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Pretty Rocks Bridge Project to Get Underway this Summer

Computer generated image of the bridge to be built over the landslide at mile 45 of the Denali National Park Road.
National Park Service
Computer generated image of the bridge to be built over the landslide at mile 45 of the Denali National Park Road.

(Denali National Park, Ak.) The 90-mile road into Denali National Park remains closed at about the halfway point, where it traverses a mountain pass. It closed there in 2021 when a melt caused landslide began moving too fast for maintenance crews to keep up with, but a soon to get underway bridge project aims to remedy the situation.

The one hundred-million-dollar, 475-foot bridge will span a melting rock glacier in Polychrome Pass that’s obliterated the gravel road at mile 45.

Looking west across the Pretty Rocks landslide on May 5th, 2023
Dan Bross
Looking west across the Pretty Rocks landslide on May 5th, 2023

Denali National Park public affairs officer Sharon Stiteler looks out over the accelerating landslide in the area known as Pretty Rocks.

“We’re standing at the edge of the original road surface, and about 70 feet below us you can see the remnants of the road.”

Stiteler says the park filled the slumping section of road with gravel, until crews could no longer keep up, forcing the road’s closure just east of the slide in September 2021.

“It got to the point of calculating how far they had to go to pick up the gravel, how much it was going to take to get out here to be able to maintain the road and make it safe enough for drivers to go across.”

Park science and resources team leader Dave Schirokauer underscores the challenging alpine terrain the section of the Park Road traverses, and how it’s changed since the road was built.

“They probably had no idea they were building a road across a rock glacier. It was completely inactive back then and it wasn’t really a problem until 2016, and so 1930 until 2016, it was a great road and just a little bit of climate warming that’s occurred in that era, along with the disturbance of creating a cut through here , really woke up this rock glacier.”

Heat dissipating thermosiphons will be installed to protect ice underlying solid rock where bridge abutments will be secured on either side of the slide.

Park engineer and bridge project manager Steve Mandt (MANT) underscored the scope of the endeavor.

“Just a massive amount of engineering and detail and thought goes into this.”

Mandt lays out a construction plan that calls for incrementally building the truss style bridge across the slide expanse without any underlying support.

“Starting at both the east and the west and then progressively work toward the center, so they will at some point be assembling bridge kind of out in space kind of hanging over the valley below.”

Work on the bridge was supposed to start in May but that’s been pushed to July, and the completion date from 2025 to 2026. Stiteler says the new timeline is primarily the result of a geotechnical issue.

“The discovery of more clay than was anticipated on the west side of the project. Originally it looked like it was going to be 30 thousand cubic yards of clay that was going to be excavated, and now it looks like it’s going to be 80 thousand cubic yards, and that’s a significant change.”

Stiteler says the more extensive clay has also resulted in some bridge design tweaks. Bridge contractor Granite Construction began mobilizing in the park this spring and is in the process of building a 50 worker camp in a gravel pit at mile 27 of the Park Road. ###