Idaho transportation officials are making major progress this month in replacing the state’s most iconic bridge.
The exploratory work, which will impact weekday travel between Boise and McCall in June, is the first in a series of efforts to build a new permanent bridge over the North Fork of the Payette River. The multi-year, $54 million project will ultimately bypass the Rainbow Bridge in Valley County, which dates back to 1933 and is on the National Register of Historic Places.
The unique concrete arch structure that connects commuters from the Treasure Valley to the northern counties and back was approved and built during the Great Depression. But after nearly a century of use, the Rainbow Bridge is nearing the end of its life and no longer meets today’s traffic requirements and safety standards for state highways, according to the Idaho Transportation Department.
The future plans for the Rainbow Bridge are part of the new bridge project, ITD said in a press release. The authority had already stated in an earlier tender that it wanted to preserve the existing river bridge.
“ITD has not yet decided what to do with the existing Rainbow Bridge,” ITD regional spokeswoman Sophia Miraglio said in an email to the Idaho Statesman. “The community will have several opportunities to provide input on the new river crossing and the existing bridge. We encourage the public to attend the public meetings in July to share their thoughts and ideas.”
ITD has not yet announced the dates and locations of the meetings planned for next month and later this year. Interested parties can request to be added to the mailing list by emailing [email protected].
“We intend to approach this project with a high level of transparency, sensitivity and community involvement,” Dan Gorley, ITD’s chief engineer, said in the release. “Community input will be an important consideration in the final recommendation for the new and existing bridge.”
The Idaho 55 highway corridor is particularly challenging
ITD will continue its preliminary work on the project this month with test drilling to collect rock and soil samples at 18 locations around the existing bridge at Smiths Ferry. Environmental and planning work for a new bridge adjacent to the current stretch of river began in the spring.
The test results from the drilling will help in the design of the foundation, columns and retaining walls of the future bridge on the existing highway, ITD said. Construction is currently scheduled to begin in 2027 or 2028.
The mountainous transportation corridor along Idaho 55 is a particularly challenging construction area due to the region’s seismic activity and geology, state geologist Claudio Berti told the Statesman. Half a mile south of Rainbow Bridge, ITD recently completed a three-year road improvement project on the state highway that experienced several rockfalls during construction, causing delays and increasing costs.
“The preparatory work before project construction is fundamental to moving the project forward in the best possible way. This allows us to understand the area as best as possible and anticipate as many complications or hazards as possible,” Berti, who also heads the Idaho Geological Survey, said in a telephone interview.
Initial estimates from the ITD put construction costs for the bridge replacement at about $54 million, the Statesman previously reported. Construction of the Rainbow Bridge in the 1930s cost $74,000 in federal funds, the U.S. Forest Service reported.
According to ITD’s latest seven-year plan, about 60 percent of the total bridge costs will be covered by federal funds this time.
ITD will begin removing trees and vegetation near the drilling sites this week so a helicopter can drop off equipment for drilling operations. Work will take place weekdays through Friday noon, and motorists should expect delays of up to 15 minutes during the work, ITD said.
After the test drilling is complete, ITD will reseed the soil. For current information on the Rainbow Bridge replacement, visit the project website at: itdprojects.idaho.gov/pages/rainbowbridge
