What is the Sharia-compliant way to deal with graves filled with the bones of the dead that cannot be deepened due to the clayey nature of the land, with a suggestion of rebuilding the grave and burying the remaining bones in a "second story above the grave"?

Every deceased person must be buried in a separate grave. It is not permissible to combine more than one deceased person in a single grave except out of necessity, with a barrier placed between the deceased. If the ground is soft, it is permissible to place the deceased in a coffin or on planks to prevent water from reaching them. In cases of extreme necessity, such as graves being full, it is permissible to create tiers within a single grave or to cover the old deceased with a vault of bricks or stones so that a new deceased can be buried above them, while honoring the dead is obligatory.

If there is fear of the vault collapsing, it must be restored, not demolished, unless it is demolished wall by wall while covering the deceased and their bones. It is not permissible to remove bones from the grave or to bury above them; rather, they should be placed to one side of the grave, and an earthen barrier should be placed between them and the new deceased.