The regulations on caskets/vaults etc. are quite variable from place to place and have changed considerably over time. Hard to say exactly what might have been done fifty or even twenty years ago. Walk through an old cemetery and see how many depressions there are over (apparent) graves. No vaults there. On a hillside, the uphill side might be 6 ft (or 5 ft) down, but the downhill end might be only 1-2 ft down.