You need to give more info than simply "rigid pipe". What kind of "rigid pipe", what pressure will it have and where you are burying it.
The threads
RSC pipe - Rigid Steel Conduit (RSC) can be installed indoors or outdoors, in dry or wet locations, exposed or concealed, in all kinds of atmospheric conditions, and in hazardous locations.
Assuming your reference to "earthing" is grounding (Grounding Electrode Conductor), a pipe is, as it sounds, a pipe (i.e. water pipe). A plate would be, as it sounds, a square (or rectangular) copper plate which would be buried in the ground.
It is not advisable to bury a galvanized gas lineMost civilized codes will not allow galvanized piping to be buried under ground even for waste or venting it is against good plumbing practices. I have seen quite a bit of buried galvanized pipe, both for water and gas. It does rust out eventually. Interestingly enough, it usually rusts through at point at which it comes up out of the ground.
a heat pump contains a water filled loop of pipe that is buried to a depth where the temperature is nearly constant
No, it is not.
The threads
Masts need to be RIGID.
A 770 Greenlee hydraulic pipe bender bend is used for rigid steel conduit from 2" to 4".
Water
Yes it should be
24 inches.
24 inches.
Yes.
Did you say im hung like a Pvc pipe? Danny Reeve x
Sir Walter Raleigh was buried with a pipe and tobacco. His coffin was also "lined throughout with the wood of my old Havana cigar-boxes."
Francis Drake