Site Investigation and Slope Stability Analysis of the Chitina Dump Slide (CDS), Alaska

We present the mapping and subsurface analysis of a slide occurring in discontinuous permafrost located near Chitina, AK. Our investigation included mapping landslide features, conducting seismic surveys, collecting tree-ring samples for dendrogeomorphic analysis, and conducting a slope stability analysis to back-calculate residual soil strength properties. Referred to as the "Chitina Dump Slide'' because of its previous use as a dump, the active slide intersects a state road and is an ongoing maintenance issue. The soils in the area consist of surficial silt overlying clayey glaciolacustrine diamicton, overlying bedrock. Drilling data indicated the deepening of the permafrost table in the active slide area from 6.1 m in 2005 to greater than 16.5 m in 2010. Field mapping indicated that the landslide extent is much larger than the active portion of the slide. The dendrogeomorphic analysis indicated that movement occurred in at least two portions of the area in the 1940s and in a third area later in the 1990s. Seismic surveys, with typical velocities ranging from 100 to 900 m/s, produced good correlation with drilling data and proved to be an effective method of subsurface imaging. From these observations, we conclude that the failure mechanism for the active portion of the slide is different than that of the overall slide area. Backcalculation of slope failure within the clay layer indicates a residual effective friction angle of 22 degrees. Permafrost degradation due to the thermal effects of the dump may be a contributing factor in the ongoing movement of the active slide area.