The dirt has to go somewhere. The new field level is still many feet below grade. They either need to stockpile the soil on site or haul it off. They’re going to need a significant amount of import to fill in the current stadium but storing it on site for three years seems a bit long. By the way…Once they reach the new field level, they’ll remove a few feet more and then reinstall it under compaction to stabilize the base and support the new structure.