When creating Surface on surfaces tab in corridor properties was "Top Links" specified for overhang correction?
Does the Code Set used contain the codes Top and Datum?
When you add the links to your surface definition make sure that the Add as Breakline box is checked.
Please use the Accept as Solution or Kudo buttons when appropriate
i used Code contain the codes Top and Datum
Only use one of the codes, top or datum. No need to add them both. Additionally, as previously mentioned, make sure you add an overhang correction to your corridor surface (in most cases it doesn't matter if you do top or bottom links).