Polychrome Area Improvements

The Pretty Rocks landslide has been active since the 1960s but began accelerating in 2014. This landslide occurred where ice-filled gaps in a porous volcanic tuff and perlite deposit, leading to progressive movement of weak and overlying rock. In 2021, the Pretty Rocks landslide began to speed up with movement accelerating from inches per year to inches per week, with little signs of slowing down.

As a result, Alaska’s Denali National Park faced a literal barrier to public access when the landslide began failing beneath a portion of Denali Park Road near Polychrome Pass faster than it could be rebuilt. Geologic conditions in the landslide zone continued to deteriorate over the past several years. The high-profile and complex project has attracted attention in several feature articles earlier this year from ASCE and Alaska Contractor.

“Higher local temperatures accelerated melting of the permafrost,” the ASCE authors explained in their article. “Eventually, road grade movement reached more than a half an inch per hour, and maintenance crews could no longer bring in material fast enough to replace the material lost to the slide. The old ways of maintenance were no longer sustainable.”

A large part of the local trade and tourist economy had been tied to the surrounding road, with an estimated $1 billion each year from tourists looking to take in Alaska’s scenic beauty. To mitigate disruption to the park and the economy, the project team brainstormed a new solution—one that did not involve replacing fill material over an active landslide. After several studies and geotechnical risk assessments, the design-build team settled on a solution, a permanent truss bridge spanning the landslide from one side of the valley to the other. With the design for the abutments and permanent 425-foot-span structure ready, contractors had to devise a way to install the bridge over an active landslide—an environmentally sensitive area—without temporary construction trestles or any crane support along the span.

GeoEngineers was hired to help devise a 100-foot micropile-supported launch bed that would allow the bridge to cantilever 425 feet over the valley before touching down on the permanent abutment on the other side of the valley. To make this monumental feat even more challenging, the same problematic landslide unit from the back of the valley dominated the geology of the launch area.

To find the most efficient and safe ground improvement solution for this challenging site, GeoEngineers’ construction design team designed and conducted a full-scale and instrumented micropile test program.

Following the test program, GeoEngineers’ construction design team designed a 34-pile system and load frame to support the bridge during launch.

To further support this project, Geoengineers’ geotechnical team modeled the effects that launching the massive bridge structure into place would have on global stability. They assessed the effects of launch loading through the micropiles and considered the impact of various support cranes and equipment loading during system erection. Reflecting on the project, Nathan Van Winkle cites three main challenges: the scale, remoteness, and compressed timeframe.

“A 425-foot cantilever span creates a massive moment arm and high lateral and vertical loads to be resisted, and the dynamic nature of the system led to complex load cases to be evaluated,” said Senior Geotechnical Engineer Nathan Van Winkle.

GeoEngineers’ work on the project may have come to a close in 2024, but the bridge was just launched this summer, with the nose portion touching down on the west abutment on July 26. The bridge now stands at the mid-stage of construction, still supported by the temporary micropiles. The construction teams are expecting that the bridge will be in its final position by the end of summer 2025, once the remainder of the structure is pulled into place across the valley and the temporary landing nose is disassembled.