If you get a hand auger and bore holes in the natural ground, and you have heavy clay from as deep as you can auger, up to the surface, there’s no reason to make coring a requirement. You are essentially replacing good clay with good clay. Dig holes along the centerline of the proposed dam and save some money. You can get a hand auger from Ben meadows or Foresty Supplier.

On the other hand, if you hit questionable material several feet down, then core it.