Is placing marble on a grave and erecting an iron fence around it for protection considered a Bid‘ah (innovation) or is there no objection to it?

If the intention is to place a piece of marble or a stone at the head of the grave to mark its location, there is no objection, and it might even be considered part of the Sunnah. However, if the grave is exposed to passage, there is no objection to placing an iron fence around it to protect it and prevent people from walking over it, out of respect for the deceased, and due to the Prophet’s (peace be upon him) saying: "Breaking the bone of a dead person is like breaking it while he is alive."