Question: In the 3D model, how do I show the portions of the corridor that are below the existing ground surface?
Answer: One of the easiest ways to show the design surface that falls under the existing ground is to create a Clipped Terrain. To see how to create a clipped terrain, view this article: Video: Terrain Model Clipping
The video below shows a different method in which the terrain reference in the 3D model is masked to show the corridor as it falls below existing ground elevations.
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.