Ordinary Method of Slices:
The simplest limit equilibrium method that assumes the interslice forces are parallel to the base of each slice. Fast but tends to be conservative.
Bishop Simplified:
More rigorous method that satisfies vertical force equilibrium and moment equilibrium about the center of the circular slip surface.
Spencer's Method:
The most rigorous method that satisfies both force and moment equilibrium for all slices, with a constant interslice force inclination.