I'm hoping the photo below will help clear up any confusion for you. In the main photo you can see what will be a HUGE concrete slab. Coming out of the concrete slab you can see what looks like they will be the "footers" that we see actually sticking out of the ground...
So the main giant concrete slab will end up being covered with dirt, and grass, and the only thing that will be sticking out will be the small "footers" that we see sticking out of the grass on most roller coasters.
Source.
Like you can kind of see in this photo,(leftish bottomish part of the photo) where the main part of the conceret is covered by dirt, while there is the "footer" part you can tell will still sticking out of the grass.
Source.
Then the end result will look like the photo below, with the main "huge" concrete part underground.
Source.
Granted I am not 100% positive this is how that works, but I am pretty sure it is!
Does this make sense?? (I hope it does)