AUTC project number: RR12.03
A slow-moving landslide, recently termed a frozen debris lobe (FDL), is approaching the Dalton Highway near MP 219. One of several FDL's within the Dalton Highway corridor, FDL-A at MP 219 is the closest to the highway (approximately 195 ft at its closest point to the northbound shoulder of the highway surface at the time of this writing). Previous analysis of images from 1955 through 2008 indicated an average movement rate of 0.4 in. per day. To better understand the movement of FDL-A, including its soil properties, the direction and rate of movement, and the nature of the shear zone, we initiated a drilling/sampling and monitoring program in 2012. We conducted a seismic survey in August 2012. The results indicated the presence of the permafrost table and cracks that penetrate from the lobe surface, resulting in areas of deeper thaw. Together with a drill crew from the Alaska Department of Transportation and Public Facilities (ADOT&PF) Northern Region Materials Section (NRMS), we conducted a drilling program during September 2012. We drilled a total of eight borings both on and off the lobe to 1) determine the soil profile and depth to bedrock typical of the greater area, 2) determine the thickness and stratigraphy of FDL-A, and 3) install instruments to measure temperature, water pressure, and slope movement. This feature, where drilled, is fairly homogeneous, consisting of silty sand with gravel. It overlies white mica schist bedrock, intercepted at a depth of 86.5 ft below the ground surface (bgs) at one location. Temperature measurements indicate that the soil of FDL-A is 30°F at depth, which is 2°F warmer than the surrounding permafrost. We measured water pressure within the lobe with a potentiometric surface 35 ft above the lobe surface. The water pressure at one boring was sufficient to enter into the sheared casing and flow up and out onto the ground surface. Additionally, we observed a significant volume of water flowing out from the lobe toe during the early winter. Since water pressure lowers the effective stress and reduces the overall soil strength, the presence of water within FDL-A suggests that it plays a major role in the lobe's rate of movement. FDL-A demonstrated at least two modes of movement, with a shear zone between 66 ft and 74 ft bgs (where measured), and slow to moderate flow above the shear zone. Combining these modes of movement, FDL-A was moving at an average rate of 1.0 in. per day over the length of this research project, which is more than twice the historic rate determined from previous remote sensing analysis. 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. These initial results are encouraging, as they provide a first glimpse into FDL-A. As Phase II of this research, we recommend the following future work at FDL-A: Continue to measure surface markers using a differential global positioning system, Continue to maintain the automated data acquisition systems installed as part of Phase I, collecting temperature and slope movement data, conduct a second drilling program on FDL-A, conduct additional geophysical surveys across the surface of FDL-A, including the DC-Resistivity method, conduct further laboratory testing of samples collected as part of Phase I, including strength testing of frozen samples,Model FDL-A in order to determine or confirm the shear surface geometry, to calculate the residual strength in the shear zone, and to identify possible mitigation strategies in order to protect the Dalton Highway.