to clarify, you have the terrain model attached to each DGN file and in each DGN file the terrain is attached to a specific model. Then when you reference in each DGN file it is also including the Terrain Model from each of the files, so you are getting multiple copies of the Terrain in the Civil Model?
Are you seeing each instance of the terrain added under a new Terrain entry in the Project Explorer > Civil model > Terrains?
Are you only wanting to reference the model that has the corridor and not the model which contains the surveyed terrain?