Old 99 (Riverside) Bridge

Bridge Replacement (1992 – 2004)

Built in 1938, the narrow, two-lane, steel-truss Old 99 Bridge was no longer adequate to support the 20,000 vehicles that cross daily between Mt. Vernon and Burlington, Washington. Spanning the Skagit River in a highly commercially developed spot that serves as the primary linkage between the twin cities, the old bridge was due for updating or replacement.

In the early 1990s, MACTEC conducted a feasibility study that helped secure design and construction funding through the Federal Surface Transportation Act and Bridge Replacement Program. We went on to perform a variety of services including biological assessment, environmental permitting, right-of-way documentation, utility relocations, and field, soil and foundation surveys. Most recently, we provided bridge design and approach roadway engineering for the widening of 1.6 miles of the Riverside Drive / South Burlington Boulevard corridor, which included replacement of the Old 99 ( Riverside) Bridge. The total construction cost for this project was $35 million.

The new Riverside Bridge is a two-stage, post-tensioned, four-lane facility that widens to five lanes on its south side. It is 847 feet long and 72 feet wide, with two 6-foot bikeways, and two 5-foot pedestrian sidewalks. The approaches on both sides were upgraded to include signal modifications, sidewalks, and landscape amenities.

In the project’s preliminary engineering phase, MACTEC evaluated various structural materials and conducted a value engineering study to consider such factors as construction cost, constructability, horizontal and vertical clearances, river hydraulics, scour and debris accumulation at the piers, traffic flow during demolition and construction, and bridge aesthetics and maintenance. The final design chosen was a precast concrete bridge consisting of three 180-foot main spans and two 150-foot outer spans using the new W95PT Supergirders.

While the bridge’s superstructure is composed of the new Supergirders, its substructure is composed of 7-foot diameter columns resting on 10-foot diameter drilled shafts that extend 90 feet below the river’s mudline. The Supergirders, weighing up to 122 tons each, were transported to the site in segments and post-tensioned into one piece on the ground. These pieces were then erected on the piers with no intermediate falsework. This process, a first in the country, was necessary to protect the salmon migrations in the Skagit River. The permissible construction window was limited to four months per year in the summer, and slender temporary falsework supports would not have survived the debris and currents caused by the usual winter and spring flooding.

Since the new bridge’s official opening in February 2004, this innovative project has won two prestigious awards for MACTEC: the Portland Cement Association 2004 National Award, and the Precast/Prestressed Concrete Institute 2004 National Award, in the category for bridges with spans greater than 135 feet.