On a crisp, late fall day last year, less than 200 feet made all the difference on Idaho Highway 55.
Just after 2 p.m. on November 18, 2021, a traffic safety vehicle guided a line of cars through the tight construction zone in the canyon alongside the picturesque Payette River. A rumbling sound rocked the air. High above the road, tons of material crashed down from the blasted cliff face and spilled across the highway.
The slide came a mere 150 feet from crushing the TrafficCorp pilot car and any others following behind on their way through the Central Idaho artery. It took weeks for the road to reopen to traffic.
This was the second of three major landslides that closed the corridor over the course of less than a year from March of 2021 through January, disrupting traffic between the Boise area and Valley County for days at a time. The slides all occurred within the construction zone for ITD’s ambitious multi-year project to flatten the curves of the winding, crash-prone highway snaking through the canyon.
Over the last six months, BoiseDev reviewed thousands of pages of public records, inspection documents, and emails from ITD officials, consultants, and engineers responsible for the design and construction of ITD’s work to blast the canyon walls and widen the road over the course of September 2020 through late fall of this year.
The records revealed warning signs of unstable slopes, raised questions about the rigor of geotechnical analysis prior to construction, and pointed to a price tag of between $15 and $20 million more than the $25.7 million original bid. An increase in the range of about 60 to 75 percent.
Prior to BoiseDev’s reporting, the general public did not know how close the November slide came to injuring or killing workers and travelers. The public also did not know the extent of the unstable rock discovered at the site where slides later occurred.
This story is the result of nearly six months of work to understand what happened along Idaho 55 near Smiths Ferry. Read our full editor’s note for more on how we produced this story – and why.
Records also turned up evidence pointing to one of the slopes that gave way being mistakenly blasted at a steeper angle than the design called for. The documents also show one ITD engineer’s extensive concerns about the project.
BoiseDev sent ITD a list of more than 30 specific questions about the project and the contents of the public records. Agency spokesman John Tomlinson responded a week later with a general statement, which did not answer the majority of the questions asked. ITD didn’t comment on why it kept the road open, even with concerns about slope stability, what sort of early testing was done on the slope or why the hillsides weren’t cut at shallower angles until after slides hit the project. The agency even declined to provide a map of the various so-called “cuts” along the construction zone.
“Safety of the traveling public is always ITD’s top priority, and we are taking the extra steps necessary to ensure this project results in a safe roadway for years to come,” ITD wrote in the email. “We appreciate the patience of communities and businesses along SH-55 that we have been reaching out to with updates during the work. We also appreciate the drivers on SH-55 for helping keep safety a priority through the work zone, and for the feedback, we received from the public and area business owners on minimizing future closures.”
“During construction of a mountain highway job of this type, it was not unexpected to learn more about the 9 cut slopes during construction and to adjust the engineering to fit the terrain, which was nearly 300 feet high and 500 feet long in less than a mile of highway distance.”