Why is the sinkhole going to be filled?

After he rapes and kills her, Mr. Harvey puts Susie's dismembered body parts into garbage bags. He locks the bags in a safe, which he then buries in a sinkhole. This makes it a very safe location for the disposal of a murder victim—one that will be very difficult to discover. The sinkhole is often used by local people to dump their garbage, so there's all kinds of stuff down there.
Later on in the story, Susie's friend Ruth—who's always had a strange fascination with the sinkhole—finds out that it's going to be filled in with concrete as part of a property development. This prompts her to go pay a visit. Much to the approval of Heaven's souls, Ruth regularly visits places where women and girls have been murdered, writing about them in her notebook.