Imagine you have a bulldozer or grader like vehicle on your terrain in Unity 3D.
How to change the terrain topography using a bulldozer or grader like earth moving machinery object in Unity 3D?
-Adamo
I believe you would start with the terrain data.
You could make some 3d geometry to mimic moving dirt then move them with blades and such then adjust the terrain height data depending if the blades are up or down.