I intend to cover a grave plot in my village with sand and pebbles to protect it from becoming muddy due to the rain
There is no objection
as to adding a layer of earth to pathways between graves for the purpose of clearing mud, thus making it easy for visitors and others to easily mover between the graves. But you are not to put anything on top of the graves that will raise them above the levels prescribed in the Shari`ah (Islamic law) which is one span.May Allah grant you success! May Peace and blessings of Allah be upon our Prophet Muhammad, his family, and Companions!