Thanks for your interest. I am constucting a short, 3 foot, extension to a 16 foot wide deck on the back of a condo, about 4 feet above ground level. The condos are about 40 feet from a lake, so the ground water is there below ground level. The local building inspector told me that the concrete in the sonotubes would displace the water (down and out), so no problem. The augered holes are 12 inch in diameter. Two decks are complete! Thanks, Ed