Effective Stress

1
Slope Stability
Text section 14.9 and 14.10 only
Failure Planes or Slip Surfaces
2
Slope Stability
W
c

• In general you have
• Driving Force Weight of Slope
• Resisting Force Strength of soil along slip
  surface
• Buttress at toe

3
Slope Stability

• In slope stability analysis we determine the
  Factor of Safety as a ratio of resisting forces
  to driving forces
• Fs Resisting / Driving
• Theoretically, any slope with a Factor of Safety
  less than one will fail and any slope with a
  factor of safety greater than one will not.
• Design focuses on the soil parameters and
  geometry that will provide the maximum factor of
  safety.
• Sometimes, the analysis of an existing slope will
  be what is called a parametric study that is
  establishing a factor of safety and performing an
  analysis that back calculates the strength
  parameters.
• The engineer will then determine his/her
  confidence level as to whether or not the soil
  has that strength through experience, lab, and/or
  field data.

4
Slope Stability
Example of Circular Slip Surface (from geoslope
software) Circular slip surfaces often used in
analysis as the most likely approximated shape of
the failure surface
5
Slope Stability
Non circular slip surfaces can also be analyzed
6
Slope Stability
In the previous slides, notice the slopes divided
into slices Common methods break the slope up
into slices for analysis
Tn
Wnsina
Tn1
Assume Side forces cancel out Do They? Now Wn
sin a (driving) N Wn cos a Tr shear strength
_at_ face c F tan f (resisting)
a
Pn
Wncos a
Pn1
Wn
Tr
N
a
Ln
7
Slope Stability
Performing this analysis on each slice and then
summing the components from each slice Fs
S (c L W cos a tan f) / S (W sin a)
8
Slope Stability
This analysis is very conducive to a tabular
solution
Fs S (10) / S (7)
9
Slope Stability - Example
Each box is 5 x 5 120 pcf c 300 psf f
32o
10
Slope Stability - Example
First, Find the areas for each slice A1 A2 A3 A4
11
Slope Stability
Fs S (10) / S (7)