Pretty Rocks Progress

n August 2021, the Pretty Rocks landslide shut off access to a portion of Denali Park Road near Polychrome Pass. The project to rebuild the road and make it safe for visitors traveling through Denali National Park in the long term is now underway and is both on time and on budget.

Work on the project has required a significant effort on the part of the Federal Highway Administration, or FHWA, National Park Service, or NPS, and Granite Construction, which is serving as construction manager/general contractor on the project.

The goal of the Polychrome Area Improvements plan, which was created by the FHWA and NPS, is to restore reliable road access west of Pretty Rocks so park visitors can reach popular destinations and facilities, including Polychrome Overlook, Toklat, Eielson Visitor Center, Wonder Lake, and Kantishna.

The plan addresses several geologic hazards in the Polychrome area that threaten public safety and infrastructure and includes the construction of a bridge to span the Pretty Rocks landslide.

To mitigate the potential for permafrost thaw, twenty-three thermosyphons will be installed in the ground around the eastern abutment of the bridge. A retaining wall will also be installed on the uphill side of the road to the east of the bridge, as will a soil nail wall below the road near the east abutment, and rock dowels below the road near the west abutment for slope stabilization.

Workers prepare the east bridge abutment. The crew plans to install the 475-foot bridge next year.

“We will assemble about one-quarter of the bridge before winter, and it will sit on the job site until next season when we will finish assembling it and push it across the opening to land on the other side,” he explains. “That will be completed by the end of next season, and we’ll tie everything together and finish the project in 2026.”

“It’s been a long road from where we started to knowing how to develop a plan for launching a bridge,” says Moren. “Basically, you’re almost building two bridges—one a launching truss and then the actual permanent truss.”

The permanent truss bridge will be built in a semi-balanced, cantilever state over the east abutment and launch frame, and a launching nose truss will be built inside the permanent truss. The launching nose truss will be launched westward from within the permanent truss to set down on a receiving tower on the west abutment, and then it will be disassembled as the permanent truss structure is launched and then jacked down to its final position.

“There are a lot of technical weights and requirements and geotechnical requirements on the pilings, and all kinds of technical information that goes into finding out exactly what you can and can’t do, as well as the use of specialty rollers and specialty motors,” says Moren. “We hired subcontractor KWH Constructors to handle it since they build steel structures similar to this and design launch schemes.

“Our way to make the job happen is to find the right subcontractor with the right expertise, and KWH ran with it,” he adds. “We’re just here to support them.”

It is expected to take about four months, or one season, to construct and launch the bridge over the landslide and then to disassemble the temporary works. Once in place, Granite Construction will tie in the abutments, then add deck panels and friction surface coating on the bridge deck. Final road grading is tentatively scheduled for the summer of 2026.

The bridge will be placed approximately 40 feet above the landslide area, so even though that area will continue to slide, it will not affect the bridge or those using it.

“We removed some of the rock that was unsafe, pulling loose rock off so that it could be brought down in a safe manner,” says Thurman, adding that the actual landslide area is still moving every day.

“We’d get to the site early in the morning and drive across the landslide to the other side to check for settlement; if it occurred, we put more material in where it was needed and cleaned up the road to make it safe to work on,” he says.

Thurman adds that the team spent a lot of time and effort choosing two solid abutments to hold the bridge that spans the landslide. “The landslide is in the neighborhood of 150 to 175 feet wide, and the bridge length is 475 feet,” says Thurman. “There is still plenty of room on each side of the bridge if the slide gets wider.”

“We’ve got a night shift as well as a day shift, and that takes a lot of people and a lot of coordination to make it work,” says Thurman, noting that between twenty-five and sixty people are on-site at any one time, depending on the current scope of work.

The bridge structure alone, including the steel for the bridge and launch nose, required moving close to fifty loads of material into the park, he says.

“The bridge material is shipped up from Oregon, loaded on a train to Fairbanks, hauled by truck to Denali National Park, off-loaded and placed on other trucks, and then hauled into the park,” Thurman says. “The logistics are pretty challenging.”

And that’s not even considering the onsite logistics entailed.

“You never think about a project as being this complicated,” adds Moren. “There’s no room out there—it’s like a postage stamp. You’re working on a one-lane road going over a landslide and you have to get the parts and pieces and Conexs and trailers and pickups out there to work. There’s nowhere to stage anything.”

Because the project is in a national park, Granite partnered with National Park Service staff and the FHWA to determine where the staging areas would be and what they would look like.

“This had nothing to do with the construction of the bridge—it was just about how we would get things out there,” says Moren, adding that the staging areas will revert back to their natural state after use.

“How we got from Day 1 to where we are now has been a pretty amazing feat,” adds Thurman. “It has taken a lot of people working together in different avenues to make it happen. And a lot has gotten accomplished in a relatively short period of time.”