All perfect praise be to Allah, The Lord of The Worlds; and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
In principle, a grave is a hole in the ground which keeps the smell of the corpse inside, and keeps the animals away.
One dead person should be buried in each grave unless there is some necessity for burying more. As regards burying the dead in what is called "Fosqiahs", the dead is placed in an underground chamber, then its door is closed and he/she is left to transform therein. This method of burial is not approved by Sharia because the Sunnah has clarified how our Prophet (PBUH) used to bury the dead Muslims.
Muslim scholars have agreed that exhuming the grave is forbidden, because that violates the sanctity of the dead. In this regard, the Prophet (PBUH) said: "Breaking a deceased body’s bones is exactly like breaking them when he is alive."{Abu Dawud}. Moreover, Allah, The Almighty, Said (What means): "We have honoured the sons of Adam" {Al-Isra`/70}; Therefore, not exhuming the grave and not violating its sanctity is honouring its dweller.
Muslim scholars have forbidden exhuming a grave to bury another person in it because this violates the sanctity of the grave`s dweller and triggers the smell of his corpse, whether both were males or females or a male and a female, even if the latter were Mahrams, such as burying a person with his mother.
In conclusion, burying the dead in "Fusqiahs" doesn`t comply with the rules of Sharia; therefore, it is forbidden because of the aforementioned violations. Moreover, resolution No.(126) of the Jordan Iftaa` Board forbids burying more than one person in each grave. However, if the corpse has transformed into earth, it is permissible to exhume the grave and bury another in it. It is also permissible to establish a cemetery in the place of the above mentioned one, provided that it complies with the rules of Sharia in that regard. Al-Imam Annawawi said: "It isn`t permissible to bury a person in another`s grave unless its dweller has already transformed into earth, and nothing was left of his corpse; neither flesh nor bone." {Al-Majmou` Shareh Al-Mohazzab 5/284}. In addition, the time that each corpse needs to reach a state of full decay varies from one area to another, and the people of each country are aware of that; however, in our country, it is well known that the bones of the dead can last for hundreds of years, as we have noticed. We would like to point out that it isn`t permissible to take out the corpses of those who were buried in the "Fosqiahs" so as to protect their sanctity. And Allah knows best.