Analysis of a Frozen Debris Lobe: A First Look inside an Impending Geohazard

Abstract

A permafrost-related geohazard is approaching the Dalton Highway near MP219. Analysis of historic remotely-sensed imagery indicated that this feature, termed a frozen debris lobe (FDL), was moving at an average rate of 0.4 in. per day between 1955 and 2008. More recent measurements indicate an increased rate of movement. As this frozen debris lobe (FDL- A) is less than 200 ft from the Dalton Highway, it is necessary to determine its current rate of movement, the nature of its movement, and its internal structure. We conducted field investigations in 2012 to determine the thickness and stratigraphy of FDL-A, and to install instruments to measure temperature, water pressure, and slope movement. Temperature measurements indicate that, although frozen, FDL-A is 2oF warmer than the surrounding permafrost. Results from the field work indicate that FDL-A is fairly homogeneous, consisting of silty sand with gravel, and is 86.5-ft thick where drilled, overlying white mica schist bedrock. It is moving mostly along a shear zone between 66 ft and 74 ft below the ground surface, at an average rate of 1.0 in. per day. As this rate was measured only during the fall season, however, it does not reflect any slowing of the lobe that may occur through the winter months. As these results are preliminary, future work will entail recording the positions of surface markers on FDL-A, conducting geophysical surveys of the feature, and modeling FDL-A to determine the geometry of the shear surface and residual strength in the shear zone, and to identify possible mitigation strategies.