Yuop, I get the Landscrapers to do my trenching too!
NEC its a structure... That is if it has wiring in/on it. The tub, I consider seperate....
A leiniant AHJ would give you an OK on it, maybe. But if theres other electrical like say lights and outlets in it, its not only a structure but a range of other violations too. (Noticing the bar area on it.... It just screams "rope light".)
Personally, I would have done a UFA in the pad and bonded it to the tub, but thats me....