In the UK, buried cable without conduit is virtually unheard of. In some cases "Alkathene" or similar pipe is used. It's pretty cheap, and very tough, so it will generally prevent an enthusiatic gardener with a spade from doing damage. Often low cost 2" server ducts are used, or if the building contractor is thinking ahead, he will run in 4" pipes. (Not that expensive, and easy to add cables later, provided he had the wit to leave a draw-in rope!)
Even cables graded for outdoor use can easily get cut through if they are under tension and/or laid over sharp stones etc., so it's always worth a bit of added protection.
