Direct Burial cable including Direct Burial electric wire needs to be buried in a trench that is a minimum of 24 inches or 2 feet deep to meet code and protect it from any damage.
If you want to go with a less deep trench then you can place the direct burial wire in conduit which will allow you to bury the electric wire at 18 inches deep.
You don't have to put direct burial cable into conduit when burying it by I've always done so just to give it added protection should someone decide to dig in that spot and make contact with the wire.