Recently I read an article regarding burial in Israel. There seems to be a shortage of graves and therefore there are those that are using this method of burying 4 deep underground in order to alleviate the shortage of space. Are there any Halachic issues with this? Is this an area of dispute amongst Halachic authorities?
(Gra and R. Akiva Eiger there cite Beis Yosef, who in turn cites Terumas Hadeshen, that indeed three tefachim is enough.)
Shach there (:4) points out that even the requirement of six tefachim of separation can be waived if there is no other way to perform the burial (as indeed was common in the European communities, where the size of Jewish burial grounds was sharply restricted by the authorities - see Pischei Teshuvah :3).
Obviously it's a question for the local rabbanim to decide on a case-by-case basis, based on their evaluation of how critical the shortage of space is.