okay guys, here's the standard. and you won't have concerns or worries about the what if's.
whether above or below, the hole itself should be 6 feet deep. the trampoline should be 18 to 24 inches from the bottom. if you have boys that do big high bar dismounts and big vaults and senior girls that do big bars and vault, the tramp should be closer to 18 inches.
if the tramp is in a cement hole, angle iron is mounted in to the the walls of the cement. the angle iron has as many holes as required to accommodate the size of the trampoline bed with springs.
if it is above ground, a trampoline structure must be custom made. it's made of tubular steel with flat iron and gets placed at the bottom. the same depth measurements apply.
the whole concept of a pit with foam cubes and it's depth is to have the ability to decelerate the body as it enters the pit. yes, there are pits 3-5 feet but they are precarious at best and cause a lot of unnecessary injuries had the pit hole been 6 feet deep.
again, there are dealers out there that can explain in fuller detail depending on what the needs of the gym are. and what kind of athletes you have training. midwest gym supply is a great resource and they have done hundreds of these kinds of applications.
i personally have in ground pits. construction described above. tramps are 18 inches from the floor bottom. have never had an injury in these pits. we also have college men and women that train in our facility over the summer and during certain breaks. no way i would have a hole less than 6 feet given some of the skills they are practicing nowadays.