This division was considered only as a mental one in order to simplify the mentioned equilibrium equations. Justification of such procedure was fully proved in further investigations with the use of a filming method, where the occurrence of soil failure according to an introduced plane of mental division of the soil lump was not found. Such procedure was also applied by Hettiaratchi and Reece (1989). Solving a set of equilibrium equations one can obtain the equation for Fp: