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August 18, 2011

New Nalley Valley viaduct untangles traffic

  • The $119 million project saw crews build 77 piers to support 10 new bridges.

    Photos courtesy of WSDOT [enlarge]
    Man-lifts were used extensively, allowing crews to access hard-to-reach spots on the new viaduct.

    When high winds and torrential rains flooded highways and closed bridges in January 2009, the Washington State Department of Transportation saw no choice but to cancel its formal groundbreaking for the SR 16 Westbound Nalley Valley project. Although the band didn’t march on that day, construction of the complex project began as scheduled.

    The task: Build a new and improved westbound viaduct, the primary access from Interstate 5 to state Route 16, and keep 131,000 drivers moving while doing it.

    Accomplishing this charge was no easy feat, especially given the unique design of the original viaduct. When it opened in 1971, the structure featured a V-shaped, tetra-pod column system to support its overpasses. Back then a scant 40,000 vehicles a day drove the route. As time went by and traffic numbers swelled, it became apparent that the structure could not be widened to add capacity while still meeting today’s seismic standards.

    The tetra-pods had to come down. In their place WSDOT and Guy F. Atkinson Construction would build 77 piers to support 10 new bridges, placing more than 10.4 million pounds of steel and 48,000 cubic yards of concrete, and installing more than three miles of storm drain to complete the $119 million project.

    Congestion is an issue in locations throughout Washington, so why build more capacity at this site?

    For decades, volumes between I-5 and Route 16 grew and bottlenecks became a daily ritual. The weave where I-5 traffic exits to Route 16 and then to Sprague Avenue’s merge with westbound Route 16 traffic became one of the highest accident locations in the state. According to WSDOT data, more than 75 percent of all accidents in this corridor were rear-end or side-swipe collisions. Traffic engineers believe that eliminating the weave will reduce collisions by more than 60 percent, or just over 16 collisions per year.

    Tetra-pod structures supporting the original viaduct made widening impossible. They were demolished to make way for new columns that not only could accommodate larger bridges, but meet today’s seismic standards as well.

    To get the job done on time and on budget, and maintain the commitment to keeping traffic moving, WSDOT and Atkinson consistently worked during off-peak hours. Late nights and early mornings were the norm on the project as ramps, lanes and full directions of Route 16 and I-5 were closed to reduce impacts on drivers while ensuring construction progressed as planned.

    With a project of this magnitude, not all impacts could be avoided. Construction necessitated closing the four ramps at Sprague Avenue, rerouting 31,000 drivers each day.

    As old structures were demolished, the new Westbound Nalley Valley viaduct grew daily. It seemed as if someone had planted a seed that quickly sprouted a forest of towering concrete columns, curving ramps and enormous, snaking overpasses.

    The public’s tolerance of the work came with awe over its complexity. And perhaps the most astonishing visual was construction of the segmental bridge that now carries northbound I-5 traffic over the new viaduct to Route 16.

    Pier 2 towers above northbound I-5 with three segments on each side. Segments were added two at a time — one to each side — to balance the middle three piers.

    For weeks drivers watched the bridge slowly come together as pieces sometimes eerily hovered over the freeway below. Construction on 112 segments for the flyover ramp began in November 2009, and crews started installing segments in June 2010.

    Work included using massive cranes to position the 60- to 90-ton segments, which were post-tensioned in pairs to ensure proper balance. Segments are 12 feet tall, 8 to 10 feet deep and 45 feet wide. The flyover ramp was completed in June 2011.

    Shortly thereafter, on June 26, nearly two and a half years since construction started, the Westbound Nalley Valley project opened and traffic is now flowing freely.

    The end result:

    Looking out of pier 1 at piers 2 and 3. There are five piers total for the bridge, with the outside two using falsework to support the initial segments.

    • New westbound bridges over Nalley Valley.

    • Eliminated traffic weave with new direct connections from northbound and southbound I-5 to Sprague Avenue and westbound Route 16.

    • New Sprague Avenue on-ramp to westbound Route 16.

    • New eastbound Route 16 off-ramp to Sprague Avenue.

    • New flyover ramp from northbound I-5 to westbound Route 16 and Sprague Avenue.

    • Improved safety with full-width shoulders and new lighting.

    • Traffic cameras, electronic signs for traveler notification, highway advisory radio transmitters and traffic data collectors to provide real-time traffic information for the Web and 5-1-1.

    Construction of the new westbound Nalley Valley viaduct is one of three projects that will rebuild the entire interchange. The next project constructs ramps for the eastbound viaduct, and the final project adds high-occupancy-vehicle lanes and direct-access HOV ramps through the area.

    Joe Irwin works for WSDOT as a communicator in the Olympic Region. Prior to joining WSDOT in 2007, he was an award-winning editor and journalist.

    Graphic courtesy of WSDOT

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