We had to lower a 60" diameter by 5' high cast iron basin into flowing sand years ago. We had to put 4x4 posts at the corners with 2x8 planks on the outside. As we dug down we drove the posts deeper and slid the boards down. We could only get it within 3" of the finished floor, so we filled it with water to keep it from floating. We also placed heavy metal "clips" over the edges and into the concrete, when the floor was poured so it could not float when the pumps emptied it.