Investigate your soils more closely. Dig more test holes, different places. Analyze the soil, and try to quantify the amount of clay you have. Compare what you have with what you need, how far it must be moved to create the core, and front side slope. Then, if you don't have enough clay, explore other options.
Do this....dig more test holes, then come back to this site and let us know what you find. Based on those findings, we'll do our best to advise your next step.

Teach a man to grow fish...
He can teach to catch fish...