Excavation involves the demolition of buildings, removal of existing concrete, asphalt, stumps, surface organics and ultimately the delivery and placement of the proper compactable fill.  A well-prepared site involves the removal of material to reach virgin grade, compacting a granular fill within limits of the compactor used, ensuring the prepared site is four feet larger than the concrete floor to be poured and ensuring the final elevation allows water to flow around and away from the new site, not through it.