We did one about 20 yrs ago, maybe more, at that time hot tubs were not real popular and putting them inground was unheard of at the time. With nothing to go on we went about installing one in a berm probably about 3 feet into the ground. We built up a pisa stone wall around the perimeter and set the hot tub sinside that. The bottom was just granular stone. I think it was two years later the owner called to say that his hot tub was floating
and part of the pisa wall was opened up! Of course we had forgot to take drainage
into consideration and never suspected that water would accumulate and actually lift the hot tub.