Not to discourage but if you ever see an actual 15 ft deep hole you realize just how deep that really is. We are talking about basically two stories deep/3 grown men standing on each others shoulders. Your soils very well might/probably will drastically change in that depth. Another issue, and I HATE to bring up the S word, but safety becomes a concern when going much past 8 ft deep.
I would really recommend start digging a 5-6 ft deep area and see if you are actually able to do that, I'm guessing you will change the size, depth, and more once you get a single width ramp down to 6 ft deep. And to try this your only out maybe $20 in diesel, and some grass seed if you decide to fill it back in.