Soil Pile is sort of a hidden resource in the game. It is not an actual item, but is carried within your mecha and added to or subtracted from as necessary.
Soil pile is gained (added to your mecha's Soil Pile storage) when you place a Building or Foundation on terrain that is elevated above the "level" terrain height. There appears to be no maximum limit to how much soil pile your mecha can carry.
Soil pile is consumed (removed from your mecha's Soil Pile storage) when you place Foundation in an area lower than the "level" terrain height. If you do not have sufficient soil pile in your mecha's storage, the game will warn you and not permit you to place the Foundation.