When Nature and Infrastructure Collide: Examining Frozen Debris Lobes and Their Impact on the Dalton Highway, Brooks Range, Alaska

Abstract:

Frozen debris lobes (FDLs) are one of the many types of periglacial features found in the Brooks Range of Alaska. We have identified over 200 FDLs in the Brooks Range, with 43 located within the Dalton Highway corridor. These slow-moving landslides in permafrost consist mostly of silty sand with gravel, incorporating trees and other organic debris as they move down slope through a combination of slower internal flow and major shearing within discrete zones at depth. FDLs move throughout the year, resulting in large cracks that are open at the surface. Snow melt and rain water entering these cracks freeze, forming zones of infiltration ice. Water that reaches the shear zone facilitates movement, with a peak in shear occurring around October 1 (approximately 140-150 days after snow melt). Over the last decade, we focused our observations on eight FDLs, selected for their proximity to infrastructure. Analysis of aerial and satellite imagery since 1955 indicates that there is an overall increase in movement rates for these eight FDLs. On-the-ground measurements collected since 2008 using a real-time kinematic global positioning system (RTK-GPS) support the long-term trends. The fastest FDLs currently move at average annual rates of over 20 m yr^-1.

FDL-A is the largest of the studied FDLs and the closest to the Dalton Highway. Its movement rate has been steadily increasing since 1955, recently transitioning from a linear to exponential movement trend, with an average annual rate of movement of 9.3 m yr^-1 in 2019. Recognizing the risk associated with FDL-A, the Alaska Department of Transportation and Public Facilities realigned the Dalton Highway to a location about 122 m down slope in 2018. Part of the original embankment was left in place in front of FDL-A, representing a unique opportunity to measure the impact of this landslide on an engineered structure through a full-scale field experiment. Anticipating collision by early 2021, in the summer of 2020 we installed instruments within the abandoned embankment to measure water pressure, temperature, horizontal displacement, and earth pressure. In this presentation, we briefly summarize the previous decade of field investigations, and provide the first look into the imminent collision of FDL-A with the Dalton Highway embankment.