Photos & Text, J. Kim, 2000
At the Jeffersonville landslide site, approximately
50' of silty clay deposited in a Pleistocene glacial lake are overlain
by approximately 50' of fine deltaic sands that prograded northward
into this glacial lake. Due to a pronounced permeability contrast,
water tends to percolate through the sand and accumulate along the
sand/silty clay interface. North of the landslide, a nearly vertical
clay bank rises from the level of the Brewster River and is overlain
by fine sand. Some geologic factors
that contributed to the Jeffersonville landslide are: 1) geologically
heterogeneous materials 2) accumulation of water in these glacial
sediments and 3) erosion of the west side of the Deer Run Heights
ridge by the Brewster River. These conditions still persist in the
vicinity of the slide.
The presence of cohesive silty clay at the base of the Deer Run Heights Ridge allows for the development of steeper banks than would occur if this ridge was composed of non-cohesive fine sand alone. Because the cohesive clay is capable of holding up large volumes of overlying sand, larger landslides will result from this situation than from sand alone. As the river erodes into the clay, the development of oversteepened unstable banks is likely. The bank may fail catastrophically, bringing the overlying sand with it.
Due to a pronounced permeability contrast
between the clay and overlying sand, water will tend to percolate through
the sand and accumulate at the sand/clay interface. This water, in addition
to increasing the weight of the clay, may lubricate the sand/clay interface
and facilitate failure along this surface. The water in the clay, if
under sufficient hydraulic head, may also lead to an increase in pore
pressure along a buried surface and contribute to slope failure.
A View from the Top:
Sand, clay and debris came to rest close to neighboring homes.
As with floods,
earthquakes, volcanos, and other geologic phenomena it is not possible
to predict with certainty when a catastrophic event will occur. Although
are often governed by complex geological factors, a set of "geoindicators"
has been suggested by the International Union of Geological Sciences.
Monitoring for the presence of ground cracks and, if present, any increase
in their spacing, the appearance of and increases in ground subsidence
or upheaval, and measuring the area of slope failure are all possible
monitoring which may provide some insight into future potential failures.
VT DEC Geology and Mineral Resources Division 1 National Life Drive, Davis 2 Montpelier, VT 05620-3920
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