I've never claimed to be an expert but here goes.
Good clay, mashed against other good clay, still doesn't make the leak tight seal that you want. Every time I find a leaking dam, I ask if it was cored. I've only found one cored one that leaked. Of course, I haven't seen them all or even very many. I've seen a lot of non leakers that were cored. You can almost raise mosquitos behind my non cored dam. But it is dry behind my cored one.
Of course, like Mike Otto says, "all dirt leaks". But, if I ever do it again, mine will be cored. It's just not worth the possible problems.
