilled shafts and roadway structures. Crews built the tunnel about 500 ft behind the TBM, sequenced to keep operations moving and compress the schedule. “The project included an advanced seismic design for the tunnel liner and interior roadways, and complex mechanical and electrical systems unique to tunnels.”Ĭreating 32-ft-wide roadways to carry two southbound lanes on top of two northbound lanes required plenty of materials and synchronized construction. “Many of the design challenges were new, since a tunnel of this size with its required safety features had never been built before,” Russell says. The piles supported foundations, while carbon fiber reinforcement of existing viaduct-beam sections added capacity for the still-in-use roadway structure, allowing differential settlement without damage. “The innovations by the contractor in terms of being able to build something repetitive and with repetitive quality, I think a lot of those construction techniques are really overlooked but are really important.”Ĭrews installed battered micropiles to form a canopy over the tunnel. “The advancement in tunneling technology is key to the future,” Russell says. 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. The project started and finished in downtown Seattle, tunneling under 157 buildings along the way. “Our biggest worry and biggest effort were to come up with a plan to launch the machine and tunnel deep enough underground to get to soils where the machine could do its job and avoid ground loss,” Russell says. While the coordination across international offices was challenging, what concerned Russell most was the soil. That said, the HNTB team tapped into knowledge from previous jobs, overall company expertise and experts from Europe with experience in tunneling. “I don’t think I have ever been on a billion-plus-dollar project before, and the enormity of the undertaking and the schedule-and the fact that it was a design-build project with a lot of complexities-was daunting for us as engineers to mobilize staff and the resources needed to meet the schedule requirements to get the construction started,” he says. The size and scope of the project-in the planning stages for nearly two decades and officially started in 2011-was entirely new to Russell. 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. “Ultimately, the SR 99 tunnel is a model for megaprojects and mega-tunnels in urban areas. “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.

Highway underpass design