I need to fill in a 12 foot by 3 foot rectangular hole in some concrete slab. The slab is about 5 inches thick. The dirt under the slab has been dug out to reach a sewer pipe and needs to be shoveled back into the hole. There will not be very much weight on the slab and it will be painted over. I need to know how I fill in the hole, if the dirt needs to be compacted and how much cement I need.