A geotechnical investigation of Pretty Rocks Landslide, mile 45.4, Denali Park Road, Denali National Park, Alaska

Stanczyk, Anna; Capps, Denny; Anderson, Douglas

doi:10.36967/nrr-2288333

As of 2016, the Pretty Rocks Landslide causes approximately 0.8 m of annual subsidence of a 90-m section of the Denali National Park Road. The dead-end road is the only access into the park and is a regionally important economic engine. Because of these reasons, combined with its location near the road’s midpoint, the landslide is currently the highest rated unstable slope among 141 sites along the road according to the Unstable Slope Management Program for Federal Land Management Agencies. The landslide's morphology and composition inform us about both its history and likely future. The surrounding bedrock comprises altered, highly fractured rhyolite and basalt. Two unconsolidated sediment lobes below the road are primarily rhyolitic and a third is basaltic. These are likely deposits from previous mass movement events. Surface and subsurface data demonstrate that clays are pervasive throughout the slope. These clays are weathered tuffs and Atterberg Limit tests show that they are highly plastic and have potential for shrinking and swelling. Bore logs and geophysical surveys indicate that ice-rich permafrost is present below the road surface in these clays and other coarser sediments. Because the permafrost is very near 0° Celsius, it is likely thawing, which would be consistent with regional trends. As a further complication, uncontrolled water drains both internally and externally down the steep slope. The landslide’s persistent creep-style movement is likely due to the steep topography, weak underlying clay, thawing ice-rich permafrost, and poor drainage. We classify this unstable slope as a soil slope deformation based on Hungr et al.’s modified Varnes classification system. This study compiles previous geotechnical data and provides new insight on failure history and mechanisms to support hazard and risk assessment, and potential risk reduction strategies by the National Park Service with technical assistance provided by the Federal Highway Administration.

A geotechnical investigation of Pretty Rocks Landslide, mile 45.4, Denali Park Road, Denali National Park, Alaska

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