What precautions will be taken to prevent the subsoil from mixing with the topsoil?

On improved lands, topsoil will be excavated and segregated into separate stockpiles to allow for the re-establishment of the original soil profile. In agricultural fields, hayfields and other fields used for crops, the top 12 inches of topsoil will be segregated into a separate stockpile. In places that have less than 12 inches, all of the topsoil will be removed and stored separately. Once construction is complete, the subsoil will be placed into the trench first, followed by the topsoil.