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All limit equilibrium methods of slope stability analysis have four characteristics in common (Duncan and Wright, 1980):

1. All use the following definition of the factor of safety (F):
   

   (1)

   Placing a factor on shear strength is appropriate because evaluation of the shear strength typically involves the greatest uncertainty in practical applications of slope stability analyses. Note, however, that by definition the factor of safety is the same at all points along the potential slip surface. This is reasonable only at failure; that is, when the factor of safety equals unity. Because the factor of safety is taken to be the same at all points along the potential slip surface even when the factor of safety is greater than unity, limit equilibrium methods of analysis cannot model the mechanism of progressive failure.

2. All assume that the strength parameters are independent of stress-strain behaviour.

3. All use some or all of the equations of equilibrium to calculate the average values of and on each slice, where is the normal stress on the base of the slice. is required to determine the shear strength using the following equation:
   

   (2)

   in which c and are Mohr-Coulomb strength parameters.

4. Since the forces involved in equilibrium methods are statically indeterminate, all methods employ assumptions to make up the balance between the number of equilibrium equations and the number of unknowns in the problem.

The most commonly used slope stability analysis methods divide the mass above an assumed slip surface into vertical slices. This is to accommodate conditions where the soil properties and pore pressures vary with location throughout the slope. The forces acting on a typical slice are shown in Figure 1.

W = weight of slice
kW = seismic force applied at center of slice
S/F = mobilized shear forces at base of slice
P' = effective normal forces on base
U = water pressure force on base
B = resultant top boundary forces
X = vertical side force
E = horizontal side force

Figure 1 Forces acting on a typical slice.

As stated previously, equilibrium methods employ assumptions to make the problem statically determinate. The most critical of these assumptions typically deals with the side forces X and E. Figure 2 shows the assumption made concerning side forces for several of the more common methods.

Figure 2 Differences in assumptions regarding side forces in common methods of slope stability analysis.

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