Investigating Movement and Characteristics of a Frozen Debris Lobe, South-Central Brooks Range, Alaska

Abstract

Frozen debris lobes (FDLs) are large masses of soil, rock, incorporated organic material, and ice moving down permafrost-affected slopes in the south-central Brooks Range, Alaska. In this paper, we focus on FDL-A, which is an impending geohazard to the Dalton Highway, located just under 40 m away from the highway embankment. We present the results of multi-faceted research, including field-based studies, laboratory testing of soil samples, slope stability analysis, and a GIS analysis. Subsurface instrumentation indicates that major movement of FDL-A occurs in a shear zone 20.6 to 22.8 m below the ground surface, with temperature-dependent internal flow as a secondary movement mechanism. Surface measurements show an overall average rate of movement of 1.2 cm per day, which is an increase over historic rates. The slope stability analysis required a back analysis to determine soil strength parameters at failure, resulting in cohesion values between 43 to 53 kPa and friction angles between 10° and 16°. The modeling results indicated a high sensitivity to pore water pressure and cohesion. This is critical since the melting of massive ice and thawing of frozen soil will increase pore water pressure and lower shear strength, resulting in the acceleration of FDL-A towards the Dalton Highway. The GIS analysis also provided insight into the movement and instability of FDL-A, and provided groundwork for a GIS protocol to examine catchment and lobe features of all FDLs along the highway corridor.