Originally posted by Emelio Lizardo:

I keep seeing these abandoned structures with a beam set well into the ground.

Well, what you can do is for example:
1. place a floor piece 3m above the ground where you want to set a beam well in to the ground.
2. attach a 4m core-wood pole to the floor piece from below. It will snap to any of the four corners and be 1m in the ground.

In principle I don't know of any other (vanilla) way to do that.