Burying the dead on top of each other

In the Shulchan Aruch Yoreh Deah, Siman 362, it is written that two coffins should not be placed one on top of the other, and if it was already done, the upper one should be pushed aside. If there are six tefachim (Biblical measure ‘handbreadth’) of earth between them – approximately 60 cm, it is permissible. In practice, there should be about a meter of earth between each coffin, since there is shrinkage after they have been placed, so that there should be a space of 60 cm left afterwards.