ABSTRACT

The largest naturally occurring landslide in New York State history began in early May 2011. Melting of winter snowpack followed by abnormally high volumes of April rain saturated the underlying glacial stratigraphy and initiated the 0.33 km2 (82 acre) landslide on Little Porter Mountain in the High-Peaks region of the Adirondack Mountains. Downslope soil movement rates between 15 and 60 cm per day demonstrated their destructive capacity as the shifting soils caused one house to be condemned because of the unstable ground and foundation while simultaneously rendering three other houses into immediate danger. The mechanisms controlling this landslide were investigated using geophysical methods, displacement measurements, and borehole logs. Nineteen electrical resistivity imaging surveys were implemented to map the subsurface glacial features. Ground penetration radar (GPR) profiles were collected along many of the resistivity lines and additional reconnaissance lines in several other locations. These surveys identified subsurface features such as the buried bedrock surface, thick sequences of loose zones of saturated sediments and clays within the glacial till, and boulder zones. The geophysical data were compared with displacement station measurements and precipitation records, which led to the interpretation that the landslide was triggered by large infiltration and the failure surface was along a clay-sand interface. The nature of the slide is revealed as a deep-seated rotational surface within the glacial deposits instead of the bedrock debris avalanche failures that are considered typical of the Adirondack region. The results of this study assist in the Little Porter Mountain landslide characterization and hazard potential classification of similar glacial deposits on slopes in the Adirondacks region.

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