Update:
I think I’ve found a way to keep the debris out of the path behind the bulldozer: I added a passive floor beneath the fractured solid and gave it a Box type collision. I also changed the collision type for the cube from the default to Sphere. That gives a much more bulldozer-like displacement of the fragments. I turned off the visibility of the new floor in both the viewport and the render so I could keep the plowed path transparent.
