There is a longstanding custom to pile a mound of dirt upon a freshly dug grave. This is considered the final step of the burial and serves as a marker of the grave. Piling the dirt upon the grave is permissible on Yom Tov.
It is forbidden to derive benefit from any part of a grave. The poskim disagree about whether the prohibition encompasses the entire pile of dirt on the grave or only the layer that covers the body. Regarding the second opinion, there is further disagreement about whether the prohibition involves only a thin layer of dirt covering the body or whether it includes the first three tefachim of dirt covering the body.
[שו"ע תקכו, ד, ומשנ"ב כח; ביאורים ומוספים דרשו, 25-26]