When driving between Flagstaff and Phoenix, certain hazards seem clear: wildlife darting across the road, vehicles recklessly zigzagging between lanes, flooding, wildfires and moving at 75 mph just feet from the edge of a mountain.
As for the dangers lurking beneath the pavement, that’s for the experts to consider.
One such risk, landslides, is a priority when building or modifying a road, especially one that traverses through mountainous regions like Interstate 17, which will be expanded starting in the 2020 fiscal year.
For use in projects like this, geologists at the Arizona Geological Survey created the first ever statewide landslide database that documents more than 6,000 landslides, debris flows and rock slides.
The same geologists will also be working with the Arizona Department of Transportation (ADOT) to map the landslides adjacent to the construction area.
The $331-million project calls for two main components: the widening of Anthem Way to Black Canyon City by one lane in each direction and the creation of two flex lanes from Black Canyon City to Sunset Point.
The flex lanes will be constructed alongside the median side of southbound I-17 and would function differently depending on the time: holding northbound traffic on Friday and Saturday and southbound traffic on Sunday.
“With gates at each end, this two-lane system will accommodate traffic in the direction of greatest need at a given time,” ADOT spokesperson Steve Elliott wrote in an email.
Last week, the Governor’s Office announced Arizona won a $90 million Infrastructure for Rebuilding America (INFRA) grant to help fund the expansion, which is scheduled to be completed in 2023.
One of the initial steps of the project, an environmental impact study, is taking place now and should be completed by the end of the summer.
Although ADOT works closely with a variety of state environmental organizations – including the Arizona State Land Department, Arizona Department of Environmental Quality and the U.S. Fish and Wildlife Service – the geological survey will play a valuable role in the I-17 expansion project because it will occur in the middle of a landslide-heavy area.
“The geology itself is inherently unstable in these areas. Right now, I-17 gets very close to the edge of this plateau, which has dozens of landslides along it. The next landslide to occur will be even closer,” said Joe Cook, a research geologist with the Geological Survey who manages the Arizona Statewide Landslide Inventory Database (AzSLID).
Using funds awarded by the state’s emergency management department, Cook and his team will create a detailed map of the landslide features in a 15-mile stretch alongside I-17 from Black Canyon City to near the Agua Fria National Monument, using direct observations from field work and drone photos for harder-to-reach areas.
According to the final design concept report for the I-17 project, ADOT will also continue to use the slope-measuring inclinometers posted between the Black Canyon City and the Bumble Bee traffic intersections, where additional landslide features are present, “to measure the stability of the cut slopes along the northbound roadway.”
Recording an unstable history
Infrastructure is no match for landslides, like the one that split U.S. Highway 89 at the Echo Cliffs near Page in February 2013. It took more than $25 million and two years to rebuild.
Incidents like this prompted the creation of AzSLID.
In a 2017 report on the landslide database, it was stated that use of the information could improve “long-term disaster recovery and resiliency” statewide.
The endeavor was time consuming, though, as geologists had to piece together information from hand-drawn geologic maps, written reports, digital elevation models and aerial photos to determine the location, scale and type of Arizona landslides.
Google Earth was an especially helpful tool.
“The bird’s eye view is much better than seeing [a landslide] from the ground. If you were driving on a highway and looked in that direction, you might not notice it,” Cook said.
Through their efforts, geologists have uncovered hundreds of never-before-recorded landslides (many on reservations) from all 15 Arizona counties. The entire database now accounts for landslide features covering more than 1,200 square miles.
Cook said the key to spotting a landslide all depends on the texture.
“It looks like bites out of a cookie,” he said.
This lumpy texture also reveals the age of the landslide: sharp, angular cuts and cracks are younger, while the rounded, more-eroded features are older.
Despite the prevalence of landslides along the I-17 corridor, Cook and his team have not found evidence of recent disruptive land movements in the area.
“There’s some slow movement on that slide that is still ongoing. We don’t know in what timeframe this landslide is unstable. It could be 100,000 years, but it’s still a hazard that’s there and should be recognized,” he said.
Landslides catalogued by AzSLID can be viewed using the landslide filter on the Arizona Natural Hazards map at http://data.azgs.az.gov/hazard-viewer/.