Landslides in the Fairbanks North Star Borough, Alaska: inventory map and Tanana 440 landslide assessment

Abstract

Landslides are geologic hazards that threaten human life, property, and infrastructure. Effective threat mitigation requires knowledge of where past landslides occurred. Until now, no published landslide inventory maps existed for any part of Alaska. Here we present an overview of our landslide mapping within parts of the Fairbanks North Star Borough (FNSB), Alaska and a thorough investigation and assessment of the Tanana 440 (T440) landslide. We mapped 1,679 landslides, and field-verified 51 landslides within the FNSB. These landslides vary in age, movement type, and material. Most are prehistoric, but we did observe some historic and active landslides. Observed slope failures include flows in soil, translational and rotational slides in bedrock, and complex features that combine multiple types of movement. Potential landslide triggers may include thawing permafrost, increased pore water pressure conditions, seismic events, and river erosion. The landslide inventory map, the first of its kind for Alaska, directly benefits the Borough, the State, and the general public, as it can be used by public agencies to make informed land management decisions and to incorporate landslides within multiple-disaster scenarios. Additionally, the map serves as the foundation for future landslide analysis within the FNSB. We also present results of in-depth mapping, subsurface exploration, soil engineering properties, and slope stability analysis of the T440 landslide. Based on analysis of stratigraphy, soil testing, and geomorphology, we determined the T440 landslide is a flow slide in loess that occurred during the late Pleistocene to mid-Holocene. Our modeling results suggest that thawing permafrost and/or seismic loading were possible triggers for the T440 landslide. We also present the first comprehensive direct shear testing of non-plastic silt with variation in moisture content, as well as the first comparison of direct shear and field vane shear measurements of silt. These results can be used for engineering design purposes for Interior Alaska silt for any gravimetric water content over 5%.