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ITD answers questions on landslides and ‘natural variability’ of construction after BoiseDev reporting on Idaho 55

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The Idaho Transportation Department released more information about the landslides on Idaho 55 along the busy corridor during the agency’s multi-year effort to straighten a portion of the highway in Valley County. 

This comes after BoiseDev published a report detailing difficulties with unstable slopes along the highway and internal emails from some department engineers raising questions about the rigor of geotechnical engineering leading up to the project. Governor Brad Little requested a report ensuring the safety of the corridor and a review of the article’s findings after its publication.

ITD responded to the over 30 questions BoiseDev submitted on June 17 prior to the publication of the initial article that went largely unanswered previously. BoiseDev staff also toured the work site on Wednesday for over an hour with project engineers.

The agency also walked back a passage in its letter to Little arguing an engineer BoiseDev quoted was just an “employee,” and didn’t have “close project participation.” In ITD’s response to BoiseDev’s questions, ITD now admits he was a geotechnical engineer and was involved in the design of the project on Idaho 55 for years.

Agency engineers say due to the amount of excavation required for the project and the heavily forested terrain on top of the canyon, they were unable to predict the quality of the rock they were expected to encounter when creating the design. ITD’s response to the questions said an initial design concept with steeper rock walls led to the first rockslide at the project in March of 2021. That slide sparked a project-wide redesign to address the weaker rock discovered once excavation began, as originally reported by BoiseDev. 

But even with a flatter redesigned slope, the hillside in one of the areas of the project still gave way and now requires an additional 100,000 cubic yards of material excavated in order to keep the hillside stable, Project Engineer Alex Deduck said. ITD says the slope has not moved since the landslide hit the area in November. 

When asked if the scale of excavation required for the project and difficulties the agency described with predicting rock quality led to risks during construction, Engineering Manager Jason Brinkman said there is “inherent uncertainty” in these types of projects where a large amount of rock needs to be cut. He said all projects involving heavy excavation, including this one, require engineering in the field to modify the design as they go along.

“Risk in some cases has the negative connotation of unmitigated risk,” Brinkman said, driving through the site. “That’s never our plan to go in with unmitigated risk, but rather take the natural variability that we know exists and put a plan in place to deal with it.”

BoiseDev will continue to research this project and the design process leading up to construction. If you have more information or witnessed the November landslide, please get in touch with us

ITD: November landslide a “slow-motion event”

Unlike the other two landslides that occurred at night during the project, the slide in November 2021 occurred in the afternoon as cars were preparing to move through the construction zone. 

Prior to BoiseDev’s reporting, it was unknown to the greater public how close the slide was to a pilot car moving through the construction site. But, the agency says although seeing the slide would likely be “intimidating” for drivers, it “did not pose an undue hazard to the traveling public.”

A photo of the landslide at Cut 8 taken right after the slide. Courtesy of ITD.

Brinkman said the low angle of the hillside at the time of the slide, and the composition of the highly eroded rock, meant the material slid off the hill instead of falling onto the highway. A statement from ITD spokesman John Tomlinson said the agency wasn’t trying to downplay the risks from the slide, but wanted to describe how it was different than a rockslide that fell directly onto the highway, like the first slide in March 2021.

“(The hillside) didn’t fall or didn’t even tumble, it just displaced,” Brinkman said. “When we talk about a slow failure…it just rolls over itself. It’s a relative term what is slow or fast in this space, but it didn’t fall on the road as much as it slid into the road.”

Multiple redesigns for Cut 8 

As originally reported, Cut 8 was a trouble spot early in the project and showed signs of instability leading up to the November slide. 

Brinkman said all of the hillsides in the project did have geotechnical analysis conducted on them, but the type of studies completed did not reveal the extent of the weakness of the rock the contractor would find once they began digging, particularly at Cut 8. He said a variety of tests were completed on the rock in the design phase, but they did not drill into the rock face until after landslides occurred and a new design for Cut 8 was required. 

Tomlinson told BoiseDev in a statement drilling into the hillside to test the rock quality before the design was not “feasible nor fiscally responsible” due to the very steep slopes, uneven surfaces, loose rocks, large trees, and difficulties with accessing the top of the canyon. He said by the time the slide happened, it was possible to get on top of the slope to drill into the slope and help inform the new design.

“The state of practice in heavy rock removal construction such as on (Highway 55), where extensive rock blasting is required, is not to drill such vertical rock slopes, but rather to rely on stereo net plots,
kinematic analysis, and seismic refraction for the project design decisions, with the intent of
being adaptive to rock conditions during construction excavation work with engineering experts
monitoring such as our geotechnical engineering support,” Tomlinson said in a statement.

