This video demonstrates how to troubleshoot when the existing ground does not show up as you cut the Cross Sections of your corridor.
You can try to fix it by detaching the terrain reference from the 3D model, and re-attaching it to the 2D model.
Note: video has no sound.
Click the video below to play it. To view in fullscreen mode, first click the YouTube link at the bottom of the video to launch the video on the YouTube site. Then, click the fullscreen button (rightmost button at bottom of video).
Recorded in 1280 X 720 resolution.