Landslide 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).
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).