In essence, ITD says it didn’t study the rock face due to cost or geologic factors involved in doing so. A mid-construction landslide made it possible to study the now exposed hillside. Ultimately, the cost of the project has increased significantly from $25 million to $45 million, in part due to the redesign and additional work involved.

Engineers noticed the rock around Cut 8 was not as stable as expected in September 2020 in the early days of construction, as originally reported by BoiseDev. At one point, emails reported by BoiseDev showed contractors drilled 30 feet into the rock face without finding material solid enough for anchor bolts to support the hillside.

Over the course of the fall and winter, Brinkman said the angle of the hillside got laid back at a shallower angle further and further until they opted for a 45-degree hill by the spring of 2021. This required more excavation, but both ITD engineers and consulting engineers from McMillen Jacobs expected this would help stabilize the loose rock in the hillside and prevent slides. 

A view of the base of Cut 8 in July 2022. Photo: Margaret Carmel/BoiseDev

But, the hillside kept shifting and eventually slid on November 18. This left the road closed for three weeks while engineers evaluated the slope. Now, the hillside has been further excavated to an even more extreme angle, laying it back 26 degrees from vertical with a large buttress of heavy rock at the bottom to keep the loose rock from shifting again long-term. This requires excavation even further back into the mountain than before with an additional 100,000 cubic yards of material removed. 

“That is a pretty unusual thing in our review and understanding to have an angle that low,” Brinkman said. “Below that, it can’t even slide. It slid on a surface that is almost as flat as possible to slide on. That is something that was unexpected. I don’t know if we did this 100 times someone could predict the feature of the slope.”

This is the last cut heavily under construction on the project, which is expected to be completed this fall. 

Designers debated flatter slopes, opted for steeper option

The steeper the angle of the rock wall on a project, the more engineering is required to keep it stable. 

The original design developed for the project called for less excavated hillsides at steeper angles stabilized with rock anchors drilled into the rock face, steel mesh pinned to wall, and other measures to keep the hillside from sliding, Brinkman said. This design was put in place at Cut 9, located at the far end of the project, with no warning signs of a slide or instability in the fall of 2020, as BoiseDev originally reported. 

But, things changed in the spring when snowmelt seeped into the rock face and fueled a slide. ITD told BoiseDev the first slide on the project was tied to the design not being properly suited to the quality of the rock on the hillside. 

“We started the excavation process in the fall when cuts are most stable,” Project Engineer Alex Deduck told BoiseDev. “We brought (the hillside) down and it was able to hold, but in the spring in March 2021 when that snow melt started and infiltrated the rock, that’s when it (slid).”

A view of a completed rock wall along the newly widened Idaho 55. Photo: Margaret Carmel/BoiseDev

This led to McMillen Jacobs redesigning every other rock cut on the project, changing the design to flatter angles with more excavation and less steel reinforcement midway through the project. Brinkman said geotechnical engineering reports in the design phase debated the pros and cons of this design and the possibility of flatter, more stable hillsides before construction began. But, ultimately he said ITD opted for the design with less excavation with more steel reinforcement and began digging. 

They had to revert to the flatter slopes after all when rock quality concerns arose on the construction site. An email quoted in BoiseDev’s original story pointed to changes required from the original design to prevent landslides. 

“This is a more inherently stable design and so we made the calculated decision to lay this slope back to mirror the natural fracture in the rock, which are things you can’t know with certainty until you get into them,” Brinkman said at the worksite on Wednesday. “In all cases, we made the decision to err on the side of safer slopes and inherently safe slopes than relying on a lot of engineering work.”

Agency walks back characterization of engineer critical of project

BoiseDev’s original article contained an email from Dave Richards, a now-retired geotechnical engineer, where he described a litany of concerns about the project and its design. 

In ITD’s response to the Governor last week, the agency referred to Richards as “an employee not involved” with the project and called his writing about the project “extensive conjecture.” On Friday, ITD changed its description of Richards, now admitting he was involved with the project for six years until his retirement a week after he penned the email BoiseDev recovered in a public records request. 

“In spite of being involved with the project from 2016 onward, during both design and construction, these ‘suggestions’ from Dave Richards didn’t come until January 2022, within a week of his retirement,” ITD wrote. “They also only went to his closest geotechnical engineering colleagues in HQ. Regardless, they are suggestions on how he now thinks the design should have been different, prior to construction, which by the time of his email was significantly complete in construction.”

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