Assumptions:- circular arc, radius R, centre O

The soil mass above a trial failure surface is divided into slices by vertical planes. Each slice is taken as having a straight line base.

The Factor of Safety of each slice is assumed to be the same, implying mutual support between the slices, ie. there must be forces acting between the slices.

To go about finding the FoS, the problem is now statically indeterminate and we need to make some assumptions about the interslice forces:- (See Fellenius' Method)