You can select corridor feature lines for a boundary via the boundary menu in corridor properties. Right click on the surface in the boundary menu to bring up the “add interactively” command. Then tap the start and end of each feature line that you would like to use for the boundary. One caveat to this is that if you have you links overlap around curves, then the feature lines are connected via the stationing order – so you get a overlapping feature line that Civil 3D will not accept as a boundary.
