Project Overview
Cascade, Idaho, is a small resort town with barely 1000 residents. But it is perched on the edge of scenic two-lane highway that is a vital north-south link for Idaho’s commercial and tourist traffic. The highway originally had a three-span steel girder bridge that was scheduled to be replaced in 2017, but when the structure was classified as structurally and functionally deficient due to excessively corroded piles, plans had to be accelerated.
“The bridge could no longer support heavy truck loads, requiring new load restrictions along the route and impeding commercial traffic,” explains Leonard Ruminski, P.E., technical engineer for the Idaho Transportation Department (ITD). And because there were few other highways in the area, any prolonged traffic interruptions would have significant impact.
To address these issues, ITD determined the entire structure had to be replaced using accelerated bridge construction methods using precast concrete elements for all of the bridge components to minimize traffic disruption while delivering a durable, sustainable high-performance replacement. “Considering all of the challenges, choosing an all-precast option was the only logical solution,” he says.
Precast Solution
The plan required traffic to be opened throughout construction, and for the project to be completed during a single construction season to minimize public inconvenience. This required a permanent shift of the roadway alignment, which called for construction of precast concrete mechanically stabilized earth (MSE) retaining walls at each end of the new bridge. MSE walls with precast concrete panel facing were also used instead of conventional wings at each abutment.
Bridge removal and replacement was then conducted in two stages. Because the bridge crosses a 20 ft deep river with limited access, the project team constructed a temporary work structure on one side of the bridge during stage 1, then used parts of the existing weakened bridge as a work platform during stage 2. “This required careful analysis and equipment placement since the new bridge pier piles were installed through cut-outs in the existing deck and between the existing girders,” Ruminski says. Installation of piles for the pier was complicated by the need to predrill through existing large boulders and layers of hard clay to ensure adequate pile embedment.
Though the most challenging part was finding a solution for pier construction in the deep river that wouldn’t require a cofferdam—a watertight enclosure that permits construction work below the waterline—which would slow construction and negatively impact the design aesthetic. The solution involved steel shell piles driven into the riverbed that cut off just above the water line, with precast concrete pier segments positioned on top so they are completely hidden from view above the water. “This solution proved to be aesthetically pleasing and allowed on-time project delivery,” Ruminski says.
Once the bridge deck was complete, it was covered in a spray-on waterproofing membrane and double layer of asphalt to protect against deicing chemicals, water, and wear-and-tear; and the exposed precast concrete faces were texturized and stained to imitate the natural surrounding rocks to maintain the aesthetic goals of the project. “All of these details resulted in a durable, long-lasting and low-maintenance bridge,” he says. |