Changes

For instance: 0 = empty (walkable tile); X = support beamed tile; R = rock tile.

In this example you can collapse all of the 0 tiles. (You can not collapse the tiles already fill with rock.)

You could collapse all 0 (empty) tiles making it look like this:

After which you could mine out all of the R tiles again. But not the X, unless you are mayor of that tile.