[LouisVanDerWright]

The metal tube you see is a temporary liner that is reusable. They sink that liner into the ground, lower in a permanent galvanized spiral tube shell, then lower in a rebar cage and pour the concrete. After that's done, they fill the top of the cage with gravel and pull the temporary liner that you see. The reason they fill it with gravel is to temporarily bury the cage so equipment can drive over that spot again once the temp liner is out. I think it also probably fills in the gap between the temporary liner and the permanent shell so that there is less risk of settlement once the liner is out. You don't want your foundation tipping out of plumb when you pull that liner out.