Twin Bridges over Snake River built to handle triple the traffic

A pair of bridges in south-central Idaho spanning the vast Snake River finished last fall, connecting Cassia and Minidoka counties along Interstate 84. These Twin Bridges are a critical link to interstate travel through the area, with nearly 18,000 vehicles passing over the crossings each day. This volume of traffic is expected to nearly triple during the life of the new structures.

Located eight miles east of Burley, the original bridges were built in 1960 during the initial construction of Interstate 80-North (now I-84). The new bridges replaced two major structures, lengthened the westbound on-ramp at the Declo Interchange (Exit 216), and reconstructed short portions of I-84 to tie into the new bridges and stormwater-retention areas.

The design of the project was done by ITD District 4 and Bridge Section designers and involved many challenges with innovative solutions. The bridge design was led by Umesh Narsinghani, a longtime employee of the Bridge Section.

The steel girders were haunched to provide an aesthetically pleasing appearance to this important crossing of the Snake River. Early on in the project, value engineering meetings were held with contractors, FHWA, ITD, and other stakeholders to optimize the project design and construction to meet project goals in the most economical way possible. The Value Engineering process was award-winning for ITD for this project.

Construction of the new Twin Bridges project was under the management of ITD District 4 Project Managers Kandace Stewart and Ken Lively. The pair worked closely with the contractor, Ralph L. Wadsworth Construction, throughout construction to ensure objectives were met.

The new bridges were constructed with a total of seven spans (five of which are in the water) — a drastic reduction from the 24 spans of the original bridges. The previous structure was twenty four 40 foot spans founded on exposed multi-pile piers. The new structure consists of six spans of approximates 165 feet in length, founded on piers that consist of two drilled shafts drilled into bedrock.

“Fewer spans really opened the area for improved recreation use by boaters and other water enthusiasts,” Stewart said. “Maybe more importantly, fewer spans reduced the environmental impact on the river, which was particularly important for the Snake River physa – a snail native to the area and listed as endangered.”

Numerous repairs had been completed on the bridges over the years leading up to the replacement, which began in 2015. They underwent deck reconstruction in 1976, but cracking was found in 2010 in the concrete pre-stressed stringers/girders at the east end of each bridge, and both bridges fell into the range of structural deficiency.

Bridge widening also allowed crews to lengthen the westbound on-ramp from the Declo Interchange, which was particularly important for safety because the original interchange had a short acceleration ramp that terminated as it entered onto the old structure. The new structure lengthened the ramp and carried it out 450 feet onto the bridge, allowing more time for vehicles to accelerate and merge into I-84 traffic. 

A temporary bridge-like platform allowed traffic flow to continue unimpeded during construction. Traffic was diverted to a single lane in each direction while the platform was used to construct the new twin structures. Use of the temporary platform also reduced impacts to the river and allowed water access to continue throughout the project.

ITD District 4 Resident Engineer Scot Stacey said, “the contractor had top-notch equipment and their abilities truly shined through with this project; we formed a great relationship with them and hope to work together again in the future.”

The project was awarded to the contractor in February 2015, with a bid price of $17.88 million. The project’s low bid was substantially below the Engineer's Estimate and project construction budget, resulting in real savings to Idaho.

Published 05-18-18