Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 6 SLOPE STABILITY
and pore-water pressures along the slip surface.
Circular arc slip surface is often used because it simplifies the calculations by just conveniently
summing up the moments or forces about the center of the circle. Also, circular slip surfaces are
generally sufficient for analyzing relatively homogeneous embankments or slopes.
Fill Surface
after Failure
L w
Fill
Weight
Force
Center
L s
Failure
Case
Soft Clay
Resistance
Force
Sum of Shear Strength
along Arc
Figure 6.4 Typical Circular Arc Failure Mechanism
The requirement for static equilibrium of the soil mass are used to compute a factor of safety with
respect to shear strength. The factor of safety is defined as the ratio of the available shear
resistance to the driving force that can cause movement of the slope. In Figure 6.4, the factor of
safety (FOS) is
Resisting Moment Total shear strength x Ls
FOS = =
Driving Moment Weight force × Lw
(6.1)
Limit equilibrium analysis assumes the factor of safety is the same along the entire slip surface. A
value of factor of safety greater than 1.0 indicates that shear resistance exceeds the required for
equilibrium and that the slope will be stable with respect to sliding along the assumed particular slip
surface analyzed. A value of factor of safety less than 1.0 indicates that the slope will be unstable.
6.5.2 Location of the Critical Slip Surface
The critical slip surface is defined as the surface with the lowest factor of safety. Because different
methods of analysis like Bishop’s, Janbu’s and Spencer’s adopt different assumptions, the location
of the critical slip surface can vary among different methods of analysis. The critical slip surface for
a given problem analyzed by a given method is found by a systematic procedure of generating trial
slip surfaces until the one with the minimum factor of safety is obtained. Searching schemes may
vary with the assumed shape of the slip surface and the computer program used.
All external loadings imposed on the embankment or ground surface should be represented in slope
stability analysis, including loads imposed by water pressures, structures, surcharge loads, anchor
forces, or other causes.
6-6 March 2009