Depending which country you live in will depend on depth. Consulte your local municipality for the laws applying to you. In South Africa, depending on the size, Diameter of pipe, less than 75mm, not less than 650mm below the surface of the ground. More than 75mm but less than 110mm, not less than 1500 below the surface of the ground. More than 110mm, not less than 2000mm below the surface of the ground.
24 inches.
24 inches.
Yes it should be
Do you mean the old black alkathene? Yes it can and should be.
Yes, PEX pipe can be buried underground as it is commonly used for underground water supply lines.
When a pipe shrinks
In a lawn irrigation application it should generally be deep enough not to be hit by the lawn-mower, or rototiller if you plan to use one
Yes.
It's not a matter of how deep electrical wire should be buried. The wire should be protected by plastic or metal conduit and marked suitably above ground - buried utility. I am certain that you will be burying a water line to the fountain "below the frost line" for your area, so consider placing you electrical wires at the same depth.
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.
A geothermal pipe for a home generally needs to be buried at least 5-10 feet deep, but this can vary based on local regulations, soil type, and climate. Deeper depths may be required in colder climates to take advantage of more stable ground temperatures for efficient heat exchange.
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."