Transcript Jon Bevan Meghan O’Donnell December 10, 2003 Introduction and Objectives • We examined an active landslide along side of the Lee River in Jericho,

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.
• 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.
Location Map
N
37 meters in height
50 meters across
Scale = 100 km
See Jon for Scale
N
N
Scale = 0.5 km
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.
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.
Pit 2 Data
Unsorted fine to medium grained
sediment, 7 meters from river’s edge
See tape measure for scale
Pockets of gravel within
unsorted material
See tape measure for scale
Organic debris
See tape measure for scale
Vegetation and Ground Water
Vegetation covers approximately
50% of the slope. The roots stabilize
the slope laterally.
See Jon for scale
Outwash of subsurface groundwater
at the base of the slope.
Scale = 6 in.
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.
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.
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.
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.