Dear PoPville – What Happens to the Dirt Excavated from Huge Construction Sites?

Dear PoPville,

Where does all of the excavated dirt from these gigantic parking lots underneath the new downtown DC buidlings go (ie City Center, Mariott Marquis, O St. Market, etc.)? – Even the smaller sites have what is otherwise a significant amount of excavated dirt. Is it contaminated? Is it recycled for other projects, of perhaps just dumped in a landfill.