Ditto on checking with the PoCo!
Also, are you referring to "Cushions" used on Primary Feeder Ducts?
SCE (So. Cal. Edison) wanted Sand and Gravel Cushions on Primary Feeders - especially when Feeders came from Poles on Hills.
SCE did not require anything special for Secondary Feeders, unless the Soil is very "Rocky", then they want some Sand.
I have the currently used "ESR" Manual from SCE (Electrical Service Requirements Manual), which I use for assistance in designs/engineering projects. If needed, let me know some specifics and I can reply with quotes from the ESR.
Although it may not be exactly what your PoCo would specify, it might be somewhat relavent - just for an idea.
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!