Fellenius / Petterson
The simplest method of slices assumes only the overall moment equation of equilibrium written with respect to the center of the slip surface. The shear and normal forces between blocks Xi and Ei are neglected. The factor of safety SF follows directly from the following expression:
effective values of soil parameters
normal force on the segment of the slip surface
inclination of the segment of the slip surface
length of the segment of the slip surface
Petterson KE (1955) The early history of circular sliding surfaces. Geotechnique 5:275-296.