Abstract:
When accelerating movement of the Pretty Rocks landslide threatened to permanently sever access to more than half of Denali National Park in 2021, engineers faced a challenge with no straightforward solution. After evaluating rerouting alternatives, the selected solution was a 475-foot steel bridge spanning between two rock outcroppings on either side of the landslide. The project design and construction is one of the most logistically complex bridge projects undertaken in the Alaskan wilderness.
The project's constraints were extraordinary: dynamic and complex geologic conditions, severe environmental and access restrictions, and a construction footprint limited to less than 100 feet located on the side of a mountain. To navigate these challenges from the outset, a Construction Manager/General Contractor (CM/GC) delivery model was adopted, enabling deep collaboration between design and construction teams throughout the design phase and ensuring constructability remained central to every decision.
The geotechnical scope required to stabilize the rock outcroppings and support the bridge and construction loading required an integrated array of elements including rock bolts, soil nails, micropiles, ground anchors, and thermosiphons installed within a severely constrained work zone. Extensive 3D subsurface, structure, and foundation modeling was used to locate each element precisely and evaluate the performance under a variety of loading, geotechnical and climatic conditions.
The site constraints mean that the steel bridge had to be constructed within the limited work area and then launched across the slide zone, cantilevering up to 425 feet during the process. This generated massive vertical and lateral forces on the steel launch frame, which were managed through a global stability analysis and a purpose-designed system of 34 micropiles transferring loads into the rock formation.
This presentation will describe the integrated geotechnical, structural, and contractor design approach, the analytical methods used to evaluate system performance, and the construction innovations that made this project achievable, offering lessons applicable to remote and access constrained infrastructure projects worldwide.