Once the foundation walls are poured, the overdig needs to be backfilled. Backfill, however, is more than pushing dirt into the overdig.
It consists of placing an appropriate amount of gravel (10-14 in.) over the tile for drainage. Leftover spoils are then placed into the overdig to complete this task. Sometimes the overdigs are filled entirely with gravel - to reduce settling in areas such as patios, porches and driveways.