Margaret M. Darrow

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

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.

Cold Region Applications for In-Place Inclinometers Based on Microelectromechanical Systems Technology: Four Evaluation Case Studies

Inclinometer probes are an industry standard for measuring ground movement; however, this technology has drawbacks, including costly trips for manual measurements, operator error, and limited measurements caused by casing deformation. Relatively new in-place inclinometers based on microelectromechanical systems (M-IPIs) consist of microelectromechanical systems accelerometer segments separated variously by flexible joints or field connection systems and encased in a watertight housing. M-IPIs provide nearly continuous ground movement measurements, accommodate greater ground movement because of their flexibility, and may contain temperature sensors, useful for applications in frozen ground. Two M-IPIs from different manufacturers were evaluated at four research sites in Alaska (a) to monitor creep in frozen ground (vertical installation), (b) to identify and monitor a slide shear zone (vertical installation), (c) to monitor thaw settlement under a newly constructed embankment (horizontal installation), and (d) to identify movement mechanisms within a slide and to evaluate the M-IPIs functionality through large amounts of shear (vertical installation). Each M-IPI was evaluated for ease of installation and subsequent retrieval, durability, and functionality in frozen ground. Measurements from both devices compared well with those from the inclinometer probe; this feature indicated that these devices were suitable for use in cold regions. However, field experience indicates that the installation procedure for each instrument is better undertaken at temperatures above freezing because of the manual dexterity required. For horizontal installations over permafrost, it is recommended that casing ends be secured to posts anchored below the permafrost table and any cold-affected plastic components be replaced between installations to avoid unwanted breakage during reinstallation.

Characterizing a Frozen Debris Lobe, Dalton Highway, Alaska

Frozen debris lobes (FDLs) are slow-moving landslides along permafrost-affected slopes. Within the Dalton Highway corridor in the southern Brooks Range, Alaska, the authors have identified 43 FDLs, with 23 occurring less than one mile uphill of the Dalton Highway. FDL-A, which is the largest and closest FDL to the Dalton Highway, was just over 42 m away from the highway embankment when measured in August 2014. Since the completion of a preliminary subsurface investigation in 2012, they continue to collect subsurface temperature and inclinometer data, to measure surface movement rates, and to make field observations, which reveal the dynamic nature of FDLs. Significant rainfall during the summer of 2014 caused a deepening of the active layer on FDL-A and exposed massive “infiltration” ice in the head scarps of two retrogressive thaw slumps on its surface. The infiltration ice occurs in bands associated with transverse and longitudinal cracks, and the 2014 observations indicate that FDL-A may contain more massive ice than previously indicated. FDL-A moves mostly through basal shearing and through slow to moderate flow, which is highly dependent on temperature. The average rate of movement of FDL-A from 2013 to 2014 was 1.3 cm day−1. Comparing this value to the trend of historic rates indicates that the overall rate of movement of FDL-A continues to increase.

Active Layer and Frost Bulb Interaction for a Full-Scale, Buried Chilled Gas Pipeline

Recent economic trends have sparked renewed interest in the development of a gas pipeline to transport natural gas from Alaskas North Slope to markets in the contiguous United States. A buried chilled gas pipeline is a suitable design choice because of safety and environmental concerns. One major drawback is the interaction of the frost bulb with unfrozen, frost-susceptible soils. From 1999 to 2003, the University of Alaska Fairbanks and Hokkaido University in Japan ran a joint experiment to study the behavior of a full-scale, buried chilled gas pipeline. The pipeline, located outside of Fairbanks, Alaska, was constructed across a permafrost/non-permafrost boundary, in order to study the upheaval and stresses induced in the pipe due to differential frost heave. This paper details a small component of this project, namely the interaction between the frost bulb and the active layer immediately above the pipe. The temperature interaction at this interface was modeled using TEMP/W, a two-dimensional finite element program. Analysis of the model results indicates that the modeled temperatures correspond well to measured temperatures. The model results also indicate that at several times during the summer months, the frost bulb above the pipe was dramatically thinned due to spikes in the pipeline gas temperature. Coupled with cumulative stresses created in the pipe due to differential heave, this may result in loss of pipe structural integrity and ultimate failure. These results suggest that burial depth and the type of foundation soil present are critical elements of the final pipeline design.

Improvements in Measuring Unfrozen Water in Frozen Soils Using the Pulsed Nuclear Magnetic Resonance Method

AbstractThe pulsed nuclear magnetic resonance (P-NMR) method is commonly used to determine unfrozen water content in frozen soils. Traditionally, unfrozen water content is calculated using first re...