Modeling Slope Stability

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Modeling Slope Stability
Jason Shelton March 15, 2004
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Outline

• Objective
• Study Area
• Project Schematic
• Data Collection
• Stability Model
• Model Design
• Analysis of Model Results
• Stereonet Analysis
• Conclusions and Suggestions

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Objective

• Model slope stability along a portion of the I-81
  corridor in southwest Virginia.
• How do we improve the model? What can we add?

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Study Area

• The study area covers five USGS 7.5 minute
  quadrangles Daleville, Villamont, Montvale,
  Roanoke, and Salem.

Daleville
Montvale
Villamont
Salem
Roanoke
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Data Collection

• Hardcopy geologic maps scanned and digitized
  using AutoCAD, converted to shapefile, then to
  grid in ArcGIS
• 10 meter Digital Elevation Models
• TIGER data

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Data Collection
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Model Design
Safety Margin Resisting Forces (RF) Driving
Forces (DF)

• Slope angle (a)
• Cohesion (c)
• Friction angle (f)
• Unit weight of rock (g)
• Slab thickness (t)

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Model Design

• slope grid was created from each DEM
• attribute tables of geologic shapefiles were
  modified to include typical values of cohesion
  (c) and friction angle (f)
• unit weight (g) of rock used in this analysis was
  150 lbs/ft2
• slab thickness (t) used in the initial analysis
  was eight feet

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Analysis of Model Results

• negative safety margin results when SDF gt SRF
  (stability issues exist that should be addressed)
  
• very low, but positive safety margin values,
  further investigation should be conducted.
  (Options include comprehensive field study and
  the use of stereonets to determine failure
  potential.)

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Analysis of Model Results
Model results covering the study area.
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Analysis of Model Results
In the initial analysis, the Salem quadrangle
showed safety margin values as low as 0.02 in
some areas. These values were obtained using a
slab thickness of 8 feet.
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Analysis of Model Results

• With a slab thickness of 5 feet, the new results
  from the safety margin equation for the Salem
  quadrangle show that decreasing the slab
  thickness caused an increase in the safety
  margin, indicating that fewer slopes are likely
  to exhibit instability.

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Analysis of Model Results

• A further revised slab thickness of 12 feet,
  shows an overall decrease in the safety margin of
  the slopes. This slab thickness value essentially
  results in a more conservative estimate of slope
  stability.

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Analysis of Stereonets
Topple Failure
Friction Angle
Slope Face
Planar Failure
Wedge Failure
Slide Direction
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Analysis of Stereonets

• Stereonets were created for areas of potential
  instability on the Salem quad. These stereonets
  show two potential failure areas in the planar
  critical zones.

Planar Critical Zone
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Analysis of Stereonets
Toppling Failure
Plane Failure
Wedge Failure
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Conclusions

• Model only deals with rock slopes
• Incorporate soil and water into model
• Best suited for daylighting slopes?
• How can we incorporate bedding?

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Conclusions

• Looks only at static safety, add critical
  acceleration and shaking intensities to obtain
  seismic susceptibility. (R. Jibson, E. Harp, J.
  Michael - 2000)