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SH-55 Bridge Replacement

SH-55 is a two-lane major scenic route connecting Boise, Idaho, with the rest of the state to the north. As such, the route provides a vital north-south link for both commercial and tourist traffic without any other convenient detour available within the state borders.

The project is located on SH-55 in a small resort town of Cascade, Idaho, 75 miles north of Boise. The superstructure of the existing 65-year old three-span steel girder bridge crossing North Fork Payette River just north of Cascade was rated as structurally deficient and originally scheduled for replacement in 2017. However more recent underwater inspections revealed that the exposed slender steel piles supporting existing piers in the 20-foot deep river were excessively corroded and as such could no longer support heavy loads, causing bridge truck load restrictions and impeding commercial traffic.

As a temporary fix, the Idaho Transportation Department (ITD) was prompted to reduce truck traffic limitations by quickly encapsulating 50% of the corroded piles with epoxy filled jackets, and then to proceed with the permanent bridge replacement as soon as possible.


In order to minimize impact on traveling public and commercial freight, the bridge replacement was to be done in as short period of time as possible, while maintaining two-way traffic through the construction site at all times. This required that the new two-span 250-foot long concrete bridge replacement was to be quickly designed and built using Accelerated Bridge Construction (ABC) methods. The bridge removal and replacement was to be done in two stages and traffic handling called for a permanent shift of the roadway alignment. This alignment shift and the right-of-way limitations called for a construction of long MSE retaining walls at each end of the new bridge. Other challenges included limited access through a deep river available only on one side of the existing bridge and low hanging electrical power lines. The new bridge solution was a two-step process. The first stage was to construct the temporary work platform and erect it within the river while during stage two the access within the river was to be provided by the remainder of the existing bridge. The pier piles of the new bridge in stage two were actually installed through openings cut in the existing deck.

Due to its location next to the recreational resort and within the natural beautiful surroundings, the new structure had to be also aesthetically pleasing. The public meeting was held in the city of Cascade to inform the local public on the proposed bridge replacement. Coordination with the local utility company was also required in order to relocate utility lines from the existing bridge onto the new structure.

The superstructure and substructure of the new bridge consists entirely of prefabricated reinforced concrete components to be quickly assembled in the field. The precast abutment pile caps are supported on a single row of steel HP piles. The piles are protruding into the CMP block-outs within the caps and concreted in-place using high early strength concrete with fibers. Precast pier segments rest on top of 30” diameter shell piles driven into the riverbed, cut off at the exact elevation just above the waterline and filled with concrete. 14” diameter centering pipes cast-in at the top of shell piles are used to align and to secure lower segments of the pier wall on top of the shell piles (similar to a Christmas tree stand). Lower pier wall segments are shaped as inverted tubs (or skirts) and are covering shell piles well below the waterline, making them invisible for aesthetic reasons. The upper hollow pier segments are connected to the lower segments with continuous vertical reinforcing bars inserted through embedded metal sleeves and grouted in place. The hollow voids in the upper segments are then filled with high early strength concrete. This innovative method of pier construction eliminated the need of deep cofferdams, significantly reduced cost and construction time while providing the look of a solid. The precast 124’-3” long Deck Bulb-T girders were quickly erected on top of the precast substructure elements and the top flanges connected by 10” wide closure pours using high strength, fast curing, reinforced repair mortar. Top flange transverse girder reinforcement was protruding into the closure pour and headed rebar terminators were used in the bottom layer to enhance rebar development. By using this innovative approach, the finished deck surface imitates the behavior and durability of the cast-in-place deck. The girders were next connected at the pier and abutments by casting high early strength concrete and creating integral diaphragms. Integral abutment and pier diaphragms increase structural redundancy and eliminate exposed bearings and expansion joints, reducing future maintenance requirements. After placement of approach slabs, sidewalks and parapets, the driving surface was covered with spray applied waterproofing membrane and AC overlay. (The top flanges of the deck girders were cast with variable thickness in order to offset the upward camber deflection and allowing constant depth of AC overlay, simplifying its placement.) This approach will provide for well protected, long lasting deck and reduce future potential maintenance issues.


The bridge replacement was accomplished within 4 months between May and September of 2015 at the cost of approximately $4,500,000. (The comparable bridge constructed by conventional methods would require at least 10 months.) The weight of all precast elements, except girders, was kept under 30 tons in order to ease transportation and handling. Fast curing, high strength mortar and concrete were used in all connections as to minimize curing times. All exposed concrete surfaces received stained textured treatment as to match surrounding rocks and to enhance aesthetics.

Project Details


Cascade, ID


Idaho Transportation Department, Boise, ID


Idaho Transportation Department, Boise, ID


RSCI, Boise, ID

Total Project Cost:

$4,500,000 (approx)

Total Project Size:



Idaho Transportation Department, Boise ID