May I make a request? This is related and may help others better grasp the concept. When you build said box, run two tests for everyone here:
one test filling the box (assuming it's at least 1/2 cy or so) with #57 stone poured rapidly out of a 5 gal bucket (will probably take two folks). pour it higher than the top then beat the top a little as you screed on the top flat. measure/weigh. the is meant to simulate $57 stone backfill placed "loosely" (ie. not compacted properly).
for the second test, backfill the box in 6-8 inch lifts with very slight tamping. repeat until full. measure/weigh. this simulates a "more proper" structural type backfill placed/compacted in thin lifts.
if possilble, please provide the results to the board to help disprove this "self compacting, it don't need no compaction" malarky that shows its face every so often. by the way, i have already done this little experiment before and posted but would like to see someone else try it. I actually suggest that everyone try it at some point.
Now how much lower are these unit weights than intact rock?