Fellenius method (1927)

With this method (only valid for circular form slide surfaces) interslice forces are ignored and thereby the unknowns are reduced to:

•	n values of normal forces Ni;

•	n values of shear forces Ti;

•	1 Safety factor.

Unknowns are (2n+1).

The available equations are:

•	n equilibrium equations at the vertical displacement;

•	n equations relative to the failure criterion;

•	1 equation of the global moments.

fellenius

Actions on the i-th slice according to Fellenius

This equation is easy to solve, but it was found that provides conservative results (low safety factors) especially for deep surfaces or at the increase in the value of the pore pressure.