No knowing what your program requires, I's suggest making a sketch of your situation and copy it to a file and add it to a post here so that there can be comments as to where you have or should show the limits of computations. There might be some other limitation in the program that you missed.
You have something defined incorrectly in your search routine. Could be anyone of a half dozen things depending on how you are specifiying your slip surface.
Thank you for the replies. Geo Studio came to the rescue (once I had sent them the file). The slip surface limits of my analysis were set at a single point.
It may be worth taking me back to first principles. Any interest?
