Haven't had to deal with that since I left Chicago 40 years ago. But at that time the level NEVER dropped so we just poured on top of the gravel. Had an interesting time sinking a 5' by 5' cast iron ejector basin when dealing with a water level 6" below the floor and "quicksand". Even pouring joints was an exercise in digging and bailing while a third person made the joint. Each length had to be plugged immediately.