Since the 1930s, the limit equilibrium (LE) approach has been used to analyse slopes. This approach makes use of a number of differing analysis methods depending on the type of problem (circular vs non-circular) to be solved and the required accuracy of the result. 

The initial method adopted for undertaking LE analysis was the Fellenius or Swedish circle method (Fellenius, 1936). This method can only be applied to circular slip surfaces and leads to significant underestimation of the factor of safety (FoS) and is now rarely used. Bishop (1955) developed a revised method for undertaking circular slip analysis which improved the accuracy of the resultant FoS. This revised method required an iterative procedure to solve and so it was suited to computer methods where this could be automated. Bishop’s methods are still routinely used in slope stability analysis software to this day.