Work is scheduled to resume this month on a bridge that will cross a slumping stretch of the road into Denali National Park. The $100 million bridge will span 475 feet across a landslide at Mile 45, in an area known as Pretty Rocks, where accelerated melting of a rock glacier has closed the road since August 2021.
During the summer of 2023, bridge project contractor Granite Construction built a work camp and staging areas, transported materials and equipment, and began site preparation that included blasting, excavation, slope stabilization and pioneering a temporary heavy equipment access route across the Pretty Rocks landslide.
Denali National Park engineer Steve Mandt called 2023 “a huge year for the project” at a recent virtual town hall event, during which he reviewed last season’s progress and previewed what’s to come.
Mandt said this spring’s work will complete excavation on the west side of the slide, followed by construction of bridge abutments, or foundations.
“These foundations are composed of these large precast concrete blocks,” Mandt said. “So each of those blocks is 15 feet long, 4 feet wide, 4 and a half feet thick. There’s 13 of those per abutment. Each one of those is about 40,000 pounds of concrete. So, this entire abutment is 52 feet long, 15 feet long and four-and-a-half feet thick, so it’s just a massive foundation that’s going to be supporting each end of the bridge.”
Mandt said the abutments will be secured into solid underlying rock on either side of the slide.
“Basically, it’s a hole drilled into rock into which a steel bar is inserted and then that’s grouted in place,” he said.
Mandt said the contractor will also begin installation this summer of thermosiphons, commonly used along the Trans-Alaska pipeline and some Alaska roads, to keep ice underlying rock and soil on the east side of the bridge from thawing.
“We all know that climate is changing and even recent trends in the park are showing just progressively warmer temperatures over time, so we want to make sure that the ice that is ultimately helping support the bridge…that ice remains frozen,” Mandt said. “So, the thermosiphon is the tool that’s going to be used to do that.”
This season’s work will also see construction of a platform on the east side abutment, and the first third of the actual bridge.
“So, on that launch platform this year the contractor is going to be erecting the first three bays of the bridge,” Mandt said. “So, the first three sort of triangles of the truss will be assembled and sitting on that launch platform at the end of this construction season.”
Mandt said the rest of the bridge bays will be constructed on site in 2025 and then the entire structure will be pushed from the east side out over the slide to a receiving structure extended from the west abutment.
“Progressively moved over to the west side and then once that’s done that launch nose, the temporary truss will be removed,” he said. “The truss will be lowered onto the foundation.”
Bridge deck and guard rail installation are slated for summer of 2026, after which the bridge will open for park service and private in-holder use. Park visitor buses are scheduled to begin driving across the Pretty Rocks Bridge in the spring of 2027.
