^ This is my opinion and not my knowledge of what is going on. I do not believe they cannot leave a bunch of junk under the sand as it would create an unstable building substructure for the future. Also, as someone else here pointed out, they cannot leave the smooth concrete surfaces or shark tanks as they would fill up with common water runnoff turning the fill into quicksand. So, the substructure needs clear drainage and stable fill. I could be generally wrong, but I doubt it.