MODELING FROZEN DEBRIS LOBE-A (FDL-A), SOUTHERN BROOKS RANGE, ALASKA

Frozen debris lobes (FDLs) are slow-moving landslides along permafrost-affected slopes. We have identified approximately 200 within the southern Brooks Range, Alaska, with 43 FDLs located within the Dalton Highway corridor, and 23 occurring less than one mile uphill of the Dalton Highway. FDL-A is the closest FDL to the Dalton Highway, which measured just over 41 m away from the highway embankment in August 2014. Based on analysis of historic imagery dating back to 1955, FDL-A’s average rate of movement increased from 0.77 cm/day to the current rate of 1.3 cm/day. Should the rate of FDL-A increase even more to match rates measured on other, faster-moving FDLs, it could cover the distance to the highway in about a year. Because of its historic movement rate, size, proximity to infrastructure, and potential as a geohazard, FDL-A was the focus of a subsurface investigation in the fall of 2012, and since that time, we have made on-going measurements of surface and subsurface movement. Here we present the results of laboratory testing of frozen samples for strength properties, and the configuration and analysis of a conventional slope stability model. We developed the model in order to: 1) identify the location of the shear zone throughout the lobe; 2) determine the effect of water pressure on slope stability; and 3) identify possible mitigation techniques. The location of the shear zone in the model results agreed with the limited subsurface information. The model was most sensitive to changes in cohesion of the frozen soil and to water pressure, suggesting that FDL stability is highly dependent on temperature and precipitation.