I've noticed an area in front of my house that is starting to slightly sink. I know that I need to backfill on two other sides of the house and some areas in the backyard. As well the concrete walkway in front, not the sidewalk, has shifted due to the sinking. What is the proper way for me to backfill the land, and level out those concrete pieces (each piece is about 3ft X 5ft)? How can I calculate how much dirt I'll need, and what kind of dirt. There will be grass grown over the backfilled areas, or should I use sod?