RMC and burial depth

[going_commando]

A discussion elsewhere made me think about this. RMC is approved to be installed in areas where it is subject to severe physical damage. It can be run on the ground, such as across a concrete slab. If that's the case, why, if you want to put it in the ground (not under a driveway), it must be buried 6" deep? Why can't it be 2" deep? It is perfectly compliant to have it 0" deep, so was 6" just an arbitrary number that someone made up? I hadn't thought of it before, and just find it to be a very silly rule.

 

[infinity]

It's likely because on the ground the conduit is visible so it can be avoided if someone were digging in that area. I never really understood why a 6" burial depth is permitted maybe it's just to deal with soil conditions that make digging deeper impracticable.