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          logoLandslide Initiation and Progression: Site Investigation, Field Monitoring, Laboratory Testing, Theoretical Framework, and Computational Analysis

Schlosser, Kenneth W.; Sherrod, Laura A. ; Kozlowski, Andrew; Bird, Brian; and Swiontek, Jarred, 2011, Exploratory Study of an Active Landslide in the Adirondacks Using Applied Geophysics [abs]: American Geophysical Union - Fall Meeting in San Francisco, California (5-9 December 2011).

Dr.
          Laura Sherrod - NY landslide project

Residents of Keene Valley, NY face a serious natural hazard in the form of a landslide on Porter Mountain in the High-Peaks region of the Adirondack Mountains. The slide initiated in early May 2011 as a result of the melting of heavy snowpack and the onset of abnormally excessive April rain on the mountain slopes. Spanning 82 acres, this is the largest documented landslide in New York state history. Although it is advancing slowly, with downslope soil movement rates between 15 and 60 cm per day, the slide has proven to be destructive. At the time of this study, shifting soils had caused one house to be condemned due to the unstable ground under the foundation. At the same time, three other houses were in immediate danger. The destructive nature of this landslide speaks to the importance of understanding the distribution and character of glacial sediments deposited on the steep slopes during deglaciation of the region, and the interaction of a complex groundwater system.

In order to understand the framework and mechanisms of the current landslide and to aid in predicting the potential for other slides in the area, geophysical methods were employed. Geophysical surveys and corresponding subsurface imaging were used to examine the amount of sediment present and the stratigraphy of the shallow subsurface. The bedrock in this area is believed to be anorthosite which underlies a surficial lithology of glacial sediments. Depth to the bedrock was measured at 76 m in a borehole at the base of the slide. However, in a well near the top of the slide, depth to bedrock was measured at 6 m, with some exposures of bedrock visible at the surface. To delineate three-dimensional trends of the bedrock in the subsurface, several of the geophysical surveys followed the surface exposures of bedrock to a depth where these features were no longer detectable. Nineteen resistivity surveys were implemented to map the subsurface glacial features and depth to bedrock using a MPT DAS-1 Electrical Impedance Tomography System. GPR profiles, using a SIR 3000 GSSI radar system with 100MHz antennae, were collected along many of the resistivity lines and through reconnaissance lines in several other locations (e.g. along roads).

Surveys identified features such as clay and sand layers as well as depth to the water table. Glacial deposits and bedrock topography were interpreted in three dimensions from the results of these surveys. These techniques were analyzed for their effectiveness in providing exploratory information about the slide. In comparison to other geophysical work on landslides, this study is unique due to the large scale of the slide and the rare opportunity to observe and measure an active landslide. Accordingly, compared to results from studies of other similarly induced inactive landslides that had occurred elsewhere, the conclusions regarding the mechanisms of slope failure on Porter Mountain are more pertinent since the results were obtained from an active slide. Likewise, the conclusions about the mechanisms of this slide can be adapted in studies on currently stable slopes believed to have a high potential for a landslide (particularly other slopes in the Adirondacks region).


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Kurt Friehauf - December 2011