Slide may not always be able to compute a valid safety factor for all slip surfaces generated by an analysis. There are several reasons why this may occur, for example:
if slip surfaces do not intersect the slope surface in two locations only
if negative driving forces or moments occur
if the limit equilibrium iteration procedure does not converge
In such cases, you may wish to see why a safety factor could not be calculated for a surface. For example, if a circular surface search has been performed, and the slip center grid is incorrectly located, blank (uncontoured) areas may appear in the grid, indicating that no valid slip surfaces could be generated at one or more grid points.
If the Query Invalid Surfaces option in the Query menu is enabled (which it is by default), then while using the Add Query option, if you hover the cursor over the slip centers (circular surfaces) or axis points (non-circular surfaces) of invalid slip surfaces, a popup textbox will display the error code and a short description of why the safety factor was not calculated for that surface.
NOTE: you cannot actually add a query for an invalid surface, since there is no data available for invalid surfaces. You can only view the error codes for invalid surfaces.
If you do not want these error codes to be displayed while you are using Add Query, then you can turn OFF the Query Invalid Surfaces option in the Query menu, the toolbar, or in the right-click menu while you are using Add Query.
See the Summary of Error Codes topic for a summary of all of the error codes which can be reported by Slide, and the probable causes for each error code.