Is it permissible to bury above the dead in a mass grave after placing a layer of sand over them?

If the bones of the deceased have decayed and turned to dust, and no trace of them remains, then it is permissible to bury others above them. This may even be preferable to excavating old graves. However, if their bones have not decayed and turned to dust, then it is not permissible to bury others above them except out of necessity, such as a large number of deaths and the difficulty of assigning a separate grave for each deceased person. It is forbidden to collect the bones of the deceased to bury others, or to place a new deceased person on top of them.