Site investigation, and GIS and slope stability analysis 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. Here we focus on FDL-A, which is an impending geohazard to the Dalton Highway, located just under 40 m away from the embankment. We present the results of multi-faceted research, including field-based studies, GIS analysis, laboratory testing of soil samples, and slope stability 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. Field observations indicate the presence of massive, infiltration ice associated with cracks on the surface of the lobe. Surface measurements show an overall average rate of movement of 1.2 cm per day, which is an increase over historic rates. 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. 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 cohesion and pore water pressure. 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.