Abstract：Although the inter-slice shear forces are ignored in the simplified Bishop method and the complete equilibrium conditions are not completely satisfied，the achieved factors of safety of circular slip surfaces are in excellent agreement with the results of those rigorous methods. This is the one of the most difficult problems that have not been solved for a long time. The study shows that the absence of inter-slice shear forces in the factor of safety equation of the simplified Bishop method does not mean they actually disappear，but a certain term involving inter-slice shear forces equals zero. A set of inter-slice shear forces could be always found where not only the sliding body satisfies the complete equilibrium conditions，but also makes that term equals zero. Therefore，it is implied that the simplified Bishop method automatically satisfies the rigorous equilibrium conditions，which can be regarded as a rigorous inter-slice method.
