Slope Stability Analysis Methods and Failure Surface Options

Our shoring design software - DeepEX, as well as, our Soil Nail Walls design program - SnailPlus, have implemented slope stability analysis capabilities. Our programs can locate fast the most critical slope surface with different methods, as well as,  calculate the slope stability safety factor according to different approaches.


In most cases, the primary purpose of slope stability analysis is to contribute to the safe and economic design of excavations, embankments, earth dams landfills and soil heaps. Slope stability evaluations are concerned with identifying critical geological, material, environmental and economic parameters, that will affect the project, as well as understanding the nature, magnitude and frequency of potential slope problems. When dealing with slopes in general and slope stability analysis in particular, previous geological and geotechnical experience in an area is valuable.

DeepEX and SnailPlus software implement all common methods of slices for the calculation of the slope stability safety factor (Bishop, General Limit Equilibrium (Morgenstern-Price) and Spencer methods).

The simplified Bishop method uses the method of slices to discretize the soil mass and determine the FS (Factor of Safety). This method satisfies vertical force equilibrium for each slice and overall moment equilibrium about the center of the circular trial surface. Since horizontal forces are not considered at each slice, the simplified Bishop method also assumes zero interslice shear forces. The effective normal and pore pressure forces, acting on the base slice, do not affect the moment equilibrium expression since they pass through the center of the circle. Thus, Bishop’s method should not be applied to compute an FS for non circular surfaces.

The General Limit Equilibrium method (GLE) and the Spencer's method take into consideration both the equilibrium of moments and the equilibrium of forces. This allows the use of the GLE method in all kinds of failure surfaces, not only circular ones like Bishop method. The GLE method adopts a function to assign the interslice force angle on the right-hand side of each slice. On the other hand, Spencer method assumes a constant interslice force angle.



DeepEX and SnailPlus software implement a number of differen failure surface search procedures. All of these options can be assigned in different construction stages on the same model, allowing us to check fast different cases and locate the most critial failure surface. The following search options are available:

  • Automatic Failure Surface Search
  • Circular Failure Surface (Single Center or Rectangle of Centers)
  • Circular Failure Surface with Active and/or Passive Wedges
  • Block Type Failure
  • User-Defined Slope Surface

According to the selected method, several parameters (possible surface centers, radious limits, left and right search limits, block limits) can be accessed and modified on the model area.

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