jon bevan meghan o’donnell december 10, 2003. introduction and objectives we examined an active...
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![Page 1: Jon Bevan Meghan O’Donnell December 10, 2003. Introduction and Objectives We examined an active landslide along side of the Lee River in Jericho, Vermont](https://reader036.vdocuments.us/reader036/viewer/2022070408/56649e605503460f94b5aa0e/html5/thumbnails/1.jpg)
Jon Bevan
Meghan O’Donnell
December 10, 2003
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Introduction and Objectives
• We examined an active landslide along side of the Lee River in Jericho, Vermont.
• The main slide occurred several years ago and recently there have been smaller slides.
• We wanted to investigate:– stratigraphy and cohesion of the sediment.– vegetative cover on and around the slope.
• Infer geologic history of the sediment.• Conclude why the slope continues to fail.
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Location Map
37 meters in height50 meters across
See Jon for Scale
Scale = 0.5 km
Scale = 100 km
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Methods
• Dig eight, 1m deep soil pits vertically up the landslide to observe stratigraphy:– Orientation of bedding,– Sediment grain size and graded bedding,– Cross-bedding and Ripples.
• A handheld device to measure cohesion of particular sediment layers.– Cohesion is the soils ability to stick to itself.
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The Landslide Data• The slope of the landslide is 28˚. • The sediment continually increases in grain size with
increased elevation, besides Pit 2.• Alternating fine to coarse layers.• All tilted layers, cross bedding, and ripples are oriented
northeast. • Finer grained sediment was more tightly packed and
exhibited greatest cohesion. • Vegetation covered the southern half of the slide was
covered with vegetation.
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Pit 2 Data
Unsorted fine to medium grained sediment, 7 meters from river’s edge
Pockets of gravel within unsorted material
Organic debris
See tape measure for scale
See tape measure for scale
See tape measure for scale
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Vegetation and Ground Water
Scale = 6 in.
Vegetation covers approximately50% of the slope. The roots stabilize the slope laterally.
See Jon for scale
Outwash of subsurface groundwater at the base of the slope.
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Geologic History• 12-13ka the Laurentide Ice mass ablated
from this region of Vermont.– Served as an ice dam for northward flowing
water
• Glacial Lake Vermont
• Creation of deltas into glacial lake
Note: Photo from Benn and Evans, 1998. And please disregard blank spots.
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How the Slide has Changed (1962 and 1999)
1962 1999*1962 landslide is almost twice as larger than present scarp. *Vegetation on farmland terrace has increased significantly.
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Explanation of why the Slope Fails
• Slope of the slide is quite steep.
• Sub-surface flow of H2O– Normal force decreases as pore pressure stays
the same, decreasing cohesion.
• Undercutting of river
• Insufficient vegetation to completely stabilize the slope.
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References• Wright, Steven. 2002 and 2003. Personal Conversations and Class
notes.• Chapman DH. 1937. Late- and Post-Glacial Champlain Valley. In
American Journal of Science. 34:53.• Bierman Paul. 2003. Class notes.• Benn, D, Evans, D. 1998. Glaciers and Glaciation. Arnold
Publishers. London, England.• Bloom, A. 1991. Geomorphology: A Systematic Analysis of Late
Cenozoic Landforms. Prentice Hall. Englewood Cliffs, New Jersey, USA.
• Kochel R, Miller J, Ritter D. 1978. Process Geomorophology. WCB Publishers. Dubuque, IA.