Separating stacked graves

Hi,

Graves which are layered require 6 tefach (21.6 inches) of height between them. This does not depend on the type of material between them; even where impermeable material separates the grave, this space is required.

(Rav Moshe Feinstein, Igrot Moshe Yoreh Deah 1:234)

המצפה לישועה,
Mordechai