"most just fill the bottom 4 to 5 feet with pea gravel "
I went down 49', 6" casing with water pressure, filled about 10 feet with gravel, then pulled the casing up 10 ft. As to sealing the casing, we have clay layers, I scooped up gallons of clay which came up, sent it down the outside the casing. You could purchase bags of bentonite, basically the same thing. You can not use soil, depending on it's content it could allow surface water to penetrate. As mentioned, you are drilling down more then a few feet with an auger, unless it has carbide tips, a rig and a decent motor driving it. Worked on one as kid.