DC, coring is kinda hard to explain without drawing a picture. Essentially, it involves using a dozer to dig a core trench in the natural earth that will be the sides of the dam, down through the bottom area. Then, filling that/those areas in with clay and packing the heck out of. It ties the dam sides and bottom together. The alternative is to just start digging dirt from the bottom area and pushing it up next to the sides. That can and often does allow leaks between where the new dirt is pushed and the natural sides. If I haven't adequately explained it, send me an E-mail with your address and I'll draw a picture.
