In the past century a large number of models to analyse the stability of slopes have been proposed, first by Fellenius [1,2], shortly followed by others, for instance May and Brahtz  and Bishop . As all models are only an idealization of reality, one should be careful to trust the results. Common idealizations for slope stability analysis were the adoption of a simplified, usually circular failure surface and the assumption of plane strain conditions. In spite of these simplifications, a slope stability analysis by hand was quite laborious, as a sufficient number of trial surfaces need to be examined to find the most critical one. This has become a lot easier by the development of computer programs.