The Global Minimum slip surface is the slip surface with the LOWEST factor of safety, for a given analysis method, of all slip surfaces analyzed.
The display of the Global Minimum slip surface can be toggled ON or OFF by selecting the Global Minimum option from the toolbar or the Data menu. Note that when the Global Minimum surface is displayed:
the factor of safety is displayed beside the axis point (center of rotation) for the slip surface
radial lines joining the axis point to the endpoints of the slip surface on the slope are also displayed (the display of these lines can be toggled on or off in the Display Options dialog).
By default, when a model is first opened in Interpret, the Global Minimum slip surface for the Bishop analysis method, will be displayed (if a Bishop analysis was one of the analysis methods used).
To view the Global Minimum surface and safety factor for other analysis methods (e.g. Janbu, Spencer, GLE etc), simply use the mouse to select a method from the drop-down list in the toolbar.
It is important to realize that the Global Minimum surface will NOT NECESSARILY BE THE SAME SURFACE, FOR DIFFERENT ANALYSIS METHODS. In general, each analysis method may find a different Global Minimum surface and safety factor.
Finding the True Global Minimum
The term "Global Minimum" should be used with caution. The Global Minimum surfaces displayed after an analysis, are only as good as your search techniques, and may not necessarily be the lowest possible safety factor surfaces for a given model. Depending on your search methods and parameters, SURFACES WITH LOWER SAFETY FACTORS MAY EXIST!!!