If the Surface Type = Non-Circular in the Surface Options dialog, the Add Surface option in the Surfaces menu allows you to define specific NON-CIRCULAR (piece-wise linear) slip surfaces. To add a specific non-circular slip surface:
Make sure the Surface Type = Non-Circular in the Surface Options dialog.
Select Add Surface from the toolbar or the Surfaces menu.
Enter the vertices defining the slip surface, graphically with the mouse, or by entering coordinates in the prompt line.
When all vertices are entered, right-click and select Done, or press Enter with nothing in the prompt line, and the slip surface will be added to the model.
If the starting and ending points of the slip surface are not entered exactly on the External Boundary, then they will automatically "snap" to the nearest point on the External Boundary.
In order to have a kinematically admissible slip surface, all points must be either increasing or decreasing monotonically in the X direction (i.e. the slip surface cannot "reverse" direction at any point, or include vertical segments)
Any number of specific non-circular surfaces may be defined for a model.
Specific non-circular surfaces and a search (e.g. Block Search or Path Search) can be defined for the same analysis. i.e. you may analyze specific surfaces, and perform a critical surface search at the same time on the same model, this does not cause any problems.
Specific non-circular surfaces created using Add Surface can be deleted using the Delete Surfaces option in the toolbar. The Delete option is also available in the Edit sub-menu of the Surfaces menu.